In this video we want to dicuss the atomic and ionic radii of Period 3 elements.

1. Atomic Radii

The atomic radii decreases across Period 3.

We can use the concept of effective nuclear charge (ENC) to explain the atomic radii.

Note that effective nuclear charge is also used to explain first ionisation energy trend across Period 3.

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Across Period 3, proton number increases, the nucleus is more positively charged hence nuclear charge increases.

Electrons are added to the same principal quantum shell, do not shield each other since they are the same distance from the nucleus, hence shielding effect is the same.

Overall the effective nuclear charge increases, there is a stronger attraction between nucleus and valence electrons, the nucleus can pull the electrons closer to itself, hence atomic radius decreases.

2. Ionic Radii

There are 2 observations for ionic radii:

a. Anions are larger than cations
b. Decrease in radii within cations and anions

a. Anions are larger than cations

The first 4 elements (Na, Mg, Al, Si) lose electrons to form cations with 10 electrons, hence will fill up 2 principal quantum shells.

The next 3 elements (P, S, Cl) gain electrons to form anions with 18 electrons, hence will fill up 3 principal quantum shells.

Therefore all anions with 3 electron shells will be larger than all cations with only 2 electron shells.

b. Decrease in radii within cations and anions

Within the cations and anions, the explanation will go back to effective nuclear charge.

Across Period 3, proton number increases, the nucleus is more positively charged hence nuclear charge increases.

All cations (and anions) are isoelectronic with the same number of electrons and electronic configuration, hence shielding effect is the same.

Overall the effective nuclear charge increases, there is a stronger attraction between nucleus and valence electrons, the nucleus can pull the electrons closer to itself, hence ionic radius decreases.

Topic: Periodicity, Inorganic Chemistry, A Level Chemistry, Singapore

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