Bond Order

In the Lewis electron structures, the number of electron bag holding two atoms together was called the bond order. Within the molecular orbital approach, bond order is identified as one-half the net number of bonding electrons:

\< \textbond order=\dfrac\textnumber that bonding electrons - \textnumber that antibonding electrons2 \label9.8.1 \>

To calculate the bond order of \(H_2\), we recognize that the \(σ_1s\) (bonding) molecular orbital contains two electrons, when the \( \sigma _1s^\star \) (antibonding) molecular orbit is empty. The bond order of \(H_2\) is therefore

\< \textbond order=\dfrac2-02=1 \label9.8.2 \>

This an outcome corresponds come the single covalent bond; twin and triple binding contain 4 or 6 electrons, respectively, and correspond come bond assignment of 2 and also 3.

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We deserve to use energy-level diagrams to explain the bonding in other pairs that atoms and also ions where n = 1, such as the H2+ ion, the He2+ ion, and the He2 molecule. Again, us fill the lowest-energy molecule orbitals an initial while being certain not to hurt the Pauli rule or Hund"s Rules.


B The He22+ ion has only two valence electron (two from every He atom minus two for the +2 charge). Us can additionally view He22+ together being created from 2 He+ ions, every of which has actually a single valence electron in the 1s atom orbital. We have the right to now to fill the molecular orbital diagram:


Exercise 9.8.1: The \(\ceH_2^2−\) Ion

However, valve der Waals Helium Dimers execute exist

Based top top molecular orbital theory debated above, the \(\ceHe_2\) molecule need to not exist since no covalent bond formed in between the helium atoms (Equation \refheliumdimer). However, the molecular orbit description over neglects the valve der Waals pressure that exists between the atoms as demonstrated by the existence of liquid helium (at 4 K). For this reason a "molecule" written of two helium atom bound through the van der Waals pressure may exist through this attractive pressure instead - and also it does.

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A helium dimer molecule bound by van der Waals pressures was first proposed by man Clarke Slater in 1928 and also observed in 1993 by Gentry and also coworkers. Interestingly, \(\ceHe_2\) is the largest recognized molecule of 2 atoms once in its floor state with an extremely long bond size with a separation of about 5,200 pm. The binding energy is only \(4.6 \times 10^−5\, kJ/mol\), therefore the \(\ceHe-He\) shortcut is 5,000 times weaker 보다 the covalent link in the hydrogen molecule (Table 9.8.1).