Orbitals are described by two sets of probability distributions. Radial probability distribution of an orbital gives an idea of its distance from the nucleus, (and hence energy) while the angular probability distribution gives an idea of its shape. The radial probability plot of 3s orbital of sodium atom has one hump of maximum probability at a greater distance than 3p, or 3d. But 3s orbital has two smaller humps very close to the nucleus, meaning it has a significant probability of being close to the nucleus. Or in other words, it ‘penetrates’ the core electron cloud and ‘feels’ closer to the nucleus. Similarly, although 3p orbitals have the largest hump closer than 3s, it has another closer hump, at a larger distance from the nucleus than those of 3s orbitals. Thus it has greater average size than 3s. 3d orbitals, on the other hand have only one hump and hence largest in size. Mean values of radial distances from the nucleus for different phosphorous orbitals are 3s = 0.47, 3p = 0.55 and 3d = 2.4 (in Angstrom units).