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D-block contraction

    '''d-block contraction''' (sometimes called '''scandide contraction''') is a term used in chemistry to describe the effect of having full d orbitals on the period 4 element| period 4 elements The elements in question are the gallium|Ga germanium|Ge arsenic|As selenium|Se and bromine|bromineBr Their electronic configurations include completely filled d orbitals (d10) d-block contraction is best illustrated by comparing some properties of the group (periodic table)|group 13 elements to highlight the effect on gallium

    Element Atomic electron config Sum 1st - 3d IP kJ/mol M3+ electron config M3+ radius (pm)
    Boron B {He}2s22p1 68874 {He}
    Aluminium Al {Ne}3s23p1 5139 {Ne} 535
    Gallium Ga {Ar}3d104s24p1 55211 {Ar}3d10 62
    Indium In {Kr}4d105s25p1 5083 {Kr} 4d10 80
    Thallium Tl {Xe}5d106s26p1 54384 {Xe}5d10 885

    Gallium can be seen to be anomalous The most obvious effect is that the sum of the first three ionization potentials of gallium is higher than that of aluminium whereas the trend in the group would be for it to be lower The second table below shows the trend in the sum of the first three ionization potentials for the elements B Al Sc Y La Sc Y La are group 3 elements and have three valence electrons above a noble gas electron core In contrast to the group 13 elements this sequence shows a smooth reduction

    Element Atomic electron config Sum 1st - 3d IP kJ/mol M3+ electron config M3+ radius (pm)
    Boron B {He}2s22p1 68874 {He}
    Aluminium Al {Ne}3s23p1 5139 {Ne} 535
    Scandium Sc {Ar}3d14s2 42567 {Ar} 745
    Yttrium Y {Kr}4d15s2 3760 {Kr} 90
    Lanthanum La {Xe}5d16s2 34554 {Xe} 1032

    Other effects of the d-block contraction are that the Ga3+ ion is smaller than expected being closer in size to Al3+ Care must be taken in interpreting the ionisation potentials for indium and thallium other effects eg the inert pair effect become increasingly important for the heavier members of the groupionization potentials The d-block contraction can be compared to the lanthanide contraction which is caused by inadequate shielding of nuclear charge by electrons occupying f orbitals

    References