Which of the following are identical in terms of their atomic number $(Z)$?
$(i)$ $Na$ and $Na^+$
$(ii)$ $F$ and $F^-$

Nuclear reactivity of $Na$ and $Na^+$ is same because both have

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) $1$ |

If a species has $16$ protons, $18$ electrons and $16$ neutrons, find the species and its charge.

$Ca$ has atomic number $20$ and atomic weight $40$. Which of the following statements is not correct about $Ca$ atom?

The number of electrons plus neutrons in ${}_{19}^{40} K^{+}$ is $.....$