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 ..........

An atom has the electronic configuration of $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^{10}, 4s^2 4p^5$. Its atomic weight is $80$. Its atomic number and the number of neutrons in its nucleus shall be

$_{18}Ar^{40}$,$_{20}Ca^{40}$ and $_{19}K^{40}$ are

An ordinary oxygen sample contains:

In the two elements, ${ }_{Z_1} A^{M_1}$ and ${ }_{Z_2} B^{M_2}$, the following relations are true. $M_1 \neq M_2$ and $Z_1 \neq Z_2$ but $M_1-Z_1=M_2-Z_2$. These elements are ($M$ is atomic weight, $Z$ is atomic number)

The nucleus of helium contains