Updated: Jul 23
Given that atoms and ions are subject to different environments and forces, it is difficult to assign atomic/ionic radii - the electron density cloud’s shape and size is constantly modified (1). Atoms and ions are also rarely considered in isolation.
However, given these limitations, there is a definite trend across the periods and down the groups of the periodic table. The atomic radii generally decrease along each period and increase down each group.
The atomic radii of the elements of Period 3 follows this trend...
Across Period 3 from left to right:
· proton number increases - the nucleus becomes more positively charged.
· greater number of electrons, but they populate the same principal quantum shell - amount of shielding doesn’t increase.
The effective nuclear charge, Zeff, is the amount of positive charge experienced by a particular electron. Although it can be calculated mathematically, it can be useful in a more simplified form.
Zeff = Z - S
… where Z is the number of protons in the nucleus and S is the number of shielding electrons (more correctly, the shielding constant).
The sodium atom has 11 electrons and 11 protons. The number of non-valence electrons is 10 (the valence electron occupies the n = 3 shell). In this case…
Zeff (Na) = 11 – 10 = 1+
For magnesium… Zeff (Mg) = 12 – 10 = 2+ (a greater effective nuclear charge)
Effective nuclear charge increases across the period - there’s a greater attraction between the nucleus and the outer, valence electrons leading to a decrease in the atomic radius.
· anions are larger than cations
· general decrease in radii for cations and anions across the period
Anions are larger than cations
The metallic elements (Na, Mg, Al, Si) lose electrons to form cations with 10 (2 + 8) electrons filling up 2 principal quantum shells.
The non-metallic elements (P, S, Cl) gain electrons to form anions with a total of 18 (2 + 8 + 8) electrons. Here, 3 principal quantum shells are filled.
Clearly, period 3 anions will be larger than the cations due to the additional shell.
Decrease in ionic radii
Anions are bigger than cations but, in each group of ions, there is a decrease in ionic radius moving across the period.
As mentioned previously, effective nuclear charge increases going from left to right along period 3. In both the cations and anions, the number of electrons remains unchanged (the ions are iso-electronic) and so electron shielding does not change.
The increasing nuclear charge leads to a greater attraction on the outer electrons and ionic radius decreases - the electrons are pulled a little closer to the nucleus.