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Trend in atomic radius of Period 3 elements

What is here?

Atomic radius decreases going across Period 3.   You can:

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Table of physical data

Element

Proton number

Symbol

Atomic radius (nm)

sodium

11

Na

0.191

magnesium

12

Mg

0.160

aluminium

13

Al

0.130

silicon

14

Si

0.118

phosphorus

15

P

0.110

sulphur

16

S

0.102

chlorine

17

Cl

0.099

argon

18

Ar

0.095

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Graph of physical data

Graph of atomic radii of Period 3 elements

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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 principal energy level ...
  • therefore the  force of attraction between the nucleus and the electrons increases ...
  • so the atomic radius decreases.

As the number of electrons in each atom increases going across Period 3, you might expect the atomic radius to increase. This does not happen,  because the number of protons also increases and  there is relatively little extra  shielding from electrons in  the same principal energy level.

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