trends on the periodic table – atomic radius
The atomic radius describes the distance between the nucleus and the outermost electron. This distance is
very small so it is often reported in picometers (1 x 10
m) or angstroms (1 x 10
m). One way of measuring the
atomic radius is to observe the distance between two bonding atoms. The distance between the nuclei of the bonding
atoms is equal to twice the atomic radius. This distance is called the bonding atomic radius and is shorter than the non-
A pattern is observed when the atomic radius is plotted versus the atomic number, as seen above. As might be
expected the largest atoms are found at the bottom of the periodic table, however, as you go across a period, the atomic
radius decreases (see highlighted region in the graph above). All of the elements in a period have the same orbitals
which have the same energy and same size. Additionally, the elements that lay on the right side of the periodic table
have nuclei which contain more protons. This results in a larger attraction of the electrons and a smaller radius. This
explains why argon (atomic number = 18) has a smaller radius than sodium (atomic number = 11).
Directions: Answer the following questions on a separate piece of paper.
1. Estimate the size of the smallest atom in the chart above. Estimate the size of the largest.
2. Which ﬁve elements are represented by the tallest ‘peaks’ in the graph above?
3. Write the approximate radius for each of the noble gases in the table below.
element atomic radius element atomic radius element atomic radius
4. What is the general trend for atomic radius as you go down the noble gas family?
5. What is the general trend as you move through period 3 (from element 11 to element 18)
6. Carbon (atomic number = 6) and neon (atomic number = 10) both have their outermost electrons in 2p orbitals.
Explain why neon is smaller than carbon even though it has more electrons.
7. The graph shows a sudden increase in atomic radius for lithium, sodium, potassium, rubidium, and cesium. Explain
why there is a spike on the graph at element numbers 3, 11, 19, 37, and 55.
8. Bromine (atomic number = 35) forms a diatomic molecule. Sketch a picture of this molecule and determine the
distance in picometers between the two bromine nuclei.