The nucleus of the element having atomic number $25$ and atomic weight $55$ will contain

Isobars are atoms with

Two atoms of elements $X$ and $Y$ are isotones of each other. Their mass numbers are $70$ and $72$ respectively. If the atomic number of $X$ is $34$,then the atomic number of $Y$ is ..........

The atomic number of an element is derived from

Among the following,the set of isoelectronic ions is:

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