Coaching Centre
Covalent molecules
How non-metal atoms share electrons in covalent bonds, single, double and triple bonds, dot-and-cross and structural diagrams, properties of covalent molecular substances, covalent networks such as diamond and graphite, and naming molecular compounds.
Two chlorine atoms both need one more electron. Neither is willing to give one up, because both pull on electrons strongly. So they compromise: each puts one electron into a shared pair that sits between them, and both atoms count that pair as their own. That shared pair is a covalent bond.
Sharing electrons
Ionic bonds form when a metal hands electrons to a non-metal. When two non-metals react, both atoms want to gain electrons, so neither gives any away. Instead they share electrons. A covalent bond is a pair of electrons shared between two atoms, held between both nuclei. Atoms joined by covalent bonds form molecules: separate, neutral particles with a fixed number of atoms.
The simplest example is hydrogen. Each hydrogen atom has one electron, and its first shell is full with two. Two hydrogen atoms share their electrons, and now each has two in its shell. That is why hydrogen gas is made of H₂ molecules, not single atoms. Seven elements exist as two-atom (diatomic) molecules: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine.
Drawing molecules
A dot-and-cross diagram shows the outer electrons of each atom, using dots for one atom's electrons and crosses for the other's, so you can see where every shared electron came from. Shared pairs are drawn where the shells overlap.
A structural formula is quicker: each shared pair is drawn as a line between the symbols. Water is H–O–H, and methane is a carbon with four lines to four hydrogens. The molecular formula just counts the atoms: H₂O, CH₄.
The number of bonds an atom usually forms equals the number of electrons it needs to fill its outer shell:
| Atom | H | Cl, F, Br, I | O, S | N | C |
|---|---|---|---|---|---|
| Outer electrons | 1 | 7 | 6 | 5 | 4 |
| Bonds formed | 1 | 1 | 2 | 3 | 4 |
Electron pairs that belong to only one atom and are not shared are called lone pairs. Oxygen in water has two lone pairs, and they push the two hydrogens down so water is a bent molecule, not a straight line. That bent shape is behind many of water's unusual properties.
Double and triple bonds
Sometimes one shared pair isn't enough. In an oxygen molecule, each O atom has 6 outer electrons and needs 2 more, so the atoms share two pairs: a double bond, O=O. In carbon dioxide, carbon forms a double bond with each oxygen: O=C=O. In nitrogen, each atom needs 3, so they share three pairs: a triple bond, N≡N. The triple bond is extremely strong, which is why the nitrogen that makes up 78% of the air hardly reacts with anything.
- Build water. How many bonds does oxygen need, and how many lone pairs are left?
- Build ammonia and methane. Predict the number of bonds before you start.
- Try oxygen with only a single bond. Which atoms are not full?
- Build nitrogen and carbon dioxide. Which one has a triple bond?
Properties of molecular substances
There are two very different kinds of force in a molecular substance. The covalent bonds inside each molecule are strong. The forces between molecules are weak. Melting or boiling only has to separate the molecules from each other, not break the covalent bonds, so:
- Low melting and boiling points. Many molecular substances are gases (oxygen, carbon dioxide, methane) or liquids (water, ethanol) at room temperature. Solids such as sugar, wax and sulfur melt easily.
- Don't conduct electricity in any state, because the molecules have no overall charge and there are no free electrons.
- Soft when solid, because the molecules are only weakly held together.
When water boils, the bubbles are water molecules (H₂O) that have escaped from each other, not hydrogen and oxygen gas. The molecules themselves are unchanged.
Covalent networks
Some non-metals form covalent bonds that go on and on in every direction, making a single giant structure instead of small molecules. These covalent network substances behave completely differently:
- Diamond: every carbon atom is bonded to four others in a rigid 3D network. It is the hardest natural substance and doesn't melt until about 3500 °C (under pressure), because melting means breaking covalent bonds.
- Graphite: carbon atoms bond to three others in flat sheets of hexagons. The sheets slide over each other, which is why pencil "lead" (graphite mixed with clay) leaves a mark, and the spare electron on each atom lets graphite conduct electricity. A single sheet, graphene, was first isolated in 2004 using sticky tape, and is one of the strongest materials ever tested.
- Silicon dioxide (silica): the network in quartz and sand.
Naming molecular compounds
Because two non-metals can combine in more than one ratio, molecular compounds use prefixes to count atoms: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5). The second element ends in -ide. So CO is carbon monoxide, CO₂ is carbon dioxide, SO₃ is sulfur trioxide and N₂O is dinitrogen monoxide (laughing gas). "Mono" is left off the first element. A few compounds keep common names: water (H₂O), ammonia (NH₃) and methane (CH₄).
Test yourself
A covalent bond is
Each atom puts one electron into the shared pair.
Which pair of elements would form a covalent compound?
Carbon and chlorine are both non-metals.
How many covalent bonds does a nitrogen atom normally form?
Five outer electrons; it needs three more.
Methane is a gas at room temperature because
The bonds inside the molecule are strong; the attractions between molecules are weak.
The correct name for PCl₃ is
Three chlorine atoms: tri-, and the second element ends in -ide.
Name each molecule.
Sort the molecules by their bonds.
A covalent bond forms when two atoms share a pair of electrons. Pairs of electrons that are not shared are called pairs. Oxygen gas contains a bond. Molecular substances usually have boiling points. Diamond and graphite are covalent substances.
Diamond and methane both contain only covalent bonds, yet diamond melts at about 3500 °C and methane at −182 °C. Explain this difference.
Methane is made of small, separate CH₄ molecules. The covalent bonds inside each molecule are strong, but the forces between neighbouring molecules are very weak. Melting methane only has to separate the molecules from each other, which takes very little energy, so it melts at −182 °C. Diamond is a covalent network: every carbon atom is joined to four others by covalent bonds that extend through the whole crystal. There are no separate molecules, so to melt diamond you have to break huge numbers of strong covalent bonds, which takes an enormous amount of energy. That is why its melting point is so high.