CHEM 103: ATOMIC STRUCTURE>notes and problem set module 4.

4.1: ATOMIC STRUCTURE Earlier in this course, we explored a very elementary view of atomic structure. We learned about protons, neutrons, and electrons (the three elementary particles that comprise an atom) and their properties. In this module, we will study these particles in detail and learn how their presence and arrangement determines the properties of all matter. The Electron During the 1800s, a number of scientists, including Sir Humphrey Davy, Michael Faraday, R. A. Milligan, G. J. Stoney, and J. J. Thompson, performed experiments that led to the discovery of the electron and measurement of its properties. Davy (1807) first suggested the idea of the electron. Faraday (1832) determined the relationship between charge and quantity of element produced by that charge in an electrochemical reaction. Stoney (1891) was the first to use the term "electron." Thompson (1897) determined the massto-charge ratio of the electron, and Millikan (1911) determined the value of the charge of the electron. The Atom There were a number of early ideas about the structure of an atom. Some popular ones included:  Thompson's (1904) "plum-pudding" model with negative charges distributed throughout an area made up of positive charge,  Ernest Rutherford's (1909) planetary atom model in which electrons occupy most of the volume of an atom around a small, dense, positively charged nucleus, and  Niels Bohr's (1913) model of electrons orbiting the positively charged nucleus in distinct energy levels. The Proton and the Neutron After the discovery of the negatively charged electron, it was deduced that there must be a positively charged particle present in equal numbers to the electrons so as to produce a neutral atom. In , Rutherford and other physicists bombarded nuclei with alpha particles (helium nuclei), transforming one atom into another. In these experiments, hydrogen nuclei were emitted, leading to the discovery that the positive charge of any nucleus was equal to a certain number of hydrogen nuclei. They determined that this particle was positive because it was attracted to a negatively charged pole (cathode) of an electric field. Rutherford coined the name proton and used it in print in 1920. In 1932, James Chadwick bombarded Beryllium atoms with helium nuclei and observed that uncharged particles with about the same mass as a proton were emitted. This particle accounted for the difference in the mass of atoms which could not be accounted for by the mass of the atom's protons and electrons. This uncharged (neutral) particle was given the name neutron. The Bohr Atom In 1913, Niels Bohr, a young Danish physicist came to work in the laboratory of Ernest Rutherford who, a few years earlier, had proposed the planetary model of the atom. He was asked to solve some of the problems with the model. The biggest problem was that the model indicated that atoms were unstable because the orbiting electrons would lose energy and quickly fall into the nucleus. Bohr used facts known about light emission by atoms to develop his new theory, which is summarized as follows: 1. Electrons can occupy only certain stable orbits around the nucleus, each of which represents a definite energy level; these energy levels are said to be quantized, or restricted to certain values, rather than a continuous set of values. 2. Light of a definite energy or frequency is emitted or absorbed by an atom when an electron jumps from one orbit to another orbit. When Bohr's theory was applied to the hydrogen atom, it produced the exact emission results given by the actual atom. In 1922, Bohr received the Nobel Prize in Physics for this work. However, Bohr's model only worked for hydrogen, which is the simplest atom. Electron Configuration In an earlier section, we learned how to determine how many electrons were present in an atom (# of electrons = # of protons = atomic #). Now we will learn how chemists describe the manner in which those electrons are arranged within the area they occupy around the nucleus. Electrons are not arranged