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Lesson: Chapter 10 — Chemical Bonds

10.2. Covalent bonds 2 of 8

Two atoms of the same kind

The simplest covalent molecules are made of two identical atoms, where neither can possibly take electrons from the other.

The fluorine molecule

A fluorine atom is 2, 7. It needs one more electron. Two fluorine atoms each put one electron into a shared pair, and each then counts eight in its outer shell.

The fluorine molecule: two fluorine atoms sharing one pair of electrons, each keeping three further pairs of its own.

Figure to be added

Two fluorine atoms drawn as overlapping shells, showing the shared pair in the region where the shells meet (figure 10.8).

Counting for the left-hand atom: six electrons in its own three pairs, plus the two in the shared pair, makes eight. The same count works from the right. The single shared pair is a single bond.

The hydrogen molecule

Hydrogen is the exception to the count of eight. A hydrogen atom has just one electron and one shell, and that shell is full at two — the helium arrangement. Two hydrogen atoms share their single electrons as a pair, and both are satisfied.

The hydrogen molecule: two hydrogen atoms sharing one pair of electrons and no others.

Figure to be added

Two hydrogen atoms drawn as overlapping shells with the shared pair between the two nuclei (figure 10.9).

Hydrogen never reaches eight

Do not look for an octet around a hydrogen atom. Its shell holds a maximum of two electrons, so a hydrogen atom in any molecule has exactly one bond and no lone pairs.