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Atomic Structure Atomic Structure

The ancient Greek philosophers Leucippus and Democritus believed that atoms existed, but they had no idea as to their nature. Centuries later, in 1803, the English chemist John Dalton, guided by the experimental fact that chemical elements cannot be decomposed chemically, was led to formulate his atomic theory. Dalton's atomic theory was based on the assumption that atoms are tiny indivisible entities, with each chemical element consisting of its own characteristic atoms.✶ ✶See Atoms article for further discussion of Dalton's atomic theory. The atom is now known to consist of three primary particles: protons, neutrons, and electrons, which make up the atoms of all matter. A series of experimental facts established the validity of the model. Radioactivity played an important part. Marie Curie suggested, in 1899, that when atoms disintegrate, they contradict Dalton's idea that atoms are indivisible. There must then be something smaller than the atom (subatomic particles) of which atoms were composed. Long before that, Michael Faraday's electrolysis experiments and laws suggested that, just as an atom is the fundamental particle of an element, a fundamental particle for electricity must exist. The "particle" of electricity was given the name electron. Experiments with cathode-ray tubes, conducted by the British physicist Joseph John Thomson, proved the existence of the electron and obtained the charge-to-mass ratio for it. The experiments suggested that electrons are present in all kinds of matter and that they presumably exist in all atoms of all elements. Efforts were then turned to measuring the charge on the electron, and these were eventually successful by the American physicist Robert Andrews Millikan through the famous oil drop experiment. The study of the so-called canal rays by the German physicist Eugen Goldstein, observed in a special cathode-ray tube with a perforated cathode, let to the recognition in 1902 that these rays were positively charged particles (protons ). Finally, years later in 1932 the British physicist James Chadwick discovered another particle in the nucleus that had no charge, and for this reason was named neutron. As a physical chemist, George Stoney made significant contributions to our understanding of molecular motion. However, this Irish scientist is better known for assigning a name to negative atomic charges, electrons, while addressing the Royal Society of Dublin in 1891. —Valerie Borek Joseph John Thomson had supposed that an atom was a uniform sphere of positively charged matter within which electrons were circulating (the "plum-pudding" model). Then, around the year 1910, Ernest Ruthorford (who had discovered earlier that alpha rays consisted of positively charged particles having the mass of helium atoms) was led to the following model for the atom: Protons and neutrons exist in a very small nucleus, which means that the tiny nucleus contains all the positive charge and most of the mass of the atom, while negatively charged electrons surround the nucleus and occupy most of the volume of the atom. In formulating his model, Rutherford was assisted by Hans Geiger and Ernest Marsden, who found that when alpha particles hit a thin gold foil, almost all passed straight through, but very few (only 1 in 20,000) were deflected at large angles, with some coming straight back. Rutherford remarked later that it was as if you fired a 15-inch artillery shell at a sheet of paper and it bounced back and hit you. The deflected particles suggested that the atom has a very tiny nucleus that is extremely dense and positive in charge. Also working with Rutherford was Henry G. J. Moseley who, in 1913, performed an important experiment. When various metals were bombarded with electrons in a cathode-ray tube, they emitted X rays, the wavelengths of which were related to the nuclear charge of the metal atoms. This led to the law of chemical periodicity, which provided refinement of the periodic table introduced by Mendeleev in 1869. According to this law, all atoms of an element have the same number of protons in the nucleus. It is called the atomic number and is given the symbol Z. Hydrogen is the simplest element and has Z = 1. Through Rutherford's work it was known that that electrons are arranged in the space surrounding the atomic nucleus. A planetary model of the atom, with the electrons moving in circular orbits around the nucleus seemed an acceptable model. However, such a "dynamic model" violated the laws of classical electrodynamics, according to which a charged particle, such as an electron, moving in the positive electric field of the nucleus, should lose energy by radiation and eventually spiral into the nucleus. To solve this contradiction, in 1913, the Danish physicist Neils Bohr (then studying under Rutherford) postulated that the electron orbiting the nucleus could move only in certain orbits, having in each a certain "quantized" energy. It turns out that the colors in fireworks would help prove him right. The colorful lights of fireworks are emitted by "excited" atoms; that is, by atoms that have absorbed extra energy. Light consists of electromagnetic waves, each (monochromatic) color with a characteristic wavelength λ and frequency v. Frequency is related to energy E through the famous Planck equation, E = hν, where h is Planck's constant (6.6256 x 10−34 J s). Note that white light, such as sunlight, is a mixture of light of all colors, so it does not have a characteristic wavelength. For this reason we say that white light has a "continuous spectrum." On the other hand, excited atoms emit a "line spectrum" consisting of a set of monochromatic visible radiations. Each element has a characteristic line spectrum that can be used to identify the element. Note that line emission spectra can also be obtained by heating a salt of a metal with a flame. For instance, common salt (sodium chloride) provides a strong yellow light to the flame coming from excited sodium, while copper salts emit a blue-green light and lithium salts a red light. The colors of fireworks are due to this phenomenon. Scientists in the late nineteenth century tried to quantify the line spectra of the elements. In 1885 the Swedish school teacher Johann Balmer discovered a series of lines in the visible spectrum of hydrogen, the wavelengths of which could be related with a simple equation: in which λ is wavelength, k is constant, a = 2, and b = 3, 4, 5, … This group of lines was called the Balmer series. For the red line b = 3, for the green line b = 4, and for the blue line b = 5. Similar series were further discovered: in the infrared region, the Paschen series (with a = 3 and b = 4, 5 … in the above equation), and much later in the ultraviolet region, the Lyman series (with a = 1 and b = 2, 3 …). In 1896 the Swedish spectroscopist Johannes Rydberg developed a general equation that allowed the calculation of the wavelength of the red, green, and blue lines in the atomic spectrum of hydrogen: where nL is the number of the lower energy level to which an electron falls and nH is the number of the higher energy level from which it falls. R is called the Rydberg constant (1.0974 x 10−7 m−1). R was later shown to be 2π 2me 4Z2/h 3c, where m is the mass of the electron, e is its charge, Z is the atomic number, h is Planck's constant, and c is the speed of light. As noted earlier, Bohr had suggested the quantization of Ruthford's model of the atom. Although he was not aware of the work of Balmer and Paschen when he wrote the first version of his 1913 article, he had incorporated Planck's constant h into his model, which turned out to be an important decision. Bohr assumed that the absorption or emission of radiation can occur only by "jumps" of the electron from one stationary orbit to another. (See Figure 1.) The energy differences between two such allowed orbits then provided the characteristic frequencies of the emitted light. ΔE = E n1 − E n2 = hν Planck's constant h was named by Bohr the "quantum of action." Bohr's theory was in close agreement with many experimental facts regarding one-electron atoms (the hydrogen


