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Lesson: Chapter 10 — Chemical Bonds

10.2. Covalent bonds 6 of 8

Dot and cross diagrams

Only the valence electrons ever take part in covalent bonding, so the diagrams drop everything else and show the outer shell alone.

The difficulty is that once two atoms share a pair, the electrons are identical and there is no way to see which atom brought which. The dot-and-cross convention solves that by marking them differently.

The convention

The valence electrons of one atom are drawn as dots and those of the other as crosses. A shared pair then shows as one dot beside one cross.

Methane, worked through

Carbon is 2, 4, so it has four valence electrons; these are drawn as dots. Each of the four hydrogen atoms brings one electron, drawn as a cross.

Methane as a dot-and-cross diagram: four shared pairs around the carbon atom, each made of one carbon electron and one hydrogen electron.

Read it and the arithmetic is visible. Around the carbon there are four pairs, eight electrons, a complete octet. Around each hydrogen there is one pair, two electrons, a full first shell.

Dots and crosses are the same thing

The two symbols are a bookkeeping device for the reader, not a physical difference. Once a bond has formed there is no experiment that could tell you which electron came from which atom.

Activity 02

Model a covalent bond

Take a sheet of styrofoam. Draw on it one of the molecules studied in this section, represent the electrons with beads of different colours — one colour per atom, exactly as dots and crosses are used here — and build up the shared pairs so the bonding is visible. Display the result in class.