Q. Explain about Atomic Radius?

The atomic size decreases with increase in atomic number  along any period in the long form of the periodic table due to increase in effective nuclear  charge. Though, the decrease in atomic radius is small when the difference in electronic configuration from one element to the next is that of an additional inner electron. This is due the additional inner electron screens the size-determining outer electrons from the nucleus much better than an additional outer electron. For example, decrease in the covalent radius from Sc to Zn; i.e., across ten elements of the 3d transition series, is 19 pm. this.  Decrease is almost one-third of the decrease in the covalent radius of the seven elements of s and p blocks of the period 3.