1.0+ATOMS




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ATOMS ** history an theories ** Atom ** n. A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, the entire structure having an approximate diameter of 10-8 and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons. The history of the study of the atomic nature of matter illustrates the thinking process that goes on in the philosophers and scientists heads. The models they use do not provide an absolute understanding of the atom but only a way of abstracting so that they can make useful predictions about them. The epistemological methods that scientists use provide us with the best known way of arriving at useful science and factual knowledge. No other method has yet proven as successful. **In the beginning** Actually, the thought about electricity came before atoms. In about 600 B.C. Thales of Miletus discovered that a piece of amber, after rubbing it with fur, attracts bits of hair and feathers and other light objects. He suggested that this mysterious force came from the amber. Thales, however, did not connect this force with any atomic particle. Not until around 460 B.C., did a Greek philosopher, Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms. Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. In fact, Aristotle dismissed the atomic idea as worthless. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. (Primates have great mimicking ability.) For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. Not until the early 1800's did people begin again to question the structure of matter. In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental.  Thomsons 'Rasin in the Pudding' model of the atom In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding.  In 1900 Max Planck, a professor of theoretical physics in Berlin showed that when you vibrate atoms strong enough, such as when you heat an object until it glows, you can measure the energy only in discrete units. He called these energy packets, quanta. Physicists at the time thought that light consisted of waves but, according to Albert Einstein, the quanta behaved like discrete particles. Physicists call Einstein's discrete light particle, a "[|photon]*."  Photoelectric effect Atoms not only emit photons, but they can also absorb them. In 1905, Albert Einstein wrote a ground-breaking paper that explained that light absorption can release electrons from atoms, a phenomenon called the "photoelectric effect." Einstein received his only Nobel Prize for physics in 1921 for his work on the photoelectric effect.

A heated controversy occured for many years on deciding whether light consisted of waves or particles. The evidence appeared strong for both cases. Later, physicists showed that light appears as either wave-like or particle-like (but never both at the same time) depending on the experimental setup.

Other particles got discovered around this time called alpha rays. These particles had a positive charge and physicists thought that they consisted of the positive parts of the Thompson atom (now known as the nucleus of atoms). In 1911 Ernest Rutherford thought it would prove interesting to bombard atoms with these alpha rays, figuring that this experiment could investigate the inside of the atom (sort of like a probe). He used Radium as the source of the alpha particles and shinned them onto the atoms in gold foil. Behind the foil sat a fluorescent screen for which he could observe the alpha particles impact.

The results of the experiments came unexpected. Most of the alpha particles went smoothly through the foil. Only an occasional alpha veered sharply from its original path, sometimes bouncing straight back from the foil! Rutherford reasoned that they must get scattered by tiny bits of positively charged matter. Most of the space around these positive centers had nothing in them. He thought that the electrons must exist somewhere within this empty space. Rutherford thought that the negative electrons orbited a positive center in a manner like the solar system where the planets orbit the sun.  Rutherford's atom  Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth (10 ) of the atomic volume, but contains almost all of the atom's mass. If an atom had the size of the earth, the nucleus would have the size of a football stadium.

Not until 1919 did Rutherford finally identify the particles of the nucleus as discrete positive charges of matter. Using alpha particles as bullets, Rutherford knocked hydrogen nuclei out of atoms of six elements: boron, fluorine, sodium, aluminum, phosphorus, an nitrogen. He named them protons, from the Greek for 'first', for they consisted of the first identified building blocks of the nuclei of all elements. He found the protons mass at 1,836 times as great as the mass of the electron.

But there appeared something terribly wrong with Rutherford's model of the atom. The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward. Moreover, physicists reasoned that the atoms should give off a rainbow of colors as they do so. But no experiment could verify this rainbow. In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. (This began a new approach to science because for the first time rules had to fit the observation regardless of how they conflicted with the theories of the time.) Bohr said, "Here's some rules that seem impossible, but they describe the way atoms operate, so let's pretend they're correct and use them." Bohr came up with two rules which agreed with experiment: RULE 1: Electrons can orbit only at certain allowed distances from the nucleus. RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.  Bohr's atom for Hydrogen  The electron can exist in only one of the orbits. (The diagram shows only five orbits, but any number of orbits can theoretically exist.)

