Originally posted by Clyde
Heavier elements have larger more diffuse orbitals hence form weaker bonds.
In order to form 5 bonds (formal oxidation state 5+), electrons in the s orbital need to be promoted to sp hybridised orbitals this takes energy, in lighter elements this is offset by the strongly stabilising effect of forming 2 extra bonds. However for heavier elements like Bismuth the energy required to promote the electrons is greater the the energy released via formation of the extra bonds, so it only forms 3 bonds (formal oxidation state 3+). This is known as the "inert pair effect".