In order to find this, we take the mass (which is 35.45 but we're just going to use 35) and take away the atomic number, making the equation 35-17=18 and this is the amount of neutrons. So now we know that m=35, a=p=e=17 and n=18.
(IF YOU ALREADY KNOW WHAT YOUR NEUTRON NUMBER IS, USE P(ROTONS)+N(EUTRONS)=M(ATOMIC MASS) TO FIND YOUR MASS. WITH THIS ATOM, IT WOULD BE 17+18=35)
More Atomic Info
(Diagram to the right is a diagram of the Chlorine atom)