Light (photons) emit whenever an electron jumps from one orbit to another. The jumps seem to happen instantaneously without moving through a trajectory. The examples above show only two possibilities from Rule 2. By the 1920s, further experiments showed that Bohr's model of the atom had some troubles. Bohr's atom seemed too simple to describe the heavier elements. In fact it only worked roughly in these cases. The spectral lines did not appear correct when a strong magnetic field influenced the atoms.  Bohr- Sommerfeld model of the atom  Bohr and a German physicist, Arnold Sommerfeld expanded the original Bohr model to explain these variations. According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Orbits can appear circular or elliptical, and they can even swing back and forth through the nucleus in a straight line.

The orbit shapes and various angles to the magnetic field could only have certain shapes, similar to an electron in a certain orbit. As an example, the fourth orbit in a hydrogen atom can have only three possible shapes and seven possible traits. These added states allowed more possibilities for different spectral lines to appear. This brought the model of the atom into closer agreement with experimental data. The conditions of the state of the orbit got assigned quantum numbers. The three states discussed so far consist of: orbit number (n), orbit shape (l) and orbit tilt (m). In 1924 an Austrian physicist, Wolfgang Pauli predicted that an electron should spin (kind of like a top) while it orbits around the nucleus. The electron can spin in either of two direction. This spin consisted of a fourth quantum number: electron spin (s). <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;"> Pauli's Exclusion principle <span style="font-family: Arial,Helvetica,sans-serif;">Pauli gave a rule governing the behavior of electrons within the atom that agreed with experiment. If an electron has a certain set of quantum numbers, then no other electron in that atom can have the same set of quantum numbers. Physicists call this "Pauli's exclusion principle." It provides an important principle to this day and has even outlived the Bohr-Sommerfeld model that Pauli designed it for. In 1924 a Frenchman named Louis de Broglie thought about particles of matter. He thought that if light can exist as both particles and waves, why couldn't atom particles also behave like waves? In a few equations derived from Einstein's famous equation, (E=mc2) he showed what matter waves would behave like if they existed at all. (Experiments later proved him correct.)

In 1926 the Austrian physicist, Erwin Schrödinger had an interesting idea: Why not go all the way with particle waves and try to form a model of the atom on that basis? His theory worked kind of like harmonic theory for a violin string except that the vibrations traveled in circles. The world of the atom, indeed, began to appear //very// strange. It proved difficult to form an accurate picture of an atom because nothing in our world really compares with it. Schrödinger's wave mechanics did not question the makeup of the waves but he had to call it something so he gave it a symbol: <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;"> The "psi" symbol of Schrödinger's wave came from the Greek lettering system. <span style="font-family: Arial,Helvetica,sans-serif;">In 1926, a German physicist, Max Born had an idea about 'psi'. Born thought they resembled waves of chance. These ripples moved along waves of chance, made up of places where particles may occur and places where no particles occured. The waves of chance ripple around in circles when the particle appears like an electron in an atomic orbit, and they ripple back and forth when the electron orbit goes straight through the nucleus, and they ripple along in straight lines when a free particle moves through interatomic space. You can think of them as waves when traveling through space and as particles whenever they travel in circles. However, they cannot exist as both waves and particles at the same time. Just before Schrödinger proposed his theory, a German physicist Werner Heisenberg, in 1925, had a theory of his own called matrix mechanics which also explained the behavior of atoms. The two theories seemed to have an entirely different sets of assumptions yet they both worked. Heisenberg based his theory on mathematical quantities called matrices that fit with the conception of electrons as particles whereas Schrödinger based his theory on waves. Actually, the results of both theories appeared mathematically the same. In 1927 Heisenberg formulated an idea, which agreed with tests, that no experiment can measure the position and momentum of a quantum particle simultaneously. Scientists call this the "Heisenberg uncertainty principle." This implies that as one measures the certainty of the position of a particle, the uncertainty in the momentum gets correspondingly larger. Or, with an accurate momentum measurement, the knowledge about the particle's position gets correspondingly less.

