Atomic radius of  Period 3 Elements

The diagram below shows how atomic radius changes across Period 3.

The figures used to construct this diagram are based on:

• metallic radii for Na, Mg and Al;
• covalent radii for Si, P, S and Cl;
• the van der Waals radius for Ar (which forms no strong bonds).

It is appropriate to compare metallic and covalent radii because they are both being measured in tightly bonded circumstances. These radii cannot be compared with a van der Waals radius, however, making the diagram deceptive. The general trend towards smaller atoms across the period is not broken at argon. For convenience and clarity, argon is ignored in this discussion.

explaining the Trend:

A metallic or covalent radius is a measure of the distance from the nucleus to the bonding pair of electrons. From sodium to chlorine, the bonding electrons are all in the 3-level, screened by the electrons in the first and second levels. The increasing number of protons in the nucleus across the period attracts the bonding electrons more strongly. The amount of screening is constant across Period 3.