An element P disintegrates by α emission and the new element suffers two further disintegrations, both by β emission, to form an element Q. Explain the fact that P and Q are the isotopes.

As we know, the atoms of same element, having same atomic number Z, but different mass number A, are called isotopes.

When the element P disintegrates by α emission, atomic number of P decreases by 2 and mass no. decreases by 4 then the atomic number increases by 1 due to first β emission and again increases by 1 due to second β emission and we get Q. We observe that atomic number of Q becomes the same as it was before the emission (i.e. P). Hence, P and Q are the isotopes.