The additional electron does not affect the atomic number nor the mass number. The atomic mass is the sum of neutrons and protons. Electrons are not accounted in atomic mass as their mass is so small, approximately 1/1,836 of a protons mass. Atomic number is the total number of protons of an atom. Chlorine taking in another electron will not change its atomic number. In an ion, the atomic number and atomic mass do not change from the original.
The working equation for this problem is average atomic mass
is equal to the summation of the product of relative abundance and mass of
isotope. In this problem, just subtract the % abundance of Ag-107 to 100. Thus
the % abundance of Ag-109 is 48.16.
