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12. Combining H with B–Ne 13. Combining H with Li 14. Combining H with Be
13. Combining H with Li

The interaction of H and Li resembles the H-H interaction substantially, at least initially. Lithium has one valence electron in the 2s1 orbital. As in H-H, we see a bound curve if the spins of the electrons in the two singly occupied orbitals are different and a repulsive curve if the spins are the same, as shown below.

13.1

The figure also shows the coupling diagram for LiH with bond pair now colored gray as usual. The unoccupied 2p orbitals are included, although they could be omitted.

The big difference between H2 and LiH is evident in the orbitals, as shown in the animations below:

13.2

LiH Formation

σLi 2s1

σH 1s1

The change in the H 1s1 orbital is very slight, while the change in the Li 2s1 orbital is enormous. It delocalizes almost completely onto H by the time the separation has decreased to Rmin. The net effect is to leave Li without most of its third electron, which means that it is very close to becoming the Li+ cation. At the same time, the H has acquired a lot of anionic character, becoming like H–. Since the Li electron is mostly transferred to H, this type of bonding is called ionic bonding.

Ionic bonding can be viewed as an extreme case of polar covalent bonding. In polar covalent bonds, electrons shift toward the atom with the strongest attraction for them. The measure of this is a property called electronegativity.

We'll now expand the bonding type table to include ionic bonding:

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Nonpolar Covalent: one electron from each atom, overlapping as allowed by the Pauli principle, with the electrons shared equally (e.g., H2 for electrons with opposite spins)

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Polar Covalent: one electron from each atom, overlapping as allowed by the Pauli principle, with the electrons shared unequally and polarized toward one of the atoms (e.g., HF for electrons with opposite spins)

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Ionic: one electron from each atom, overlapping as allowed by the Pauli principle, with the pair mostly residing on an electron hungry atom (e.g., LiH)

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Dative: a shared electron pair provided by just one of the atoms (e.g., HeH+)

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One-electron: a single, shared electron (e.g., H2+)

Now that we've investigated the H + Li interaction, that leaves us just one second row element to explore: Be.

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