explain why ₁ ³H AND ³ ₂He are not considered isotopes.

The atoms of 1,3 hydrogen atom and helium 1,3 atoms are not considered pair of isotopes.An isotopic pair have same atomic number and different mass due to different number of electrons. But in this case the atomic mass is same but atomic number is different.hence they are isobars. Like.

3H1 and 3HE2 are not considered as isotopes as they have different atomic no. but same mass no..but isotopes have same atomic no. and different mass no.