In the Lewis electron structures, the variety of electron pairs holding two atoms together was called the bond order. Within the molecular orbital method, bond order is characterized as one-half the net variety of bonding electrons:
< extbond order=dfrac extnumber of bonding electrons - extvariety of antibonding electrons2 label9.8.1 >
To calculate the bond order of (H_2), we understand that the (σ_1s) (bonding) molecular orbital includes two electrons, while the ( sigma _1s^star ) (antibonding) molecular orbital is empty. The bond order of (H_2) is therefore
< extbond order=dfrac2-02=1 label9.8.2 >
This result corresponds to the single covalent bond; double and triple bonds contain four or six electrons, respectively, and correspond to bond orders of 2 and also 3.
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We can usage energy-level diagrams to define the bonding in various other pairs of atoms and ions where n = 1, such as the H2+ ion, the He2+ ion, and also the He2 molecule. Again, we fill the lowest-power molecular orbitals initially while being certain not to violate the Pauli principle or Hund"s Rules.
B The He22+ ion has only two valence electrons (two from each He atom minus two for the +2 charge). We deserve to additionally view He22+ as being developed from two He+ ions, each of which has a single valence electron in the 1s atomic orbital. We can currently fill the molecular orbital diagram:
Exercise 9.8.1: The (ceH_2^2−) Ion
However before, Van der Waals Helium Dimers do exist
Based on molecular orbital concept discussed over, the (ceHe_2) molecule must not exist considering that no covalent bond developed between the helium atoms (Equation
efheliumdimer). However, the molecular orbital summary over neglects the van der Waals force that exists between the atoms as demonstrated by the presence of liquid helium (at 4 K). So a "molecule" written of two helium atoms bound by the van der Waals pressure might exist by this attractive force instead - and also it does.
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A helium dimer molecule bound by Van der Waals pressures was initially proposed by John Clarke Slater in 1928 and oboffered in 1993 by Genattempt and also coworkers. Interestingly, (ceHe_2) is the largest recognized molecule of two atoms as soon as in its ground state with an extremely lengthy bond length with a separation of about 5,200 pm. The binding energy is only (4.6 imes 10^−5, kJ/mol), so the (ceHe-He) bond is 5,000 times weaker than the covalent bond in the hydrogen molecule (Table 9.8.1).