β⁻ emission is an
emission of an electron from the parent atom while converting a neutron into a
proton. Hence, the formed daughter nucleus has the same
number of mass but the atomic number is higher by 1 and number of neutrons is
lower by one than the parent atom.
After a β⁻ emission of ¹⁰⁶Ru, the formed daughter nucleus
should have 106 as mass number but (44 + 1) = 45 as the atomic number.
Hence the daughter nucleus is Rh (Rhodium) which has 45 protons
(atomic number).
