Variable valency explanation and cause

[adi1998]

many transition elements have variable oxidation state like Fe.what is the exact cause for this?they may not obey the octet rule,but what is the reason behind this?Also,elements like nitrogen are not transition elements and still show variable valency.what will be the electronic configuration of such elements?is it always according to aufbau principle?then which electrons will take part in the reaction,like in Fe---1s¹,2s²,2p⁶,3s²,3p⁶,4s²,3d⁷

 

[Borek]

Octet rule is obeyed mostly by 2nd row elements, and even then not always, so don't treat it as a starting point. The real force behind is the energy - the lower the energy, the more stable the configuration. Sometimes energy is particularly low for some configurations and these are the only ones we observe, but more often there are several configurations, each with their own energy minimum (not much different from others), and all are observed.

 

[chill_factor]

octet rule is a very crude approximation for main group elements. for a quantitative reason why electron shells are what they are, you need computational chemistry.