a) Chlorine atom (Cl) has atomic number 17. It contains 17 protons and 17 electrons.
Chlorine ion (Cl-) is formed when Cl gains one electron.
So, Cl- has 18 electrons and 17 protons.
Therefore, the electronic configuration of Cl- = 2, 8, 8.
b) Atomic number of Cl- = Number of protons = 17
Mass number of Cl- will be the same as Cl i.e. 35.
c) Valency is defined as the combining capacity of an atom.
For a non-metallic element, it is equal to eight minus the number of electrons present in outermost shell. Here, Cl- has 8 electrons in the outermost shell, therefore valency of Cl- = 8 - 8 = 0