The History of Atomic Structure

1. The History of Atomic Structure GPS Standards: SC3. Students will use the modern atomic theory to explain the characteristics of atoms. a. Discriminate between the size, charge, and position of protons, neutrons, and electrons in the atom. f. Relate light emission and the movement of electrons to element identification January 2011

2. Part I: The Road to Modern Atomic Theory • Early Theories - 400 B.C. • common Greek theory was that all matter consisted of four "elements"—earth, air, fire, and water. • Democritus (460-360 BC) = a philosopher who theorized that all matter was made of indivisible particles (atoms). • his theory was based on logical reasoning, not data. • philosophers/scientists of this time period did not think doing experiments was necessary—you could reach the truth by pure logical reasoning. • alchemy (the process of changing base metals to gold) was the main form of experimentation from this time period until the late 1600's. Democritus

3. alchemy (the process of changing base metals to gold) was the main form of experimentation from this time period until the late 1600's. • Modern Era Theories/Laws—1600’s – early 1800’s • Robert Boyle, in the mid-1600’s, defined “element” as a substance that cannot be broken down into simpler substances. • Antoine Lavoisier – mid 1700’s – considered to be the pioneer of modern chemistry. • used the scientific method and did careful, controlled experiments involving the combustion of metals. • law of conservation of mass: mass is neither created nor destroyed in an average chem. rxtn. (this means burning, rusting, etc.  destroying matter) Boyle Lavoisier

4. law of conservation of mass: mass is neither created nor destroyed in an average chem. rxtn. (this means burning, rusting, etc.  destroying matter) • Joseph Proust—published Law of Definite Proportions in 1794: • a chemical compound contains the same elements in exactly the same proportions (by mass) regardless of the size or source of the sample. (this means water = H2O wherever you get it, however much you have) • law of multiple proportions: picture two different compounds with the same two elements in them—i.e. carbon monoxide, carbon dioxide—these compounds will have the same small, whole number ratio in their required masses as they do in their chemical formulas. Proust

5. law of multiple proportions: picture two different compounds with the same two elements in them—i.e. carbon monoxide, carbon dioxide—these compounds will have the same small, whole number ratio in their required masses as they do in their chemical formulas. Part II: Early Atomic Models • Dalton, John—“Solid marble” model • English schoolteacher (1808), proposed explanation for the 3 laws above • thought elements were composed of atoms, which were indivisible CO = 1 oxygen, CO2 = 2 oxygens, 1:2 ratio in oxygen atoms CO = 16g of oxygen, CO2 = 32 g of oxygen, 1:2 ratio in oxygen grams Dalton

6. Part II: Early Atomic Models • Dalton, John—“Solid marble” model • English schoolteacher (1808), proposed explanation for the 3 laws above • thought elements were composed of atoms, which were indivisible • Dalton’s Atomic Theory: (five parts) • all matter is composed of very tiny particles called atoms • atoms of a given element are identical in size/mass/other properties; other elements’ atoms are different • atoms cannot be subdivided, created, or destroyed • in chemrxtn, atoms are combined/separated/rearranged • atoms of diff. elements combine in small, whole-# ratios to form compounds • “black box” model of the atom. Even after 2000 years of the atomic concept, the structure of the atom was still unknown. Dalton

7. “black box” model of the atom. Even after 2000 years of the atomic concept, the structure of the atom was still unknown. • at this point, the atom was still thought to be the smallest unit of matter. • Thomson, Joseph John—“Plum Pudding” model (or “cookie dough”) • English physicist (1897), performed cathode-ray tube experiments • discovered the electron and its charge • cathode-ray tube: glass vacuum tube (attached to a voltage source) through which electricity flows, producing a glow. • electrons travel towards anode (opposite charges attract) • paddle wheel placement shows that particles were moving, had mass, and had a negative charge Thomson

8. paddle wheel placement shows that particles were moving, had mass, and had a negative charge • cathode-ray tube: glass vacuum tube (attached to a voltage source) through which electricity flows, producing a glow. • electrons travel towards anode (opposite charges attract) • paddle wheel placement shows that particles were moving, had mass, and had a negative charge • this model shows the electrons on the outside of the positively-charged atom (if there were negative parts to atoms, then there must be positive parts as well to balance it, because most matter is neutral).

9. this model shows the electrons on the outside of the positively-charged atom (if there were negative parts to atoms, then there must be positive parts as well to balance it, because most matter is neutral). • cathode rays were not like ordinary light—they could be deflected by electric or magnetic fields, and they could pass through thin metal foils, since they were so small.