From Yahoo Answers

Question:Hi, I am an eighth grade student. I have to do a science project making a model for the sodium atom. I really want to do well on this project, but I'm not quite sure how to do it. I do not want this to be too complicated, but I do want to make it look like used a lot of effort. Not only look, but i do want to make a decent effort. Please help me out here. Thanks!

Answers:You want to show that the atom has a nucleus with protons and neutrons has three electron layers with 2+8+1 electrons You make the nucleus of marbles or of playDoe -- one color for protons, another for neutrons. Get them hold together -- with glue, plastic film, or alike to show they are held together by sub-atomic forces. make electrons as balls of another color. Show that they belong to different layers -- e.g. by drawing the bands on the mat (you are probably want to attach every to a mat). Hope this helps Look also at http://galileo.phys.virginia.edu/education/outreach/8thgradesol/AtomicConstruct.htm

Question:i am doing a project on atoms and molecules. ive run into a big problem. im making a 3d model of ions and ionic bond formation. ive already made my models but now i need to show the structure and describe their functions. ive searched and searched on different sites but no luck. could someone please help me? (breakdown)- what does it mean when it says "show the structures and describe their functions"?

Answers:"Show the structure" usually refers to the arrangement of the atoms within the molecule or the crystal lattice, but "describe their functions" is very vague and not a topic usually discussed. If this project is for a class, talk with the instructor about this. Your 3D model will "show the structure." This might be very simple for a substance like Sodium chloride. The Sodium cations (positive charge) are smaller than the Chloride anions (negative charge). If "describe their functions" is a reference to what is the function of the cations and anions in the crystal lattice, then one could say that the attraction of the ions is what makes the Ionic bonds which hold the ions in place. These ionic bonds are broken when the salt is dissolved in water.

Question:The project im doing is Sodium from the periodic table. My 2 questions are. *What are the correct number of shells? *What are the correct number of electrons in each shell?

Answers:there are three shells and there is one electron on its outer shell, two electrons on its first shell, and 8 electrons on its second shell

Question:please show the 5 models of atom

Answers:Go to this web site and you will see the image of the 5 models of atoms. 1) The first model was called the chocolate pudding model. An Atom was solid as chocolate pudding and as uniform. 2) They then discovered there were electrons in atoms. So they call this the pudding model with raisins. Think of it as a pudding model with raisins stuck to it. 3)The Rutherford model was the previous model with a nucleus added. 4) The next model was simply electrons orbiting a nucleus like planets orbiting the sun. 5) The last model was the nucleus with only a high probability of where you will find electrons.

From Youtube

3D Bohr Model of a Sodium-26 Isotope Atom :This is a 3D Bohr model of a sodium-26 isotope atom. The red particles represent protons, the blue are neutrons, and the yellow particles orbiting the nucleus are electrons. The electron shells are 2-8-1. I created this in the Maya 2009 software.

Atom Model :WEBSITE: www.teachertube.com A simple model of an atom.