See the atomic mass of 59 is the result of the sum of protons and neutrons present in the stable state. As for electrons, they are qual to the number of protons in stable state. If you count through the periodic table you will get to know that the atomic number for cobalt (Co) is 27. As per the following formula the number of neutrons would be 32. Atomic Mass (A)= Atomic Number (Z) - Neutrons (N) 59 = 27 - ? 59 - 27 = 32Neutrons Now as the Co2+ is a cation it means that there are two electrons less in comparison to the neutral stable state. Since the number of protons is 27, number of electrons would also be 27 in stable state. Therefore, Co2+ would have 27-2 electrons i.e. 25 electrons. You will have to go the same way about all other chemical compounds. Buy a complete periodic table showing the compounds in their stable state. Test and check it yourself. Hope this Helps:)
