• The atomic number of an element is defined by the number of protons it has.

• Each element has a unique atomic number.

Atomic Mass

• The atomic mass of an element is the sum of the numbers of protons and neutrons it has.

• It does not include electrons, because electrons have a negligible mass.

Number of Electrons

• The number of electrons in an element is equal to the number of protons it has, so that the element is neutral.

• If the number of electrons does not equal the number of protons, the element has a net charge.