Question:
Why do hydrogen compounds from the third period down (that are electron deficient or have completed octets) readily decompose?
Thoughts:
Apparently, heavier group 14 elements form hydrides which decompose readily because they have valence orbitals with which $\ce{O_2}$ and $\ce{H_2O}$ can coordinate. But I've tried and tried and cannot think of why this makes $\ce{CH_4}$ more stable than $\ce{SiH_4}$. What is the difference between the two (except for the fact that silane contains a larger central atom)?
As for the remaining groups on the periodic table, I cannot fathom how this trend holds.