What Is Atomic Number?

Atom has a nucleus which can carries positive charge on it. The positive charge carries by the nucleus is called nuclear charge which is equal to the total number of positive charges on the nucleus of the atom of that element. This total number of unit positive charges on the nucleus is called the atomic number of the element. Now since each proton has +1 charge and neutron has no charge, the total number unit positive charge carried by the nucleus is equal to the number of protons present in the nucleus of the atom. Thus atomic number of element can also be defined as the number of protons present in the nucleus. Thus:

'Atomic number of an element is defined as the number of  unit positive charges on the nucleus (nucleus charge) of the atom of that element or as the number of protons present in the nucleus.'

Now since the atom as a whole is electricity neutral, the number of protons (positively charged particles) in the nucleus is equal to the number of electrons (negatively charged particles) revolving in orbits around the nucleus. Consequently atomic number of an element is also equal to the number of electrons revolving in orbits round the nucleus. Thus:

Atomic Number (Z) = No. of unit positive charges on the nucleus (Nuclear charge).
                               = Total number of unit positive charges carried by all protonsnt in the nucleus.
                               = No. of protons in the nuclens (p)
                               = No. of electrons revolving in orbits (e)
or,

Z = p = e

Atomic  number of an element is also defined as the serial number of its position in the periodic table, starting from hydrogen as the first element.