The visual concept of the atom now appeared as an electron "cloud" which surrounds a nucleus. The cloud consists of a probability distribution map which determines the most probable location of an electron. For example, if one could take a snap-shot of the location of the electron at different times and then superimpose all of the shots into one photo, then it might look something like the view at the top. Note: Just as no map can equal a territory, no concept of an atom can possibly equal its nature. These models of the atom simply served as a way of thinking about them, albeit they contained limitations (all models do).

Although the mathematical concept of the atom got better, the visual concept of the atom got worse. Regardless, even simplistic visual models can still prove useful. Chemists usually describe the atom as a simple solar system model similar to Bohr's model but without the different orbit shapes. The important emphasis for chemistry attemps to show the groupings of electrons in orbital shells. (The example above shows the first eleven elements.) <span style="font-family: Arial,Helvetica,sans-serif;">Chemical behavior of the elements form together to create molecules. Molecules may share electrons as the hydrogen and water molecules above illustrates. (Atoms which share electrons have the name "ions.") The outer electron shell of an atom actually does the sharing and bonding of the atoms. This in turn allows chemists to describe the interactions of chemistry. Even though the orbit model of the atom does not provide an accurate model, it works well for describing chemistry. <span style="display: block; font-family: Arial,Helvetica,sans-serif; font-size: 80%; text-align: right;">[|//taken from : http://www.nobeliefs.com/atom.htm//]

=<span style="color: #ff0000; font-family: Tahoma,Geneva,sans-serif;">Origin of scientific theory =

<span style="font-family: Arial,Helvetica,sans-serif;">Further progress in the understanding of atoms did not occur until the science of chemistry began to develop. In 1661, natural philosopher Robert Boyle published The Sceptical Chymist in which he argued that matter was composed of various combinations of different "corpuscules" or atoms, rather than the classical elements of air, earth, fire and water. In 1789 the term element was defined by the French nobleman and scientific researcher Antoine Lavoisier to mean basic substances that could not be further broken down by the methods of chemistry.

In 1803, English instructor and natural philosopher John Dalton used the concept of atoms to explain why elements always react in a ratio of small whole numbers—the law of multiple proportions—and why certain gases dissolve better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms can join together to form chemical compounds. Dalton is considered the originator of modern atomic theory.

Additional validation of particle theory (and by extension atomic theory) occurred in 1827 when botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically—a phenomenon that became known as "Brownian motion". J. Desaulx suggested in 1877 that the phenomenon was caused by the thermal motion of water molecules, and in 1905 Albert Einstein produced the first mathematical analysis of the motion. French physicist Jean Perrin used Einstein's work to experimentally determine the mass and dimensions of atoms, thereby conclusively verifying Dalton's atomic theory. Taken from :[]

HISTORY of **//<span style="color: #000080; font-family: Arial,Helvetica,sans-serif; font-size: 190%;">AtOMs //**1.ATOMISM:

Further progress in the understanding of atoms did not occur until the science of chemistry began to develop. In 1661, natural philosopher Robert Boyle published //The Sceptical Chymist// in which he argued that matter was composed of various combinations of different "corpuscules" or atoms, rather than the classical elements of air, earth, fire and water.In 1789 the term //element// was defined by the French nobleman and scientific researcher Antoine Lavoisier to mean basic substances that could not be further broken down by the methods of chemistry.

