Answer :
To find the mass number of the species, you need to calculate the total number of protons and neutrons in the nucleus of the atom.
1. Given that the ion has a charge of -2, it means that the number of protons is 2 less than the number of electrons. So, you can calculate the number of protons by subtracting 2 from the number of electrons.
2. The number of protons in an atom is the same as its atomic number. Therefore, to find the number of protons, you can use the atomic number of the element that corresponds to the given ion.
3. The total number of protons and neutrons (mass number) can be calculated by adding the number of protons (atomic number) and the number of neutrons together.
Given that the ion has 66 electrons and 64 neutrons, the number of protons can be calculated as 66 - 2 = 64.
Since the ion has a charge of -2, it indicates an extra 2 electrons compared to the number of protons. Therefore, the atomic number (number of protons) is 64.
To calculate the mass number, you add the number of protons (64) and neutrons (64) together. Thus, the mass number of the species is 64 + 64 = 128.
Therefore, the correct answer is b) 128.
