Isoelectronic species are atoms, ions, or molecules that have the same number of electrons and therefore, the same electronic configuration, even if they are different elements.
For 8X2−
Number of protons = 8, while Number of electrons = 10, because X gains 2 electrons and outnumbers the number of protons.
Also, for 10Y,
Number of protons = 10, while Number of electrons = 10, because Y has no charge, it is a neutral atom. Hence, the number of protons = Number of electrons =10
Since the number of electrons are the same in both atoms of element X and element Y, even though they are of different elements, they are said to be isoelectronic species.
Therefore, 8X2− and 10Y are Isoelectronic