When polonium 210 emits an alpha particle the child isotope has an atomic mass of

The child isotope has an atomic mass of 206.

Explanation:

Isotope are element that have the same number of protons but different number of neutrons i.e they have the same atomic number but different mass number. All isotopes generally have a short half life, which is the time required for individual element to decay to half of its initial size.

An alpha particle has an atomic number of 2 and mass number of 4. So that alpha particle emission by polonium 210 yield a child isotope of lead with atomic mass of 206.