The Group No . represents the number of electrons in an atom that are in the outermost layer of the atom, or the outer shell. The Atomic Wt . represents the relative weight of atoms in amu. These are responsible for the chemical characteristics of that element. This is known as the Octet Rule. OR it can gain 7 to be like Ar.
Atoms are like people and do the easiest thing energy-wise.
It s easier to lose 1 than to gain 7. Chlorine, on the other hand, having 7 valence electrons finds it easier to gain 1 than to lose 7. Once we know how an atom will act, we now know what it will do when it combines with other atoms. Here are some terms to note.