An isotope is defined as the specie which contains same number of protons but different number of neutrons.  

For example, ^{16}_{8}O and ^{17}_{8}O 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 ^{17}_{8}O is an isotope of oxygen atom whose atomic number is 8 and atomic mass is 17.

In ^{17}_{8}O, there are 8 protons and 9 neutrons.