Explain the size of atomic radii observed in the periodic table

The size of atomic radius increases down the group but decreases from left to right across the period. Atomic radius is defined by the distance of protons in nucleus to surrounding electron shells. Mainly, two factors influence the size of the atomic radius: (a) atomic number and (b) shielding. First, increasing atomic number suggests a smaller atom due to higher number of protons in its nucleus. The higher density of protons increases the positive nucleus charge and allows to attract electrons more efficiently. Secondly, phenomena known as shielding increases the atomic radius due to electrons repulsing each other. Electrons that are closer to the nucleus will repulse the outermost ones due to possessed negative charges, therefore increasing the average distance from nucleus to electrons.