Define atomic radius. Explain the variation of atomic radius along a period and down the group.

Atomic radius refers to the size of an atom. It is usually defined as the distance from the center of the nucleus to the boundary of the surrounding cloud of electrons. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius.

Variation of Atomic Radius Along a Period: As we move from left to right across a period in the periodic table, the atomic radius generally decreases. This is because, within a period, the outer electrons are in the same energy level. However, as we move across the period, each atom has more protons in its nucleus and more electrons. The increased number of protons causes the electron cloud to be drawn in closer to the nucleus, resulting in a smaller atomic radius.

Variation of Atomic Radius Down a Group: As we move down a group in the periodic table, the atomic radius generally increases. This is because, with each step down a group, an electron shell is added. Even though the number of protons also increases, the effect of the additional electron shell is greater, resulting in a larger atomic radius.