2.2 Atomic Theory

The properties of elements are determined by atomic number and atomic mass.

Study Guide

Explain each Concept

The number of protons in the nucleus of an atom is the atomic number.

è The atomic number identifies the atoms of a particular element.

è The mass number of an atom is the sum of the atom’s neutrons and protons.

è The atomic mass is an average of all naturally occurring isotopes of an element.

è Democritus argued that atoms were the smallest particles in all matter.

è Dalton proposed an atomic theory that stated the following:

è Thomson discovered electrons.

è Rutherford discovered the small, dense, positively charged center called the nucleus.

è Bohr suggested that electrons move around the nucleus at only certain distances.