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Covalent bonding

Simple molecules and dot and cross diagrams. Topic 2 of 8 in Bonding, structure and properties of matter.

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Most of the substances around you are held together by covalent bonds: the water you drink, the oxygen you breathe in and the carbon dioxide you breathe out. This topic explains what a covalent bond is, how to draw one, and why substances made of small molecules melt and boil so easily.

1. What a covalent bond is

Why atoms share: atoms are most stable with a full outer shell, like the noble gases. Non-metal atoms are short of electrons. When two non-metal atoms meet, neither can pull electrons right off the other, so they share instead.

Covalent bond: a shared pair of electrons between two atoms.

What holds the atoms together: both atoms’ nuclei are positive, and the shared pair of electrons is negative. Each nucleus is attracted to the shared pair. This electrostatic attraction holds the two atoms together, and it is strong.

Molecule: a group of atoms joined by covalent bonds. A molecule has a fixed number of atoms, e.g. water is always H2O. Molecules have no overall charge.

Slide to bring two hydrogen atoms together

Hydrogen atom1 electronHydrogen atom1 electron H H Each nucleus (+) is attractedto the shared pair (−) Shared pair of electrons:the covalent bond
Each hydrogen atom has one electron. Once the shells overlap, both electrons are shared, so each atom has 2 in its shell, the same as the noble gas helium.

Where you find covalent bonds:

  • most non-metal elements: hydrogen (H2), oxygen (O2), nitrogen (N2), chlorine (Cl2)
  • compounds made only of non-metals: water (H2O), carbon dioxide (CO2), methane (CH4), ammonia (NH3), hydrogen chloride (HCl)

Spotting it from a formula: if every element in the formula is a non-metal, the bonding is covalent. A metal with a non-metal (e.g. NaCl) is ionic.

How small: atoms are about 10−10 m (0.1 nanometres) across. A small molecule is only a few atoms across.

Some covalent substances are very large molecules (polymers) or giant structures (diamond, silicon dioxide). Those have their own topics. This one is about small molecules.

2. How many bonds does an atom make?

An atom keeps sharing until its outer shell is full: 8 electrons, or 2 for hydrogen. So the number of bonds an atom makes is the number of electrons it is short of.

AtomGroupOuter electrons NeedsCovalent bonds
Hydrogen (H)1111
Carbon (C)4444
Nitrogen (N)5533
Oxygen (O)6622
Chlorine (Cl)7711

Lone pairs: pairs of outer-shell electrons that are not shared in a bond. They still count towards the 8. In ammonia, nitrogen has one lone pair; in water, oxygen has two; in hydrogen chloride, chlorine has three.

Outer shells, ready to share

H

Hydrogen

1 bond

C

Carbon

4 bonds

N

Nitrogen

3 bonds

O

Oxygen

2 bonds

Cl

Chlorine

1 bond

An unpaired electron: one the atom can share. Each one makes one bond. A lone pair: two electrons already paired up. The atom keeps them; they are not shared in a bond.

3. Drawing dot and cross diagrams

A dot and cross diagram shows one atom’s electrons as dots and the other atom’s as crosses. The electrons are all identical. The dots and crosses only show which atom each electron came from.

  • Usually you draw outer shells only. A question will say if it wants the inner shells too.
  • Two layouts are accepted: overlapping circles with the shared pair inside the overlap, or just the symbols with the electrons between them (no circles).

How to draw one:

  1. Work out how many outer electrons each atom has.
  2. Put the atom that makes the most bonds in the middle.
  3. For each bond, put one dot and one cross in the overlap between two atoms.
  4. Add the rest of each atom’s outer electrons as lone pairs.
  5. Check: 8 electrons around every atom, 2 around every hydrogen.

Molecules with single bonds:

MoleculeFormulaCovalent bonds Lone pairsDisplayed formula
HydrogenH21noneH–H
ChlorineCl213 on each ClCl–Cl
Hydrogen chlorideHCl13 on ClH–Cl
WaterH2O22 on OH–O–H
AmmoniaNH331 on NN joined to 3 H
MethaneCH44noneC joined to 4 H

Six molecules, built pair by pair

  1. HH

    Dot and cross (outer shells)

    HH

    Displayed formula

    1 of 6 Hydrogen, H2

    One shared pair: a single covalent bond. Each hydrogen now has 2 electrons in its shell.

