Atomic Structure

As a result of the work of Thomson, Rutherford and others, we know a great deal about the makeup of the atom. We know that each atom has a small positive center called the nucleus surrounded by electrons. Further, we know that the nucleus is comprised of smaller particles called protons and neutrons.
Information on:

• protons in the nucleus
• neutrons in the nucleus
• electrons in the atom

So, in simplest terms, the protons determine the identity of the atom, the neutrons (along with the protons) determine the mass of the atom and the electrons determine the charge (in comparison to the number of protons) of the atom or ion.

There are several numbers that are used to keep all of this organized. Since the identity of an atom is determined by the number of protons, this number has a special name - atomic number. Charge represents the relationship between the positive protons and the negative electrons. Mass number is the number of particles present in the nucleus (protons and neutrons).

Knowing these relationships allows us to do some simple calculations.

For the rest of the unit, we will use the standard shorthand way of representing atoms and particles which is described here.

It is also important that you understand the difference between mass number and atomic mass, how atomic mass is determined (mass spectroscopy), and the meaning of average atomic mass.