Question

Why Fe^+2 and chromium don't have same electronic configuration??
Guys please answer it fast...❤​

Answer 1

Cr(0) and Fe(II) have the same number of valence electrons. You might have heard of the concept of isolobality, which, briefly explained, suggests that compounds with similar electronic configurations (isolobal configurations) will react similarly; so you could replace F with OMe or even [Co(CO)4]. The concept has its values and its strong simplifications which I will not go into here.

However, even assuming the isolobality concept’s validity, it would not count Cr(0) and Fe(II) as isolobal to each other. The reasoning is rather simple: One is a neutral metal, the other is a cation with +2 charge. For every electron an atom looses, the atom’s orbitals will contract and the atomic (or ionic) radius will decrease. This also changes the energy levels between orbitals and will affect lower d-orbitals differently than higher s-orbitals.

The result is that the 4s and 3d orbitals are now so dissimilar in energy, that electrons hopping back or forth is now no longer an option

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Answer 2

Answer:

Another way to look at the transition in electronic configuration is to consider species with the same number of electrons but different numbers of protons.

Start with Cr0 with its electron configuration [Ar] 3d54s1. We add a proton to make Mn+, throwing in any necessary neutrons to keep the nucleus stable. We repeat this process to make Fe2+, Co3+, etc., and eventually we get to let us say, Xe30+. In all these species the electromagnetic interactions between the 24 electrons are intrinsically the same, but the excess positive nuclear charge is building up and is bound to overwhelm the electron-electron interactions at some point. By the time we get to the huge excess positive charge in the xenon ion above, the orbitals become roughly what they would be with isolated electrons, like the familiar hydrogen spectrum multiplied by the effect of a greater positive charge. The 3d orbital energy level sinks down to meet its shell-mates at 3s and 3p, leaving the higher-shell 4s orbital at a higher level.

So at some point the outer electrons all fall into the 3d orbitals and the electronic structure shifts from whatever it was in the neutral chromium atom to [Ar] 3d6. In my example I predicted that we might have to go all the way to Xe30+ for this transition, but in reality the 3d and 4s orbitals are so closely balanced in the first row of transition metals that we need not go that far. Adding two protons, and a few neutrons for nuclear balance, to chromium is enough.

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