Trend in Atomic Radii

Variation along Period

In a period, as we move from left to right the atomic radius decreases.

Reason

This is because with the increase in atomic number, (i.e., on moving from left to right along a period, the atomic number increases by one unit at each interval) the nuclear charge increases while the number of shells remain same. The electrons are attracted more strongly towards nucleus. The result is decrease in atomic radius. Thus in a given period, the alkali metals has the largest atomic radii and halogens, the smallest atomic radii.

Noble gases which are at the end of each period, are expected to have smallest radii, but they have comparatively larger atomic radii. This is due to the reason that in case of inert gases, the outer shell is complete and it has hence maximum electronic repulsion. Moreover in case of inert gases, the atomic size is expressed in terms of Vanderwaal’s radius because, noble gases do not form covalent bonds  while other form covalent bonds and hence their atomic radii is expressed in covalent radius. Thus as, Vanderwaal’s radius is greater than covalent radius, the atomic size of noble gases is larger than other elements of same period.

Variation down the group

As we move down the group, the atomic radius of the corresponding elements increases.

Reason

This is due to the reason that as we move down the group, the number of shells increases, though the nuclear charge also increases. The effect of increase in number of shells is more than the increase in nuclear charge, therefore, the effective nuclear charge decreases. Consequently, the distance of the outermost electron from the nucleus increases on going down the group.

On Short,

Atomic radii increase down the group.

Atomic radii decrease along the period.