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Chromium and copper have electron configurations [Ar] 3d 5 4s 1 and [Ar] 3d 10 4s 1 respectively, i.e. one electron has passed from the 4s-orbital to a 3d-orbital to generate a half-filled or filled subshell. In this case, the usual explanation is that "half-filled or completely filled subshells are particularly stable arrangements of electrons".
Each orbital in an atom is characterized by a set of values of three quantum numbers n, ℓ, and m ℓ, which respectively correspond to electron's energy, its orbital angular momentum, and its orbital angular momentum projected along a chosen axis (magnetic quantum number). The orbitals with a well-defined magnetic quantum number are generally ...
However there are numerous exceptions; for example the lightest exception is chromium, which would be predicted to have the configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d 4 4s 2, written as [Ar] 3d 4 4s 2, but whose actual configuration given in the table below is [Ar] 3d 5 4s 1.
Molecular orbital diagram of dinitrogen. With nitrogen, we see the two molecular orbitals mixing and the energy repulsion. This is the reasoning for the rearrangement from a more familiar diagram. The σ from the 2p is more non-bonding due to mixing, and same with the 2s σ. This also causes a large jump in energy in the 2p σ* orbital.
The energy level of the bonding orbitals is lower, and the energy level of the antibonding orbitals is higher. For the bond in the molecule to be stable, the covalent bonding electrons occupy the lower energy bonding orbital, which may be signified by such symbols as σ or π depending on the situation.
Assignments can be made to these signals indicated by the transition of electrons moving from one orbital at a lower energy to a higher energy orbital. The molecular orbital diagram for the final state describes the electronic nature of the molecule in an excited state.
In this case, it is easier to put electrons into the higher energy set of orbitals than it is to put two into the same low-energy orbital, because two electrons in the same orbital repel each other. So, one electron is put into each of the five d -orbitals in accord with Hund's rule, and "high spin" complexes are formed before any pairing occurs.
Cotton and Harris formulated a molecular orbital rationale for the bonding that explicitly indicated a quadruple bond. [3] The rhenium–rhenium bond length in this compound is only 224 pm . In molecular orbital theory , the bonding is described as σ 2 π 4 δ 2 with one sigma bond , two pi bonds and one delta bond .