\tOn moving down a group, the number of shells increases at each succeeding element. As a result, the valence electrons lies farther away from the nucleus. Therefore, the force of attraction between nucleus and the electrons decreases and tendency to lose electrons increases down the group.\tAs we move left to right in a period, number of shells remain same but number of valence electrons increases. As a result, effective nuclear charge increases and the tendency to lose electrons decreases.