Atomic structure

The mass number of an atom is its total number of protons and neutrons. Isotopes are atoms of an element with different numbers of neutrons. The relative formula mass of a compound is found by adding together the relative atomic masses of all the atoms in the formula of the compound. The relative formula mass of a substance in grams is one mole of that substance.

Subatomic particles

Structure of the atom

Each atom consists of a nucleus containingprotons and neutrons, with electronsarranged around it in energy levels.

Relative masses of subatomic particles

Name of particle	Relative mass
Proton	1
Neutron	1
Electron	Very small (1/1836)

Mass number and atomic number

The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger:

• The mass number of an atom is the total number of protons and neutrons it contains
• The atomic number of an atom is the number of protons it contains

Notice that most of the mass of an atom is found in the nucleus.