Patterns and trends in the periodic table
Chemists observe patterns in different properties of elements as they are arranged in the periodic table.
Covalent radius
The covalent radius (a measure of how large individual atoms are) shows different trends if you are moving across a period or down a group.
A comparison of the relative covalent radii of atoms is shown in the diagram below.
Across a period from left to right, the covalent radius decreases.
As you move from left to right across the periodic table, atoms have more electrons in their outer energy level and more protons in their nucleus.
The greater attraction between the increased number of protons (increased nuclear charge) and electrons, pulls the electrons closer together, hence the smaller size.
As you move down a group in the periodic table, the covalent radius increases. Atoms increase in size.
This is because of the screening effect of the filled inner electron levels. These electrons screen or shield the outer electrons from the nuclear charge.