5.1 The Nature of Covalent Bonds

Earlier I introduced covalent bonds, but didn't really explain what they are. Atoms' electrons are arranged in shells, which we can think of as concentric layers around the nucleus. The outermost shell is called the valence shell, and its electrons are the valence electrons. Most atoms' valence shells can hold 8 electrons, except for hydrogen and helium that are so small their one and only shell can only hold 2 electrons. (That's not quite accurate either, because quantum physics, but it'll do.) The fact of the goal being 8 electrons is called the octet rule.

Atoms generally try to fill their valence shell, except for metals since they have so few valence electrons that they find it easier to say "forget this garbage" and give away their valence electrons to something like oxygen or a halogen whose valence shell is already almost full.

Carbon has four valence electrons, so to get to 8, it likes to share its electrons with other atoms. In a molecule of methane, the four electrons from the carbon and the four electrons from the four hydrogens form four electron pairs. Each pair represents a single bond. The electrons mutually orbit the nuclei of both atoms, satisfying both atoms' valence shells.

When atoms form a double bond, they share two pairs of electrons; a triple bond, three pairs. Quadruple bonds are rare and don't occur between carbon atoms, for reasons we've already seen, but the semiconductor gallium arsenide can form a quadruple bond between its gallium and arsenic atoms.

Electrons like to form pairs in atoms, and any unbonded electrons form lone pairs. Nitrogen has five valence electrons, so if it shares three of these with other atoms to make a full octet, its last two electrons form a lone pair. Oxygen has six valence electrons, so divalent oxygen kepps two lone pairs. Lone pairs are what's on the smooth non-bonding parts of atoms; a full octet consists of four electron pairs arranged in a tetrahedron; this is why carbon, nitrogen, and oxygen are often tetrahedral.

The octet rule also explains how carbon monoxide (CO) can exist. The carbon atom contributes two electrons, leaving the other two as a lone pair. The oxygen atom contributes four electrons, also keeping a lone pair. The result is a triple bond: C≡O. Even though neither element has a valence of 3, the triple bond nevertheless does form two complete octets.

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