[antimatter101]

I have read that elements of group 13-14 are affected by the inert-pair effect, which causes them to have a valency 2 less than normal. But why is the effect greater for elements with higher mass than lighter ones? eg. Thallium (I) compounds are stable but Boron (I) compounds cannot exist.

Is it because the heavier atoms' s electrons are more easily ionised into p orbitals, or is it the bond strength?

Thanks for replying.

[Corribus]

Usually it's described by a relativistic effect - the s orbitals contract for heavy atoms, making s-electrons of heavier atoms more stable.  Heavier atom = greater the effect.  Similar effects are usually attributed to gold's color and mercury's liquid state at room temperature..

[antimatter101]

You haven't answered my question yet.