Matter and Atoms

Most matter is made up of atoms. That includes gases, like He or air, where atoms are free and go zipping around with an average speed proportional to the square root of the temperature of the gas (see post on heat and T); solids where atoms are stuck together on a semi-rigid lattice; and the in between state called liquids where atoms are close together, like atoms in a solid, but have a sense of disorder, freedom and flow reminiscent of the gas state.

All known atoms can be presented in something called the Periodic Table of Elements. This organizes the elements in a profound and amazing way. Some degree of familiarity with the nature of the periodic table and its organizing principles is an essential part of your education.

One of the puzzling things we noticed about atoms has to do with their size. The size of an atom is determined by its electrons, but it does not increase in any simple or systematic way with the number of electrons in the atom. For example, Hydrogen, which has one electron, has a radius of about 0.1 nanometers (nano means 10^-9). Neon, which has 10 electrons, also has a radius of about 0.1 nanometers --about the same as Hydrogen. The graph of atom size (radius) as a function of its number of electrons is very interesting and perplexing. This is a nice graph to be familiar with and to appreciate the subtlety of its origins, which lie in the wave theory of the electron (Edwin Schrodinger, 1928).

This complex and interesting behavior of the size of atoms contrasts with the relative simplicity of the mass of atoms, which is basically equal to the number of protons and neutrons in the nucleus of the atom time the mass of a single proton (or neutron), which is 1.67 x 10^-27 kg. All this mass is concentrated in very tiny nucleus, which is much much smaller than the size of the atom. So an atom consists of a positively charge nucleus which is very tiny but contains essentially all the mass of the atom, surrounded by a light, wave-like electron.