What is the trend in Atomic Radii? Which elements have a bigger Radius?
Down a group, the atomic radius increases. Across a period, the atomic radius decreases.
#1. Down a group
THE ATOMIC RADIUS INCREASES
• The number of energy levels increases by 1 as we go down a group. This makes the atom larger.
• An increasing number of electron shells increases the nuclear shielding. Thus, outer e- are protected from the positive charge of the nucleus by filled inner e- shells.
• The nuclear charge (ie. the number of protons) does increase down a group and the nucleus becomes more positive. However, the increased nuclear shielding protects the outer e- from the increased pull.
For example, Rb has a bigger atomic radius than Na.
#2. Across a period.
THE ATOMIC RADIUS DECREASES
• From L to R, no new energy levels are added to the atom (ie. electrons are not farther away).
• This means that the nuclear shielding stays the same (ie. there number of filled inner e- shells stays the same from L to R).
• However, the nuclear charge increases. More protons are added to the nucleus, making it more positive. As such, it is able to pull the outer e- inwards. The atom becomes smaller.
For example, C has a smaller atomic radius than Li.