Step-by-step explanation:
An isotope is defined as the specie which contains same number of protons but different number of neutrons. 
For example, 
 and
 and 
 are isotopes.
 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.
 is an isotope of oxygen atom whose atomic number is 8 and atomic mass is 17. 
In 
 , there are 8 protons and 9 neutrons.
, there are 8 protons and 9 neutrons.