A covalent bond is a shared pair of electrons

1. Two non-metal atoms can each contribute one electron to a pair that sits between them.
2. Both nuclei attract that shared pair, and that shared attraction is what holds the atoms together.
3. Neither atom loses an electron outright, so no ions are formed.
4. Each atom counts the whole shared pair towards its own outer shell.
5. One shared pair makes one covalent bond, and two shared pairs between the same atoms make a double bond.
6. Atoms joined this way form particles with a fixed number of atoms, unlike the open-ended ionic lattice.

Atoms and small molecules are about $10^{-10}m$ across

1. A typical atom measures roughly $10^{-10}m$ across.
2. A small molecule such as $H_{2}O$ is only a few times wider, so it shares the same order of magnitude.
3. An order of magnitude is the nearest power of ten, which is all the precision these sizes are quoted to.
4. The unit is the metre, so $10^{-10}cm$ is a hundred times too small.
5. That scale is why single molecules cannot be seen through a light microscope.

Dot-and-cross diagrams show shared pairs and lone pairs

1. A dot-and-cross diagram draws only the outer-shell electrons of each atom.
2. Dots mark the electrons of one atom and crosses those of the other, so each shared pair shows one of each.
3. A shared pair is drawn in the overlap between the two atoms.
4. Outer electrons that are not shared stay on their own atom, drawn as pairs.
5. Those unshared pairs form no bond, so they are not counted when the bonds are counted.
6. Two shared pairs drawn between the same two atoms represent a double covalent bond.

Single bonds: $H_2$, $HCl$, $H_{2}O$ and ${CH}_4$

1. $H_2$: each hydrogen contributes its one electron to a single shared pair, leaving both atoms with two electrons and no lone pairs.
2. $HCl$: hydrogen and chlorine share one pair, and chlorine's six remaining outer electrons stay as three lone pairs.
3. $H_{2}O$: oxygen forms a shared pair with each of two hydrogen atoms and keeps four electrons as two lone pairs.

1. ${CH}_4$: carbon forms a shared pair with each of four hydrogen atoms and keeps no lone pairs.
2. Hydrogen is full at two electrons, while chlorine, oxygen and carbon are full at eight.
3. Counting the shared pairs against the formula is the quickest check that a diagram is complete.

Double bonds: $O_2$ and ${CO}_2$

1. $O_2$: the two oxygen atoms share two pairs with each other, which makes one double bond.
2. Each oxygen atom in $O_2$ keeps two lone pairs, so each reaches eight outer electrons.
3. ${CO}_2$: carbon shares two pairs with each oxygen atom, making two double bonds.
4. Carbon dioxide therefore holds four shared pairs altogether, and carbon keeps no lone pairs.
5. Each oxygen atom in ${CO}_2$ keeps two lone pairs.

1. A double bond is drawn as two separate shared pairs in the overlap, not as one crowded group.

Checking a covalent diagram

1. Start from the outer-shell electron counts: hydrogen $1$, carbon $4$, oxygen $6$ and chlorine $7$.
2. Work out how many electrons each atom has to share to fill its outer shell.
3. Draw the shared pairs first, then place the leftover electrons as lone pairs.
4. Check the totals: two electrons around each hydrogen and eight around each of the others.
5. Check that the number of atom symbols matches the molecular formula.