The total of numbers of proton and neutron s in the nucleus of an atom is its

The total number of protons and neutrons in nucleus of an atom is its atomic mass.

Explanation:

An atom consist of electron, protons and neutrons. The neutrons and protons are present with in nucleus while electrons are present out side the nucleus. The sum of protons and neutrons are the mass number of an atom while the number of protons are number of electrons are atomic number of an atom. All these three subatomic particles construct an atom. The number of protons and electrons are always equal.

A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons.

Consider the carbon atom it has six protons six neutrons and six electrons. So its mass will be sum of neutron and protons which is 6+6 = 12

while the number protons are number of electrons are its atomic number which six.