More Trends in the Properties of the Elements of Period 3

This is because noble gases do not form covalent bonds and therefore do not have covalent radii

Down a group, atomic radii increases because a new outer main level of electrons is added, which increases the distance between the nucleus and the outer electrons

Across a period, atomic radius decreases

This is because more protons are added to the nucleus which increases the nuclear charge. This increase causes the electrons to be pulled closer to the nucleus. There are no additional electron shells to provide more shielding. so the size of the atom decreases across the period

The number of protons in the nucleus increases

This is because no additional electron shells are added; all new electrons go into the same outer shell

The increased nuclear charge pulls the electrons closer, so without extra electron shell shielding the atomic radius becomes smaller

It generally increases across a period from left to right

It is the energy required to remove one electron from each atom in a mole of isolated gaseous atoms to form a mole of single positively charged gaseous ions

As you move across a period, the number of protons in the nucleus increases, but the electrons enter the same main energy level. The increased nuclear charge makes it harder to remove an electron

It decreases going down any group

The number of filled inner electron levels increases, resulting in increased shielding. The outer electron is also further form the nucleus, so it is held less strongly and is easier to remove

There is a sharp drop in ionisation energy when moving from between periods because a new main energy level starts, increasing the atomic radius. The outer electron is further from the nucleus, less strongly attracted, and easier to remove