  2. ClCl

    Dot and cross (outer shells)

    ClCl

    Displayed formula

    2 of 6 Chlorine, Cl2

    One shared pair. Each chlorine also keeps 3 lone pairs, so each has 8 outer electrons.

  3. HCl

    Dot and cross (outer shells)

    HCl

    Displayed formula

    3 of 6 Hydrogen chloride, HCl

    One shared pair. Chlorine keeps 3 lone pairs: 8 electrons around chlorine, 2 around hydrogen.

  4. OHH

    Dot and cross (outer shells)

    OHH

    Displayed formula

    4 of 6 Water, H2O

    Two single bonds, one to each hydrogen. Oxygen keeps 2 lone pairs, so it has 8 outer electrons.

  5. NHHH

    Dot and cross (outer shells)

    NHHH

    Displayed formula

    5 of 6 Ammonia, NH3

    Three single bonds. Nitrogen keeps 1 lone pair, so it has 8 outer electrons.

  6. CHHHH

    Dot and cross (outer shells)

    CHHHH

    Displayed formula

    6 of 6 Methane, CH4

    Four single bonds. Carbon has no lone pairs: all 4 of its outer electrons are shared.

electrons from one atomelectrons from the other

4. Double and triple bonds

Sometimes two atoms need to share more than one pair to fill their shells.

  • Double bond: two shared pairs (4 electrons) between the same two atoms. Drawn as =.
  • Triple bond: three shared pairs (6 electrons). Drawn as .
MoleculeFormulaBonds Lone pairsDisplayed formula
OxygenO2one double bond2 on each OO=O
Carbon dioxideCO2two double bonds2 on each O, none on CO=C=O
NitrogenN2one triple bond1 on each NNN

Why: each oxygen needs 2 electrons, so two oxygens share 2 pairs. Carbon needs 4, so it makes a double bond to each of two oxygens. Each nitrogen needs 3, so two nitrogens share 3 pairs. The triple bond in nitrogen is very strong, which is one reason nitrogen gas is so unreactive.

Count the lines: each line is one shared pair, and the number of lines to each atom matches the table in section 2 (C has 4, N has 3, O has 2).

Sharing more than one pair

  1. OO

    Dot and cross (outer shells)

    OO

    Displayed formula

    1 of 3 Oxygen, O2

    Each oxygen needs 2 more electrons, so the two atoms share 2 pairs: a double bond. Each oxygen keeps 2 lone pairs.

  2. OCO

    Dot and cross (outer shells)

    OCO

    Displayed formula

    2 of 3 Carbon dioxide, CO2

    Carbon needs 4 and each oxygen needs 2, so carbon makes a double bond with each oxygen. Carbon has no lone pairs; each oxygen keeps 2.

  3. NN

    Dot and cross (outer shells)

    NN

    Displayed formula

    3 of 3 Nitrogen, N2

    Each nitrogen needs 3, so the two atoms share 3 pairs: a triple bond. Each nitrogen keeps 1 lone pair.

electrons from one atomelectrons from the other

5. Other ways to show a molecule, and what each leaves out

Molecules are 3D, and every drawing leaves something out. Exam questions ask you to say what a model shows and what it doesn’t.

ShowsLeaves out
Dot and crosswhich electrons are shared and which are lone pairs; where each electron came fromthe 3D shape; the size of the atoms; electrons are all the same really, and they don’t sit still in fixed places
Displayed formula (a line for each bond)which atoms are joined to which, and single, double or triple bondsthe electrons, including lone pairs; the 3D shape
Ball and stickthe 3D shape and the angles between bondsthe electrons; bonds aren’t really sticks, and the atoms are much closer together than drawn; sizes aren’t to scale
Space-fillingthe relative sizes of the atoms, and how closely they pack togetherthe bonds are hard to see or count; the electrons

Shape in 3D: a flat diagram of methane looks like a cross, but in 3D its four hydrogens point as far apart as possible, making a pyramid with a triangular base. Water looks straight in a displayed formula but is really bent.

Reading a formula from a model: count each type of atom. A model with one nitrogen joined to three hydrogens is NH3. Your answer is the molecular formula: the actual number of each atom in one molecule.

One molecule, four ways: ammonia, NH3

  1. NHHH

    1 of 4 Dot and cross

    Shows: Which electrons are shared and which are lone pairs, and which atom each electron came from.

