Two nuclides X and Y are isotonic to each other with mass numbers 70 and 72 respectively. If the atomic number of X is 34, then that of Y would be:

${}_{34}\mathrm{X}^{70}$Number of neutrons in X

n = 70 − 34

=36 Neutrons

${}_{36}\mathrm{X}^{72}$ number of neutrons in Y

n=72-36

=36

Isotones are the atoms of different elements which have the same number of neutrons.