(a) How and why does the size of an atom vary on moving from left to right in a period ?
(b) Why does the chemical reactivity of metals increase on moving down a group?
(a) Size of the atom decreases on moving from left to right in a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
(b) Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus. Thus, these can be lost easily. Hence, metallic character increases down a group.