Mass number = No. of protons + No. of neutrons = 31
∴ Number of neutrons = 31 – Number of protons
= 31 – 15 = 16
Mass number = No. of protons + No. of neutrons = 31
∴ Number of neutrons = 31 – Number of protons
= 31 – 15 = 16
| Elements | Protons | Electrons | Neutrons | Atomic Number | Mass Number |
| A | – | – | 10 | 8 | – |
| B | 15 | – | – | – | 31 |
| C | 1 | – | – | – | 3 |
| D | – | 11 | 12 | – | – |
| X | Y | |
| Protons | 8 | 8 |
| Neutrons | 8 | 10 |