Electronic Configuration - 2,8,6. The no. of shells occupied by the electrons indicates the period it belongs to. It occupies 3 shells, and hence it will be in the 3rd period. It will be in the 16th group as it has 6 valence electrons and lies in the 3rd period. ( We add 10 to the no. of electrons for elements in the 2nd and 3rd period when they have more than 1 or 2 electrons in the outermost shell to get the group no.) Atomic no. = no. of protons = no. of electrons. It has 6 electrons and hence it will have a tendency to gain 2 electrons to complete its octet as it is easier to do so and will require less energy than losing 6 electrons and hence will have valency 2. The atomic no. thus helps to determine the valency of an element by indicating the no. of valence electrons present. If the valence electrons are <4 it is likely to gain electrons while if valence electrons are >4 then it more likely to lose electrons.