Structural Biochemistry/Atomic Radius

Sizes of Atoms

Definition

The atomic radius is defined as the measured length from the atomic nucleus to the electron orbital that is in the outermost shell of the atom. Typically in practice, this length is measured by obtaining the distance between identical, adjacent atomic nuclei of the sample element, and this distance is then divided by half. In many cases, the size of the atom of a compound varies depending on the substances that are near it, hence it can be said that the atomic radius can vary slightly depending on what is around it.

The atomic radius is a term used to describe the size of the atom, but there is no standard definition for this value. Atomic radius may refer to the ionic radius, covalent radius, metallic radius, or van der Waals radius. In all cases, the size of the atom is dependent on how far out the electrons extend. The atomic radius for an element tends to increase as one goes down an element group. The electrons become more tightly packed as you move across the periodic table, so while there are more electrons for elements of increasing atomic number, the atomic radius actually may decrease.

Ionic radii are difficult to measure with any degree of certainty, and vary according to the environment of the ion. For example, it matters what the co-ordination of the ion is (how many oppositely charged ions are touching it), and what those ions are.

There are several different measures of ionic radii in use, and these all differ from each other by varying amounts. It means that if we are going to make reliable comparisons using ionic radii, they have to come from the same source.

Trends

Across the Period

The atomic radius usually decreases in a period from left to right, due to the fact that nuclear charge increases, which then attracts the electrons that are orbiting and pulls them closer to the nucleus.

Across the Group

The atomic radius usually increases in a group from top to bottom, due to the fact that an additional energy level is added to the shell, which increases the n value.

References

1. Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.