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# Structure of Matter

"If I have seen further it is by standing on the shoulders of giants"
Isaac Newton

- It is well known today that all matter is comprised of atoms. The concept of atoms was postulated by the ancient Greeks , but their existence was not confirmed until a little more than 100 years ago. All atoms consist of a nucleus made of protons and neutrons around which "orbits" one or more electrons.
- The basic properties of protons, neutrons, electrons and the atom as a whole are described in the following table.

  Particle | Electric charge | Mass | Approximate Size
  electron (e-) | - 1.6 x 10-19 C | 9.1 x 10-31 kg | < 10-18 m
  proton (p) | + 1.6 x 10-19 C | 1.6 x 10-27 kg | 10-15 m
  neutron (n) | 0 | 1.6 x 10-27 kg | 10-15 m
  Atom | 0 (neutral) | (Np + Nn) x 1.6 x 10 -27 kg | 10-10 m

  where Np, Nn are the number of protons and neutrons, respectively, in the nucleus of the atom.

- Other important properties:
  - Mass of proton (mp) is approximately 2000 times the mass of the electron (me)
  - More than 99% of the volume of an atom is empty space. If the nucleus of the Hydrogen atom was one inch in diameter the orbit of the electron would have a radius of ½ mile.
  - Current experiments indicate that the electron has no structure, it is a true point particle.
  - Protons and neutrons are each comprised of quarks. With the same experimental limit as the electrons, quarks also have no sub-structure.
  - The wavelength of visible light is of order 5 x 10-7 m. Atoms are typically more than 1000 times smaller than this wavelength.
- The properties of the different elements in nature, e.g. Hydrogen, Sodium, Uranium etc. are defined by their "atomic structure". These elements can be arranged in a form known as the "periodic table" which illustrates the similarities and differences between them. Dmitri Mendeleev is generally considered the creator of the modern periodic table.
  Nottingham University has created the Periodic Table of Videos, which features a short video clip describing the properties, history, production etc of each element.

- Starting with the Hydrogen atom, which consists of a single proton nucleus and a single electron, different elements can be created by adding additional protons to the nucleus while at the same time adding the same number of electrons to keep the atom neutral. For light elements (excluding Hydrogen) the number of neutrons in the nucleus is equal to the number of protons; for heavier elements a neutron excess develops. For example the most common Lead atom comprises 82 protons and 126 neutrons. The heaviest naturally occuring element is Uranium , having 92 protons and 146 neutrons. Certain elements with more protons can be created but undergo radioactive decay with a lifetime shorter than that of the earth.

- For this course, the it is important to realize that the negatively charged electrons are held in "orbit" around the positively charged nucleus by the electric force of attraction. In fact all atomic and molecular forces of any significance are electrical (or electromagnetic) in nature. It could be said that all of Chemistry is due to the electromagnetic force. In fact virtually all everyday physical phenomena are electromagnetic in nature, friction, thermodynamics, elasticity etc etc. Only those phenomena involving very large masses (where the gravitational force becomes significant) and certain very small scale processes (e.g. radioactive decay, fission, fusion) are not primarily electromagnetic in nature. Understanding electricity and magnetism - electromagnetism - is critical to our understanding of nature.

A neutron walks into a bar. "I'd like a beer" he says. The bartender promptly serves up a beer. "How much will that be?" asks the neutron. "For you?" replies the bartender, "no charge"

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu