What is the Atomic Radius?

Let’s discuss the definition of the atomic radius, also called atomic size, and the atomic radius trend on the periodic table. The atomic radius is measured as half the distance between two nuclei of the same atoms that are bonded together. While your initial thought may have been to measure the distance from the center of an atom’s nucleus to the edge of its electron cloud, this is inaccurate and not feasible. This is because the borders of orbitals are quite fuzzy, and they also change under different conditions. Thus the atomic radius is measured as shown in the diagram below.

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Atomic Radius Trend on the Periodic Table


Atomic radii increase toward the bottom left corner of the periodic table, with Francium having the largest atomic radius. Atoms decrease in size across the period and increase in size down the group. Let’s break down the trend into its period and group trends.

Period Trends

There are many trends on the periodic table. For example, ionization energy, electronegativity, and of course atomic radius which we will discuss now. Across a period, atomic radii decrease. This is because while the number of electrons increases down the period, they only add to the same main energy level, and therefore do not expand the electron cloud. Down the period, however, the number of protons also increases. This increased positive charge attracts or pulls, the electrons in closer to the nucleus, decreasing the atomic radius.


One thing to note is that the effect of the attraction between the positively charged nucleus and the electrons is slightly countered by the repulsion of electrons as they are successively added. This is why the difference in atomic radii decreases down each period.

Group Trend

Down a group, atomic radii increase. This is because between each group, electrons occupy successively higher energy levels. As electron cloud sizes increase, so do atomic radii.


The Trend on a Graph

As shown in the graph below, the atomic radius is largest at the first element in each period, and it decreases down each period.

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Smallest and Largest Atomic Radius

Francium has the largest atomic size on the periodic table, and helium has the smallest atomic size.