As atom has a larger size than P atom. In moving down a group, the nuclear charge increases with increase in atomic number but at the same tire there is a progressive increase in the principal quantum number, the number of electrons in the outermost shell, however, remains the same. Consequently, the distance of the outermost electrons from the nucleus gradually increases down a group because the effect of additional energy level is more pronounced than the effect of increased nuclear size. Therefore As atom has a larger size than P atom.
