Subatomic particles in the nuclear model

An atom has a tiny central nucleus, with electrons outside it. The nucleus contains protons and usually neutrons. Protons, neutrons and electrons are the three subatomic particles in the nuclear model.

Each particle has a relative electrical charge:

• A proton has charge $$+1$$.
• A neutron has charge $$0$$, so it is uncharged.
• An electron has charge $$-1$$.

How an atom's charge balances

A proton's positive charge and an electron's negative charge are equal in size and opposite in sign. An atom has equal numbers of protons and electrons, so their charges cancel. Neutrons add no charge. The atom therefore has no overall electrical charge.

For example, an atom with three protons has a total proton charge of $$+3$$. It also has three electrons, with a total electron charge of $$-3$$. These charges cancel, giving the atom an overall charge of $$0$$.

Atomic number and mass number

An atom has a nucleus containing protons and neutrons, with electrons around the nucleus. Protons have positive charge, electrons have negative charge, and neutrons have no charge.

In a neutral atom, the number of electrons equals the number of protons. Their negative and positive charges balance, so the atom has no overall charge.

Atomic number

The atomic number is the number of protons in an atom’s nucleus. It fixes which element the atom is: every atom of the same element has the same number of protons. Because a neutral atom has equal numbers of protons and electrons, its atomic number also gives its number of electrons.

Mass number and neutrons

The mass number is the total number of protons and neutrons in the nucleus. It is a count of these particles, not the mass of the atom.

Since the atomic number gives the number of protons, subtracting it from the mass number gives the number of neutrons:

For example, a neutral carbon atom with atomic number $$6$$ and mass number $$12$$ has $$6$$ protons and $$12 - 6 = 6$$ neutrons. It also has $$6$$ electrons because it is neutral.