The number of protons in the nucleus of an atom is its atomic number (Z). It is the defining trait of an element: Its value determines the identity of the atom. For example, any atom that contains six protons is the element carbon and has the atomic number 6, regardless of how many neutrons or electrons it may have. A neutral atom must contain the same number of positive and negative charges, so the number of protons equals the number of electrons. Therefore, the atomic number also indicates the number of electrons in an atom. The total number of protons and neutrons in an atom is called its mass number (A).

The number of neutrons is, therefore, the difference between the mass number and the atomic number:

A – Z = number of neutrons.

Atomic number (Z) = number of protons

Mass number (A) = number of protons + number of neutrons

A−Z = number of neutrons

Atoms are electrically neutral if they contain the same number of positively charged protons and negatively charged electrons. When the numbers of these subatomic particles are not equal, the atom is electrically charged and is called an anion.

The charge of an atom is defined as follows:

Atomic charge = number of protons − number of electrons