1gCovalent bonding

When two non-metals meet, neither will give up electrons — both need to gain. So they share instead.

A covalent bond is a shared pair of electrons.

Say pair. "Shared electrons" on its own is often not credited, because the pair is the unit.

What holds the atoms together

The shared pair is negative. Each nucleus is positive. The pair sits between the two nuclei and is attracted to both at once, and that attraction is the bond.

Notice it is not an attraction between the two atoms. Both nuclei are positive and would repel each other; it is the shared electrons in the middle that hold everything together.

How many bonds an atom forms

Count how many electrons it needs to fill its outer shell.

Bonds formed equals electrons needed, not electrons already there. Nitrogen has five outer electrons but forms three bonds.

Hydrogen is full with two electrons, not eight. The rule of eight does not apply to it.

Double and triple bonds

If one shared pair is not enough, atoms share more.

• O₂ — each oxygen needs 2, so they share two pairs: a double bond.
• N₂ — each nitrogen needs 3, so they share three pairs: a triple bond.
• CO₂ — carbon forms a double bond to each oxygen, so four shared pairs in total, two to each.

Work out what each atom needs first; the number of shared pairs follows.

Carbon dioxide is a gas at room temperature. The covalent bonds inside each molecule are very strong. Both statements are true, and reconciling them is the whole of this note.

Two different kinds of attraction

Melting and boiling separate molecules from each other. They do not break the bonds inside a molecule.

So a simple molecular substance has a low melting point because only the weak intermolecular forces need to be overcome. The strong covalent bonds survive untouched — which is why chlorine gas is still made of Cl₂ molecules after it has boiled.

Never say covalent bonds are weak

This is the single most penalised sentence in the topic.

"Carbon dioxide has a low boiling point because its covalent bonds are weak."

They are not weak. Write intermolecular forces whenever you are explaining a melting or boiling point of a molecular substance.

Why bigger molecules boil higher

Within a homologous series, boiling point rises as relative molecular mass rises:

• Methane (Mr 16) boils at −162 °C
• Propane (Mr 44) boils at −42 °C
• Hexane (Mr 86) boils at 69 °C

Larger molecules have stronger intermolecular forces, so more energy is needed to separate them.

One caution: this comparison holds within a series. Across different families, polarity matters more than mass — ethanol boils far above propane despite a similar Mr.

Why they do not conduct

Covalent substances contain no ions and no delocalised electrons, so there are no charged particles free to move and carry a current.

"It has no electrons" is wrong — it has plenty. The point is that none of them are free.

Graphite is the exception, and it is worth knowing why: it is covalent, but its structure leaves one electron per atom delocalised.

Diamond and graphite are both pure carbon. They behave completely differently, and every difference comes from one number: how many other carbon atoms each atom bonds to.

Diamond: four bonds each

Every carbon atom bonds to four others, forming a rigid three-dimensional network.

• Very hard, because there are no layers to slide and every atom is locked in place.
• Very high melting point, because melting means breaking many strong covalent bonds.
• Does not conduct, because all four outer electrons are used in bonding. None are free.

Graphite: three bonds each

Every carbon atom bonds to three others, forming flat layers of hexagonal rings.

• Conducts electricity, because the fourth outer electron is not used in bonding. One delocalised electron per atom is free to move.
• Soft, and useful as a lubricant, because the forces between the layers are weak, so layers slide over one another.
• Still a high melting point, because within each layer the covalent bonds are as strong as diamond's.

Graphite being soft does not mean its bonds are weak. The bonds are strong; the forces between the layers are not.

C₆₀ fullerene

Each molecule holds sixty carbon atoms — large, but still a separate molecule. C₆₀ is therefore simple molecular, not giant covalent.

So it melts at a low temperature, because melting only overcomes the weak forces between whole C₆₀ molecules.

A large molecule is not the same as a giant structure. What makes a structure giant is being covalently bonded throughout, with no separate molecules at all.

Why giant covalent melting points are so high

Unlike a molecular substance, there are no molecules to pull apart. Melting requires breaking covalent bonds themselves, and there are vast numbers of them.

This is the one case where melting does break covalent bonds — the opposite of the simple molecular case, and worth keeping straight.