<span style="color: #ff0000; font-family: Tahoma,Geneva,sans-serif; font-size: 140%;">3. quantum theory :media type="youtube" key="45KGS1Ro-sc" height="332" width="462"11111111: is a set of scientific principles describing the known behavior of energy and matter that predominate at the atomic scale. QM gets its name from the notion of quantum, and the quantum value is the Planck constant. The wave–particle duality of energy and matter at the atomic scale provides a unified view of the behavior of particles such as photons and electrons. While the notion of the photon as a quantum of light energy is commonly understood as a particle of light that has an energy value governed by the Planck constant, what is quantized for an electron is the angular momentum it can have as it is bound in an atomic orbital. When not bound to an atom, an electron's energy is no longer quantized, but it displays, like any other massy particle, a Compton wavelength. While a photon does not have mass, it does have linear momentum. The full significance of the Planck constant is expressed in physics through the abstract mathematical notion of action.

=[|http://en.wikipedia.org]=

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<span style="color: #bb1b1b; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 150%;">**<span style="color: #922f2f; font-family: Tahoma,Geneva,sans-serif; font-size: 130%;">COMPONENTS OF ATOMS ** <span style="color: #ab2b17; font-family: Georgia,serif; font-size: medium; font-weight: normal;">**-** <span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: medium; font-weight: normal;">**The modern atom as viewed by scientists today consists of three main particles located in two regions**

1) **The first of these two regions is the nucleus, or central core of the atom which is composed of positively charged protonsand neutrons with a neutral charge.** **<span style="font-family: Arial,Helvetica,sans-serif;">It is believed that the neutrons are needed to hold the positively charged protons together in the nucleus. The force that holds these particles together is termed the nuclear binding force and it is believed to be one of the strongest forces that exists in nature. The nucleus takes up a very small portion of the atom. If the atom was the size of a football field the nucleus would be the size of a household fly on the 55 yard line. ** <span style="font-family: Arial,Helvetica,sans-serif;">**2) The second region surrounds the nucleus and is termed an electron cloud. The cloud holds the** **<span style="font-family: Arial,Helvetica,sans-serif;">third particle which is a negatively charged electron **<span style="font-family: Arial,Helvetica,sans-serif;">**. The electrons in a many electron atom are arranged in energy levels about the nucleus. The electrons in their lowest energy state (termed ground state) occupy these energy levels from lowest (closest to nucleus) to highest energy. Only certain numbers of electrons can be placed in each energy level**.

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 * [[image:http://www.saskschools.ca/curr_content/chem30/images/e_orbits.jpg width="208" height="201" caption="picture of energy levels"]] ||
 * picture of energy levels ||

the atoms

The atom **is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged eleelectrons ctronstrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number number of protonsof protons determines the chemical element, and the number of neutrons determine the isotope of the element.

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<span style="color: red; font-family: 'Arial','sans-serif'; font-size: 22pt; line-height: 115%;">The atoms <span style="font-family: 'Arial','sans-serif'; font-size: 13.5pt; line-height: 115%;">T <span style="font-family: 'Arial','sans-serif'; font-size: 10pt; line-height: 115%;">he atom is the fundamental building block of all stuff, or what scientists like to call "matter". An individual atom is very small. In fact, the smallest type of atom, hydrogen, has a diameter of 10 <span style="font-family: 'Arial','sans-serif'; font-size: 7.5pt; line-height: 115%;">-8 **<span style="font-family: 'Arial','sans-serif'; font-size: 10pt; line-height: 115%;"> **cm. This means that if the hydrogen atom was the size of a soccer ball, then a soccer ball would be 6450 kilometers (4008 miles) high. Every single object is composed of atoms. Your body is made up of many, many individual atoms. There are also many different types of atoms. In fact, there are over a 100. These different types are called elements. Examples of some elements are hydrogen, oxygen, iron, copper, and helium. Under normal conditions many atoms can stick together to form larger, different stuff. Scientists call material that results from the joining of different types of atoms "compounds". An example of a compound is water, which is a group of two hydrogen atoms and one oxygen atom. Notice that we said that these types of compounds can only form under what we called "normal conditions". In the type of environment in which nuclear fusion occurs, the joining of atoms, also known as bonding, can't happen. We will explain why later. From:** [|**http://library.thinkquest.org/17940/texts/atom/atom.html**]

