Data given
Number of electron = 108
Number of neutron = 158
Required
Mass number=?
Solution
As we know that mass number = No of proton + No of neutron
In any neutral atom the number of proton is always equal to number of electron
So number of proton will be = 108
However the atom has +1 charge which means that one proton is less than no of electron so proton number is 107.
Put values in above formula, we will get
Mass number= 107+108
= 215 Mass number