10. cathode rays were not like ordinary light—they could be deflected by electric or magnetic fields, and they could pass through thin metal foils, since they were so small. • Millikan, Robert A.—see Thomson model • American physicist (1909), performed experiments that determined the size of electron to be about 1/1837 the mass of the hydrogen atom, or about 9.109 x 10-31 kg. • because atoms are electrically neutral, they must contain some + charge to offset all the - charges from the e-. • because e- have so much less mass than the total atom itself, there must be other, much heavier particles make up most of the mass. Millikan

11. Rutherford, Ernest—“Electron cloud, positive nucleus” model • New Zealander, along with Hans Geiger and Earnest Marsden (1911) performed the gold-foil experiment, leading to the discovery of the positively-charged nucleus. • experiment = bombard a thin gold foil with alpha-particles (He+). • they did this assuming that the charge and mass within each atom was uniformly distributed throughout the atom, and expected the alpha particles to go right through. • most did, but some (1/8000) actually bounced back toward the particle source! Rutherford

12. they did this assuming that the charge and mass within each atom was uniformly distributed throughout the atom, and expected the alpha particles to go right through. • most did, but some (1/8000) actually bounced back toward the particle source! • Rutherford concluded that the reason some particles bounced back was that there was a very dense positively-charged area that occupied only a very small amount of space within the core of the atom (nucleus). • to explain this, Rutherford proposed a new model for the atom. He imagined the atom as a miniature solar system with a nucleus as the sun and electrons orbiting like planets.

13. to explain this, Rutherford proposed a new model for the atom. He imagined the atom as a miniature solar system with a nucleus as the sun and electrons orbiting like planets. • although the nucleus was only one million millionth the volume of the atom, it had over 99.9% of the mass. • Rutherford: "It was as though you had fired a fifteen-inch shell at a piece of tissue paper and it had bounced back and hit you."

15. Bohr, Neils—“Quantized energy level” model • Danish physicist (1913), new model that explained why negative electrons do not fall in towards the positive nucleus • Bohr used Rutherford's ideas to explain the behavior of the simplest atom with one electron—hydrogen. • he suggested that an electron was restricted to certain allowed orbits round the nucleus. • if it jumped from one orbit to another, the atom emitted or absorbed light. • thus, electrons only exist in specific energy levels (orbits around the nucleus) similar to the rungs of a ladder. • these levels reflect quantum amounts of energy that are required to move electrons from one level to another. Bohr

16. these levels reflect quantum amounts of energy that are required to move electrons from one level to another. • unfortunately, this model only works for 1 atom—hydrogen. • Chadwick, James—“2 subatomic nucleus particles” model. • discovered the neutron in the nucleus • he used work previously done by Ruther- ford to establish the existence of the proton: • using α particles to bombard N, he found nuclei of H which had been chipped off. • in 1920 he speculated that hydrogen nuclei were a building block of all nuclei. He suggested the name proton for this unit. • other evidence for the proton came from the electric charges and masses of nuclei which change by whole units from element to element. Chadwick

17. other evidence for the proton came from the electric charges and masses of nuclei which change by whole units from element to element. • these new positive particles in the nucleus were found to be quite heavy (about 2000 times the mass of the e-). • However, the total mass of most atoms did not add up to match the weight of just the protons and electrons together. Therefore, there must be another, non-charged particle in the atom that adds the additional weight. • around 1930, Irene Joliot-Curie (daughter of Pierre and Marie Curie) performed an experiment in which beryllium was bombarded with alpha-particles, and a beam of particles with high penetrating power emerged. Joliot-Curie

18. around 1930, Irene Joliot-Curie (daughter of Pierre and Marie Curie) performed an experiment in which beryllium was bombarded with alpha- particles, and a beam of particles with high penetrating power emerged. • she was conducting many radiation experiments during this time. However, she was not searching for neutrons. • Chadwick realized that some of her experiments might be used to find the elusive neutron, so he repeated her experiments with this goal in mind. • he was able to determine that these particles had approximately the same mass as a proton. When they were exposed to electric and magnetic fields, and they were not deflected, he determined that they had no charge, either. Joliot-Curie

19. he was able to determine that these particles had approximately the same mass as a proton. When they were exposed to electric and magnetic fields, and they were not deflected, he determined that they had no charge, either. • he called these high-mass, uncharged particles “neutrons.” • Modern Atomic Model • e- in the modern model are arranged in “orbitals,” not orbits (Bohr model). • Orbitals are cloud-like regions around the nucleus of the atom in which 1 or 2 electrons are most likely to be found. They are arranged about the nucleus on the x, y, and z axis, which we will discuss in further detail tomorrow.

20. The models of the atom over the years: The Modern Model ?