Question

An atom X has atomic number 15 it accepts 3 electrons in other to become stable. Determine the charge on the atom before accepting electrons

Answer 1

15X30 + 3 e = 12Y30

Above an atom with 15 electrons (15 neutrons) accepts 3 electrons and thus reduces its charge by 3, but the number of neutrons remains the same

If the atom had a charge of 18 before accepting the electrons, it's charge would be reduced by three to 15

18X30 + 3e = 15Y30

From the question I am not sure the atomic number of 15 is before or after accepting the electrons

In any case the charge is reduced by three and the atomic mass is unchanged