The oxidation states of p-block main group elements like groups 13, 14, 15 and 16 become more stable at lower oxidation states further down the column. For group 14 elements C, Si, and Ge, the +4 oxidation state is most stable while the +2 state requires kinetic stabilization. For Sn, both +2 and +4 states are important but +4 is more common, and for Pb, the +2 state is most stable due to the inert pair effect caused by relativistic contraction of the s orbital. For transition metals, higher oxidation states also become less favorable down the column as ionization energies increase.

1. The compounds with low oxidation states (O.S.) become progressively more stable
down the column for p block main group elements such as group 13, 14, 15 and 16 elements. For
example, for group 14 elements C, Si, and Ge, (valence electron configuration: ns2np2) +4
oxidation state is the most stable oxidation state. Low oxidation state such as +2 for these three
elements can only be achieved using kinetic stabilization strategy (i.e., using bulky ligand to
protect the element with a low oxidation state. disilenes are a good example. the first one was not
synthesized until to 1981 by Robert West). For Sn, both +2 and +4 oxidation states are
important, however, +4 O.S. is still more predominant than +2 O.S. For Pb, +2 O.S. become the
most stable O.S. This O.S. change trend has been interpreted as "inert pair effect". This inert
pair effect is due to a relativistic effect which contracts the valence s orbital thereby lowering its
energy and decreasing the reactivity of the electron pair in valence s orbital. for transition metals,
down a column, high O.S. is also become less popular. for example, it is more difficult to oxidize
Mo than Cr. This may be explained as follows: while atomic size increases slightly down a group
of transition elements, the nuclear charge increase much more. This results in the first ionization
energy generally increases. Mo's ionization energy is higher than that of Cr. So it is more
difficult to remove electrons.
Solution
The compounds with low oxidation states (O.S.) become progressively more stable
down the column for p block main group elements such as group 13, 14, 15 and 16 elements. For
example, for group 14 elements C, Si, and Ge, (valence electron configuration: ns2np2) +4
oxidation state is the most stable oxidation state. Low oxidation state such as +2 for these three
elements can only be achieved using kinetic stabilization strategy (i.e., using bulky ligand to
protect the element with a low oxidation state. disilenes are a good example. the first one was not
synthesized until to 1981 by Robert West). For Sn, both +2 and +4 oxidation states are
important, however, +4 O.S. is still more predominant than +2 O.S. For Pb, +2 O.S. become the
most stable O.S. This O.S. change trend has been interpreted as "inert pair effect". This inert
pair effect is due to a relativistic effect which contracts the valence s orbital thereby lowering its
energy and decreasing the reactivity of the electron pair in valence s orbital. for transition metals,
down a column, high O.S. is also become less popular. for example, it is more difficult to oxidize
Mo than Cr. This may be explained as follows: while atomic size increases slightly down a group
of transition elements, the nuclear charge increase much more. This results in the first ionization
energy generally increases. Mo's ionization energy is higher than that of Cr. So it is more
difficult to remove electrons.