= THE ATO M =
 * THE ATOM IS THE SMALLER THING OF THE MATTER.( MATTER IS ANITHING THAT CAN BE TOUCHED PHYSICALLY). ATOMS HAVE ELECTRONS, PROTONS AND NEUTRONS. THE ELECTRONS HAVE NEGATIVE CHARGE, THE PROTONS HAVE POSITIVE CHARGE AND THE NEUTRONS DONT HAVE CHARGE. THE NUCLEUS IS SORRUNDING BY A CLOUD OF NEGATIVELY CHARGED ELECTRONS. ALL IS CONFORMED BY ATOMS. EXAMPLE: LIVING THINGS AND NOT LIVING THINGS.**

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POR: DANIEL RIVERA LONDOÑO 7B
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 * 1) **<span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: 160%; font-weight: normal;">history of the atom: **
 * 1) **<span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: 160%; font-weight: normal;">history of the atom: **

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​​<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 169%;">THE DEFINITION OF ATOM <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 130%;"> The atom is a basic unit of ma​tter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1. which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.


 * 1) <span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: 120%; font-weight: normal;">jhon dalton:
 * 1) <span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: 120%; font-weight: normal;">jhon dalton:

<span style="font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">From his experiments and observations, he suggested that atoms were like tiny, hard balls. Each chemical <span style="cursor: pointer; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px; visibility: visible;">element <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">had its own atoms that differed from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. In chemical reactions, the atoms would rearrange themselves and combine with other atoms in new ways. <span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;"> thompson: <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">At the end of the nineteenth century, a scientist called J.J. Thomson discovered the <span style="font-family: Arial,Helvetica,sans-serif; font-size: medium; line-height: 24px;">**<span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">electron ** <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">. This is a tiny negatively charged particle that is much, much smaller than any atom. When he discovered the electron <span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;"> ernest rutherford: <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">The next development came about 10 years later. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described. However, they got a big surprise he notice that the atom had a positive charged nucleus with electrons rotating arround it <span style="color: #ff0000; font-family: 'Comic Sans MS',cursive;"> <span style="color: #ff0000; font-family: 'Comic Sans MS',cursive; font-size: 120%;">neils bohr: <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;">The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the <span style="color: #000000; cursor: pointer; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px; visibility: visible;">nucleus <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: medium; line-height: 24px;"> in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high

<span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: 130%;">by: julian peñarredonda 7°b
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<span style="color: #ff0000; font-family: Tahoma,Geneva,sans-serif; font-size: 160%;">structure of the atom
<span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 108%;"><span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 108%;"> <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 118.8%;">**In the atom we distinguish two parts: the core and the crust. - The kernel is the core of the atom and contains positively charged particles, protons, and particles have no electric charge, ie are neutral neutrons. The mass of a proton is approximately equal to that of a neutron. All atoms of a chemical element in the core have the same number of protons. This number, which characterizes each element and distinguishes it from others is the atomic number and is represented with the letter Z.** <span style="font-family: Arial,Helvetica,sans-serif; font-size: 118.8%;">**- The cortex is the outside of the atom. It contains the electrons negatively charged. Here, arranged in different levels, revolving around the nucleus.** <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 108%;">**<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">The mass of an electron is about 2000 times smaller than a proton ** **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">. ** <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 90%;">

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 80%;">[]

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">luis fernando quintero v.
QUARKS:INSIDE THE ATOMS

