Atomic Theory


For millennia, there has been a great debate among the natural philosophers as to the nature of matter on the smallest levels.  This debate goes back to the pre-Socratic days of Democritus, Leucippus, and Aristotle.   Democritus and Leucippus promoted the ideas of atoms, and between the atoms there was the void, the vacuum.  Atoms came in all shapes, sizes, and orientations and could arrange themselves to make complex objects.  Aristotle promoted the idea of a plenum or continuum or fluid nature to matter and detested the idea of the void. As Descartes would say “Nature abhors a vacuum.”  One might say that history is written by the victors, as we do not have any surviving works of Democritus except what was written about him (unfavorably) by Aristotle.  Click here to read the main ideas of Democritus.

The idea of a plenum or continuous fluids persisted for centuries and the atomic theory of matter did not start to regain a respectable foothold until the time of Newton.  Even then, heat was considered to be a fluid called caloric, and electric charge was also considered a fluid.

Brownian Motion

The debate still raged into the 20th century. The source of debate over atomic theory can be tied to developing ideas of  thermodynamics and the origin of thermal energy.  The final nail in the coffin of the plenum was placed by Einstein when he explained Brownian motion. It is worth it to skim the article by David Cassidy on Eistein’s connection to Brownian motion.  There is also a nice simulation from Dr. Wolfgang Christian of Davidson University.

I first heard of Brownian Motion when reading the Hitchhiker’s Guide to the Galaxy and for a long time thought there was a connection to a hot cup of tea. Although Robert Brown observed this with grains of pollen we can easily observe it using some milk under a microscope.

From middle school physical science, you are probably familiar with the with the basic ideas of the Bohr model of the atom, where negatively charged electrons circle a nucleus composed of neutrons and positively charged protons.

A few numbers you will need for calculations this semester:

Charge of an electron      qe=1.6 10-19 Coulombs

Coulomb of charge          1 C = 6.25 x 1018 electrons

Mass of electron  me = 9.1 x 10–31 kg

Mass of proton  mp = 1.6 x 10–27 kg

The Atomic Number is the number of protons in an atom and is how we define which atom we are talking about.

The Atomic Mass Number is the number of protons plus the number of neutrons in a given atom.

A common unit for measuring mass is the amu or atomic mass unit.  For our purposes we can say that an amu is approximately the mass of one proton.

Example:  Suppose we have a sample of 14 grams of Carbon Monoxide (CO).  How many grams of Carbon are in this sample?

Carbon has an atomic mass number of 12 because it has 6 protons and 6 neutrons.

Oxygen has an atomic mass number of 16 because it has 8 protons and 8 neutrons.

One atom of CO would thus have a mass of 28 amu.

The amount of carbon in any sample of CO would be 12/28

Thus we could have a total of 6 grams of carbon in our sample.


On the period table, you can read the atomic mass number for any atom.  The table gives the most common atomic mass found in nature for a given atom.  Atoms may have extra (or a deficit) of neutrons, and these different versions of an atom are called isotopes.  The number listed on the periodic table is actually an average of these isotopes that we find in nature.

For instance, two common isotopes of Uranium are U235 and U238.  Uranium has 92 protons.  How many neutrons does each isotope have?

So U 235 has 235 – 92  = 143 neutrons

U238 has 238 – 92 = 146 neutrons


Generally, in a neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons in the nucleus.  When there are extra electrons we say we have a negative ion for that atom.  When there is a deficit of electrons, we have a positive ion.