Atomic & Ionic Radii

Chemistry: IB Diploma Higher Level, topic 3.2.1

Trends in Atomic Radii

Trends are seen in atomic radii across periods and down groups as the strength of the attraction between the nucleus and the outer electrons changes.

An atomic orbital is a mathematical function that gives a probability of locating an electron within a defined volume known as an atomic orbital.
Due to the uncertainty principle, the limits of an atom are difficult to define and measurements of atomic radii are not straightforward.
To overcome this, chemists measure half the distance between two bonded atoms.

On going across a period, the effective nuclear charge increases as protons are added to the nucleus whilst electrons are added to the same principal energy level (constant shielding).
Therefore the outer electrons experience a greater attraction by the nucleus, leading to a decrease in atomic radius.
Example: Sodium is at the start of period 3 with 11 protons and an atomic radius of 1.86 x 10^-10 m. Chlorine at the end of period 3 has 17 protons and a smaller atomic radius of 9.90 x 10^-11 m.

On going down a group an extra principal energy level is added to the atomic structure and therefore the atomic radius increases.
The effective nuclear charge is constant for elements in the same group (although the nuclear charge increases so does the number of shielding electrons) so this does not influence the change in atomic radius on descending a group.
Example: In group 1, lithium has two principal energy levels and an atomic radius of 1.57 x 10^-10 m, whereas potassium has four principal energy levels and a larger atomic radius of 2.35 x 10^-10 m.

Trends are seen in ionic radii across periods and down groups as the strength of the attraction between the nucleus and the outer electrons changes.

Ionic radius increases on descending a group as the ions have the same charge, but more principal energy levels.
The trend in ionic radii going across a period is a little more complicated as there are positive and negative ions to consider:

Positive ions are smaller than their parent atom as the highest principal energy level is removed when forming the cation.
From group 1 to group 14, in period 3, all ions have the same electron configuration (1s²2s²2p⁶).
As nuclear charge increases there is greater attraction to the outer electrons and therefore ionic radius decreases from group 1 to 14.

Negative ions are larger than their parent atom as electrons are added to the highest principal energy level, but the nuclear charge is unchanged. The increased repulsion between electrons in the negative ion causes electrons to move further apart and increases the radius.
From group 15 to group 17, in period 3, all ions have the same electron configuration (1s²2s²2p⁶3s²3p⁶).
As nuclear charge increases there is a greater attraction to the electrons and therefore ionic radius decreases from group 15 to group 17.

The overall trend in ionic radius across a period is summarised above, using period 3 as an example.

What is the trend in atomic radius on going across a period?
What is the correct reason for why atomic radius increases on going down a group?
What is the trend in ionic radius for positive ions on going across a period?