The sum of the number of neutrons in the isotopes of hydrogen is 0+1+2=3 Hydrogen has three isotopes. Formula to get number of neutrons, N=M−Z where, N= number of neutrons, M= mass number and Z= atomic number.
Isotope
Number of protons
Number of neutrons (M−Z)
Hydrogen ‌1H1
1
0
Deuterium ‌1D2
1
1
Tritium ‌1T3
1
2
The sum of the number of protons =1+1+1=3. The sum of the number of neutrons =0+1+2=3.