Usually we come across anomalies in the trends of periodic properties for example the size [Atomic radius value from Raymond Chang Chemistry 9th Edition] of Gallium (135 pm) is smaller than Aluminium (143 pm) as opposed to the basic trend that atomic size increases down the group. This is usually attributed to the fact that the poor shielding of nuclear charge by intervening D orbitals leads to a greater $Z_\mathrm{eff}$ hence the radius is less as opposed to the trend.

If that's the reason why don't we observe similar effects for other groups like group 15 having Phosphorus (110 pm) and Arsenic (120 pm) [Here the size doesn't increase much but is still significant as compared to changes taking place in group 13] , where the size actually increases even when there are D orbitals to intervene. Or in short why are the effects so profound for group 13 and not the other members?

There's another example of In (166 pm) and Tl (171 pm). Both belong to group 13. As compared to other groups these two members have the lowest increase in atomic radius.


1 Answer 1


As a property showing d-block contraction, the question cited atomic radius. However, d-block contraction can manifest itself in other properties as well:

  • $\ce{GeO}$ is described as much more stable than $\ce{SiO}$. $\ce{Sn(II)}$ compounds are described as reducing. [Riedel]
  • The pentachlorides of $\ce{P, As, Sb}$ show a kink in their stability: $\ce{PCl5}$ melts at 167°C, $\ce{AsCl5}$ decomposes at $-50\mathrm{°C}$, $\ce{SbCl5}$ melts at 4°C (and decomposes at 140°C) [HoWi]
  • The redox potentials in acidic solution for the (0)/(IV) oxyanions and oxides of chalkogens show a peak at $\ce{Se}$ [HoWi]:

\begin{align} \ce{SO2} + \ce{4e-} + \ce{4H+} &\leftrightharpoons \ce{S} + \ce{2H2O} \quad &E^0 = +0.50 \ \mathrm{V} \\ \ce{SeO_3^{2-}} + \ce{4e-} + \ce{6H+} &\leftrightharpoons \ce{Se} + \ce{3H2O} \quad &E^0 = +0.74 \ \mathrm{V}\\ \ce{TeO2} + \ce{4e-} + \ce{4H+} &\leftrightharpoons \ce{S} + \ce{2H2O} \quad &E^0 = +0.57 \ \mathrm{V} \end{align}

  • the oxidative strength (at both low and high pH) of halogen oxyacids follows the pattern $\ce{BrO4-} > \ce{H_5IO6} > \ce{ClO4-}$. [HoWi]

I think that stability of oxidation states, stability of comparable compounds, and redox potentials of similar compounds are valid examples of d-block contraction. Of course, other chemists may arrive at different conclusions.

[Riedel] E Riedel, Anorganische Chemie, Vol. 6, de Gruyter (2004).
[HoWi] AF Holleman, E Wiberg, Lehrbuch der Anorganischen Chemie, Vol. 101, de Gruyter (1995).


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