The nuclei of a tritium $({^3H})$ atom contain how many neutrons?

An atom has $26$ electrons and its atomic weight is $56$. The number of neutrons in the nucleus of the atom will be

Which of the following species does not have the same number of electrons as the other three species?

Find the number of neutrons,electrons,and protons in the following species:
$(i)$ $_{15}^{31}P$
$(ii)$ $_{11}^{23}Na^{+}$
$(iii)$ $_{16}^{32}S^{2-}$
$(iv)$ $_{101}^{235}X$
$(v)$ $_{20}^{40}Ca^{2+}$
$(vi)$ $_9^{19}F^{-}$

$Be^{2+}$ is isoelectronic with which of the following ions?

An element $M$ has an atomic mass $19$ and atomic number $9$. Its ion is represented by