$_{20}X^{40}$ and $_{21}X^{40}$ are

An atom which has lost one electron would be

Which of the following particles has more electrons than neutrons?

Which of the following is correct?

Give the relation between the following pairs of species:
$(i)$ $_{20}^{40}Ca$ and $_{19}^{40}K$
$(ii)$ $_{17}^{35}Cl$ and $_{17}^{37}Cl$
$(iii)$ $_{18}^{40}Ar$,$_{20}^{40}Ca$ and $_{19}^{40}K$
$(iv)$ $_{38}^{87}Sr$ and $_{38}^{90}Sr$

From the following pairs of ions,which one is not an iso-electronic pair?