    Leaves out: The 3D shape. The size of the atoms. And electrons are all the same really, and don’t sit still in fixed places.

  2. NHHH

    2 of 4 Displayed formula

    Shows: Which atoms are joined to which, and whether each bond is single, double or triple.

    Leaves out: The electrons, including the lone pair. The 3D shape: ammonia is not flat.

  3. 3 of 4 Ball and stick

    Shows: The 3D shape: a pyramid, with nitrogen at the top. And the angles between the bonds: in ammonia, every H–N–H angle is 107°.

    Leaves out: The electrons. Bonds are not really sticks, the atoms are much closer together than drawn, and the sizes are not to scale.

  4. 4 of 4 Space-filling

    Shows: The relative sizes of the atoms, how closely they pack together, and the overall 3D shape.

    Leaves out: The bonds are hard to see or count, and the electrons are not shown.

6. Simple molecular substances

A substance made of lots of separate small molecules is a simple molecular substance: water, carbon dioxide, methane, oxygen and the halogens, for example.

Two kinds of force:

  • covalent bonds hold the atoms together inside each molecule. These are strong.
  • intermolecular forces attract one molecule to the next. These are weak.

Melting and boiling: only the weak intermolecular forces are overcome. The covalent bonds don’t break. Ice, water and steam are all H2O molecules; only the distance between them changes.

So they have low melting and boiling points. Little energy is needed to overcome the weak forces between molecules, so most are gases or liquids at room temperature.

Slide to heat the water

Covalent bond (strong): inside each molecule. It never breaks. Intermolecular force (weak): between molecules. Overcome when the water melts and boils.

Bigger molecules, higher boiling points. Intermolecular forces get stronger as molecules get bigger, so more energy is needed to separate them. The halogens show this:

HalogenFormulaRelative molecular mass Melting point (°C)Boiling point (°C)At room temperature
FluorineF238−220−188gas
ChlorineCl271−101−34gas
BromineBr2160−759liquid
IodineI2254114184solid

Where room temperature falls for each halogen

−200 −100 0 100 200 Temperature (°C) Fluorine: melts at −220 °C, boils at −188 °C Chlorine: melts at −101 °C, boils at −34 °C Bromine: melts at −7 °C, boils at 59 °C Iodine: melts at 114 °C, boils at 184 °C −188−220 F2fluorineMr = 38 −34−101 Cl2chlorineMr = 71 59−7 Br2bromineMr = 160 184114 I2iodineMr = 254 gas gas liquid solid
Each bar runs from the melting point (bottom) to the boiling point (top): the range in which the halogen is a liquid. Room temperature, 20 °C. Above a bar, the halogen is a gas; through it, a liquid; below it, a solid.

They don’t conduct electricity. The molecules have no overall charge, so there is nothing charged that can move through the substance and carry a current.

7. The mistakes that cost marks

  1. “The covalent bonds break when water boils.” They don’t. Write: “the weak intermolecular forces are overcome.”
  2. “Simple molecules have weak covalent bonds.” The covalent bonds are strong. It is the forces between the molecules that are weak.
  3. Leaving out lone pairs. Water drawn without oxygen’s two lone pairs loses the mark. Count 8 around every atom (2 around hydrogen) before you move on.
  4. Shared electrons outside the overlap. The pair in a bond goes where the circles overlap.
  5. A single bond in O2, CO2 or N2. Count the electrons: each oxygen needs 2, each nitrogen 3.
  6. “They don’t conduct because they are covalent.” That isn’t a reason. Write: “the molecules have no overall charge, so nothing charged can move to carry a current.”
  7. Calling a metal and non-metal compound covalent. Metal + non-metal is ionic; only non-metals together is covalent.

Quick checklist

  • I can say what a covalent bond is, and why it holds two atoms together.
  • I can tell from a formula whether a substance is made of molecules.
  • I can work out how many bonds H, C, N, O and Cl make.
  • I can draw dot and cross diagrams for H2, Cl2, HCl, H2O, NH3 and CH4.
  • I can draw double and triple bonds for O2, CO2 and N2.
  • I can say what each type of model shows and leaves out.
  • I can write a molecular formula from a model.
  • I can explain low melting and boiling points using intermolecular forces.
  • I can explain why larger molecules have higher boiling points.
  • I can explain why simple molecular substances don’t conduct electricity.

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