How do atomic radius generally vary in a period and in a group for s-and `p-`block elements ? How do you explain the variation?

Answer

Within a group, the atomic radius increases down the group. This is because a new energy shell is added (i.e. principal quantum number increases by unity) at each succeeding element while the number of electrons in the valence shelll remains the saem. In other words, the electrons in the valences shell of each succeeding element lie farther and farther away from the nucleus. As a result, the force of attraction of the nucleus for the valence electrons decreases and hence the atomic size increases. In contrst, the atomic size decreases as we move from left ro right in a period. This is becuase within a period, the outermost electrons remain in the same shell but the nuclear charge increases by one unit at each succeeding element. Due to this increased charge, the attraction of the nucleus for the outermost electrons increases and hence the atomic size decreases.