Atomic radius across period 3
After studying this page, you should be able to:
- describe and explain the trend in atomic radius across period 3
The table shows atomic radius values for the elements Na to Ar.
|Element||Symbol||Atomic number||Atomic radius /nm|
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.
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.