We know that the number of protons = Atomic number
Number of protons = Number of electrons
Mass number = Number of protons + number of neutrons
Using these relationships, we can fill up these gaps as follows.
Elements | Protons | Electrons | Neutrons | Atomic Number | Mass Number |
A | 8 | 8 | 10 | 8 | 18 |
B | 15 | 15 | 16 | 15 | 31 |
C | 1 | 1 | 2 | 1 | 3 |
D | 11 | 11 | 12 | 11 | 23 |