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Although atomic resolution has been attained previously for AFM imaging of individual atoms in nanotubes and in unit cells of crystals on surfaces, this is the first time all the atom positions and bonds of a single molecule, including its hydrogens, have been visualized with atomic-scale precision. Only a few weeks ago, Gross and coworkers published a paper showing that AFM can be used to measure charges with single-electron resolution on individual atoms (//Science// **2009,** //324,// 1428). Scanning tunneling microscopy (STM) has been used to image molecular orbitals of individual molecules, but the technique has limited ability to distinguish individual atoms and bonds. In the past, AFM has done no better at resolving atomic-scale features, producing only fuzzy images of overall shapes of individual molecules. The IBM team overcame the atomic resolution barrier by trying a range of atoms and molecules as AFM tip terminations. They achieved their best imaging results by putting carbon monoxide onto the AFM tip. Distances between pentacene atoms are only 1.4 Å, and the new images of them are “state of the art in terms of lateral resolution,” Gross says. In the images, it’s possible to deduce the positions of hydrogen atoms, which are too tiny to have ever been visualized before. The study “is a major advance in the field of scanning probe microscopy and characterization of molecules at surfaces,” says Óscar Custance of the National Institute for Materials Science, in Tsukuba, Japan. It is “astonishing and groundbreaking,” he adds. Until now, “molecules at surfaces have been seen by AFM as structureless protrusions,” Custance says. “These new results blast away any other molecular resolution limit accomplished to date by scanning probe microscopy”—the AFM and STM family of techniques—“and demonstrate that, with the right tip, the resolution achievable with AFM greatly surpasses that of STM. The work truly opens up new avenues to explore the behavior and properties of molecules at surfaces, and even the possibility of functionalizing them.” Physicist Alexander Shluger of University College London notes that attaching the very tiny and chemically inert CO molecule to the AFM tip made it possible for Gross and coworkers to visualize atomic features in the larger pentacene molecule. The IBM team helped confirm the findings by showing that density functional theory predicts the same type of image, he adds. The study “is a very important step forward and will have strong implications for our understanding of adsorption of molecules at surfaces,” Shluger says. “We hope we can image a large number of molecules with atomic resolution,” Gross says. Among remaining questions, he says, are: “Can we differentiate carbon, sulfur, nitrogen, and other atoms? And can we say something about bond order—how many electrons are in each bond?” By combining single-electron and single-molecule capabilities of AFM, it may also eventually be possible to study single-electron transport. []
 * By functionalizing** an atomic force microscope tip, researchers have greatly enhanced the resolution of atomic force microscopy (AFM), making it possible to view the entire structure of a single molecule. Physicist Leo Gross of IBM Research, in Rüschlikon, Switzerland, and coworkers accomplished the feat on the aromatic compound pentacene (//Science// **2009,** //325,// 1110).



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=History of the Atomic Bomb & The Manhattan Project= On August 2, 1939, just before the beginning of World War II, [|Albert Einstein] wrote to then President Franklin D. Roosevelt. Einstein and several other scientists told Roosevelt of efforts in Nazi Germany to purify uranium-235, which could be used to build an atomic bomb. It was shortly thereafter that the United States Government began the serious undertaking known then only as "The Manhattan Project." Simply put, the Manhattan Project was committed to expediting research that would produce a viable atomic bomb.

Making Enriched Uranium
ATOMS

The Columbia Encyclopedia, Sixth Edition | 2008 | The Columbia Encyclopedia, Sixth Edition. Copyright 2008 Columbia University Press. ([|Hide copyright information]) [|Copyright] atom [Gr.,=uncuttable (indivisible)], basic unit of [|matter] ; more properly, the smallest unit of a chemical [|element] having the properties of that element.


 * Structure of the Atom**

The atom consists of a central, positively charged core, the [|nucleus], and negatively charged particles called [|electrons] that are found in orbits around the nucleus.


 * The Nucleus**

Almost the entire mass of the atom is concentrated in the nucleus, which occupies only a tiny fraction of the atom's volume. The nucleus of an atom consists of neutrons and protons, the [|neutron] being an uncharged particle and the [|proton] a positively charged one. Their masses are almost equal. Atoms containing the same number of protons but different numbers of neutrons represent different forms, or [|isotopes], of the same element.


