The nitride ion in lithium nitride is composed of:

Match the following species with their respective number of electrons:
| Species | Number of electrons |
| :--- | :--- |
| $(a)$ $O^{2-}$ | $(i)$ $18$ |
| $(b)$ $Li^{2+}$ | (ii) $2$ |
| $(c)$ $He$ | (iii) $10$ |
| $(d)$ $Ca^{2+}$ | (iv) $1$ |

What is the representation of an element having a mass number of $40$ and $21$ neutrons in it?

If $A^{+}$ has $11$ protons,then the number of electrons in it is...........

Addition of two neutrons in an atom $A$ would

Number of neutrons in ${}^{12}C$ is