An isotope is defined as the specie which contains same number of protons but different number of neutrons.
For example, and are isotopes.
In a neutral oxygen atom, there are 8 protons and 8 neutrons.
As it is known that in a neutral atom the number of protons is equal to the number of electrons.
This means that in a neutral oxygen atom there are also 8 electrons.
Whereas in is an isotope of oxygen atom whose atomic number is 8 and atomic mass is 17.
In , there are 8 protons and 9 neutrons.