 * The Electrons**

Surrounding the nucleus of an atom are its electrons; for a neutral atom, the number of electrons is equal to the atomic number. The outermost electrons of an atom determine its chemical and electrical properties. An atom may combine chemically with another atom in various ways, either by giving up or receiving electrons, thus setting up an electrical attraction between the atoms (see [|ion] ), or by sharing one or more pairs of electrons (see [|chemical bond] ). Because metals have few outermost electrons and tend to give them up easily, they are good conductors of electricity or heat (see [|conduction] ).

The electrons are often described as revolving about the nucleus as the planets revolve about the sun. This picture, however, is misleading. The quantum theory has shown that all particles in motion also have certain wave properties. For a particle the size of an electron, these properties are of considerable importance. As a result the electrons in an atom cannot be pictured as localized in space, but rather should be viewed as smeared out over the entire orbit so that they form a cloud of charge. The electron clouds around the nucleus represent regions in which the electrons are most likely to be found. The shapes of these clouds can be very complex, in marked contrast to the simple elliptical orbits of planets. Surprisingly, the sizes of all atoms are comparable, in spite of the large differences in the number of electrons they contain.


 * Atomic Weight and Number**

The atomic number of an atom is simply the number of protons in its nucleus. The atomic weight of an atom is given in most cases by the mass number of the atom, equal to the total number of protons and neutrons combined. An atom may be conveniently symbolized by its chemical symbol with the atomic number and mass number written as subscript and superscript, respectively. For example, the symbol for uranium is U (atomic number 92); the isotopes of uranium with atomic weights 235 and 238 are indicated by 23592 U and 23892 U.


 * Development of Atomic Theory**


 * Early Atomic Theory**

The atomic theory, which holds that matter is composed of tiny, indivisible particles in constant motion, was proposed in the 5th cent. BC by the Greek philosophers Leucippus and Democritus and was adopted by the Roman Lucretius. However, Aristotle did not accept the theory, and it was ignored for many centuries. Interest in the atomic theory was revived during the 18th cent. following work on the nature and behavior of gases (see [|gas laws] ).


 * From Dalton to the Periodic Table**

Modern atomic theory begins with the work of John Dalton, published in 1808. He held that all the atoms of an element are of exactly the same size and weight (see [|atomic weight] ) and are in these two respects unlike the atoms of any other element. He stated that atoms of the elements unite chemically in simple numerical ratios to form compounds. The best evidence for his theory was the experimentally verified [|law of simple multiple proportions], which gives a relation between the weights of two elements that combine to form different compounds.

Evidence for Dalton's theory also came from Michael Faraday's law of [|electrolysis]. A major development was the [|periodic table], devised simultaneously by Dmitri Mendeleev and J. L. Meyer, which arranged atoms of different elements in order of increasing atomic weight so that elements with similar chemical properties fell into groups. By the end of the 19th cent. it was generally accepted that matter is composed of atoms that combine to form molecules.


 * Discovery of the Atom's Structure**

In 1911, Ernest Rutherford developed the first coherent explanation of the structure of an atom. Using alpha particles emitted by radioactive atoms, he showed that the atom consists of a central, positively charged core, the [|nucleus], and negatively charged particles called [|electrons] that orbit the nucleus. There was one serious obstacle to acceptance of the nuclear atom, however. According to classical theory, as the electrons orbit about the nucleus, they are continuously being accelerated (see [|acceleration] ), and all accelerated charges radiate electromagnetic energy. Thus, they should lose their energy and spiral into the nucleus.

This difficulty was solved by Niels Bohr (1913), who applied the [|quantum theory] developed by Max Planck and Albert Einstein to the problem of atomic structure. Bohr proposed that electrons could circle a nucleus without radiating energy only in orbits for which their orbital angular [|momentum] was an integral multiple of Planck's constant //h// divided by 2π. The discrete spectral lines (see [|spectrum] ) emitted by each element were produced by electrons dropping from allowed orbits of higher energy to those of lower energy, the frequency of the [|photon] of light emitted being proportional to the energy difference between the orbits.

