Home A Level Inorganic Chemistry - Core Periodicity Atomic radius across period 3

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Atomic radius across period 3

Learning outcomes

After studying this page, you should be able to:

  • describe and explain the trend in atomic radius across period 3

Atomic radius

The table shows atomic radius values for the elements Na to Ar.

Element Symbol Atomic number Atomic radius /nm
sodium Na 11 0.191
magnesium Mg 12 0.160
aluminium Al 13 0.130
silicon Si 14 0.118
phosphorus P 15 0.110
sulfur S 16 0.102
chlorine Cl 17 0.099
argon Ar 18 0.095

Atomic radius is the distance from the centre of the nucleus to the edge of the surrounding electron cloud. It is measured or calculated in different ways, so values vary from source to source.

Description of trend

The graph shows how atomic radius varies across period 3:

  • as the atomic number increases, the atomic radius decreases.
When you click on the download symbol, you will be able to download the graph as an image file or pdf file, save its data, annotate it, and print it. Note that graphs will be watermarked.
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Explanation of this trend

Going across period 3:

  • the number of protons in the nucleus increases so …
  • the nuclear charge increases …
  • there are more electrons, but the increase in shielding is negligible because each extra electron enters the same shell …
  • therefore the force of attraction between the nucleus and the electrons increases …
  • and the atomic radius decreases.

You might expect the atomic radius to increase because the number of electrons in each atom increases going across period 3. However, this does not happen: the number of protons also increases and there is relatively little extra shielding from electrons in the same shell.