An effect seen especially in groups 13 and 14 of
periodic table, in which the heavier elements in the
group tend to form compounds with a valency two lower
than the expected group valency.
Used to account
for the existence of thallium (I) compounds in group
13 and lead (II) in group 14. In forming compounds,
elements in these groups promote an electron from
a filled s-level state to an empty p-level.
the heavier elements, the bond strengths or lattice
energies in the compounds are lower than those of
the lighter elements.
Consequently the energy compensation
is less important and the lower valence states become