Around the same time, experiments on x-ray spectra (see [|X ray] ) by H. G. J. Moseley showed that each nucleus was characterized by an atomic number, equal to the number of unit positive charges associated with it. By rearranging the periodic table according to atomic number rather than atomic weight, a more systematic arrangement was obtained. The development of quantum mechanics during the 1920s resulted in a satisfactory explanation for all phenomena related to the role of electrons in atoms and all aspects of their associated spectra. With the discovery of the neutron in 1932 the modern picture of the atom was complete.


 * Contemporary Studies of the Atom**

With many of the problems of individual atomic structure and behavior now solved, attention has turned to both smaller and larger scales. On a smaller scale the atomic nucleus is being studied in order to determine the details of its structure and to develop sources of energy from nuclear fission and fusion (see [|nuclear energy] ), for the atom is not at all indivisible, as the ancient philosophers thought, but can undergo a number of possible changes. On a larger scale new discoveries about the behavior of large groups of atoms have been made (see [|solid-state physics] ). The question of the basic nature of matter has been carried beyond the atom and now centers on the nature of and relations between the hundreds of [|elementary particles] that have been discovered in addition to the proton, neutron, and electron. Some of these particles have been used to make new types of exotic "atoms" such as positronium (see [|antiparticle] ) and muonium (see [|muon] ).


 * Bibliography**

See G. Gamow, The Atom and Its Nucleus (1961); H. A. Boorse and L. Motz, ed., The World of the Atom (2 vol., 1966); B. H. Bransden and C. J. Joachain, Physics of Atoms and Molecules (1986). ** Cite this article ** Pick a style below, and copy the text for your bibliography.

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atom [Gr.,=uncuttable (indivisible)], basic unit of [|matter] ; more properly, the smallest unit of a chemical [|element] having the properties of that element.


 * Structure of the Atom**

The atom consists of a central, positively charged core, the [|nucleus], and negatively charged particles called [|electrons] that are found in orbits around the nucleus.


 * The Nucleus**

Almost the entire mass of the atom is concentrated in the nucleus, which occupies only a tiny fraction of the atom's volume. The nucleus of an atom consists of neutrons and protons, the [|neutron] being an uncharged particle and the [|proton] a positively charged one. Their masses are almost equal. Atoms containing the same number of protons but different numbers of neutrons represent different forms, or [|isotopes], of the same element.


 * The Electrons**

Surrounding the nucleus of an atom are its electrons; for a neutral ​ ​ ATOMS Basic model of the atoms

All matter consists of particles called atoms. This is a list of the basic characteristics of atoms:
 * Atoms cannot be divided using chemicals. They do consist of parts, which include protons, neutrons, and electrons, but an atom is a basic chemical building block of matter.
 * Each electron has a negative electrical charge.
 * Each proton has a positive electrical charge. The charge of a proton and an electron are equal in magnitude, yet opposite in sign. Electrons and protons are electrically attracted to each other.
 * Each neutron is electrically neutral. In other words, neutrons do not have a charge and are not electrically attracted to either electrons or protons.
 * Protons and neutrons are about the same size as each other and are much larger than electrons. The mass of a proton is essentially the same as that of a neutron. The mass of a proton is 1840 times greater than the mass of an electron.
 * The nucleus of an atom contains protons and neutrons. The nucleus carries a positive electrical charge.
 * Electrons move around outside the nucleus.
 * Almost all of the mass of an atom is in its nucleus; almost all of the volume of an atom is occupied by electrons.
 * The number of protons (also known as its atomic number) determines the element. Varying the number of neutrons results in isotopes. Varying the number of electrons results in ions. Isotopes and ions of an atom with a constant number of protons are all variations of a single element.
 * The particles within an atom are bound together by powerful forces. In general, electrons are easier to add or remove from an atom than a proton or neutron. Chemical reactions largely involve atoms or groups of atoms and the interactions between their electrons.
 * Does the atomic theory make sense to you? If so, here's a [|quiz] you can take to test your understanding of the concepts.

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