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K223 / K225 / 2027
Bonding and Structure

Lesson 3 of 5

Covalent bonding and molecules

Read shared electron pairs and distinguish bonds within molecules from attractions between them.

SEC G2 2027

  • I can explain a covalent bond.Syllabus

    Syllabus K223 / K225, 3.2(a). Explain a covalent bond

  • I can draw the named covalent molecules.Syllabus

    Syllabus K223 / K225, 3.2(b). Draw the named covalent molecules

  • I can deduce another molecule from its electrons.Syllabus

    Syllabus K223 / K225, 3.2(c). Deduce another molecule from its electrons

  • I can connect covalent structure and properties.Syllabus

    Syllabus K223 / K225, 3.2(d). Connect covalent structure and properties

Covalent bonding: count shared pairs

Each shared pair is one bond. Unshared electrons still count towards the outer shell.

Non-metal atoms form a covalent bond by sharing a pair of electrons. Usually each atom contributes one electron. The shared pair counts towards both outer shells, commonly giving a noble-gas arrangement: two electrons for hydrogen, eight for the other atoms shown here.

Each dot or cross is one outer-shell electron. A pair between atoms is shared; a pair beside one atom is a lone pair.

Hydrogen (H2)

Hydrogen dot-and-cross diagramTwo hydrogen atoms share one pair of electrons: one dot from the left hydrogen and one cross from the right. Each hydrogen has two electrons in its shared first shell.HH
  • Left H
  • Right H
One shared pair. Each hydrogen has a full first shell of two electrons.

Oxygen (O2)

Oxygen dot-and-cross diagramTwo oxygen atoms share two electron pairs, a double bond. The left oxygen contributes six dots and the right six crosses. Each oxygen has two lone pairs as well as the two shared pairs, giving eight electrons around each atom.OO
  • Left O
  • Right O
Two shared pairs form a double bond. Each oxygen also has two lone pairs.

Water (H2O)

Water dot-and-cross diagramOxygen contributes six dots: one in each of two bonds and four in two lone pairs. Each hydrogen contributes one cross. Oxygen has an octet and each hydrogen a duet. The flat layout shows electron accounting; a water molecule is bent.HOH
  • Oxygen
  • Hydrogens
Two shared pairs and two lone pairs around oxygen. Water is bent; this layout only counts electrons.

Methane (CH4)

Methane dot-and-cross diagramCarbon contributes four dots, one to each of four shared pairs. Each of four hydrogens contributes one cross. Carbon has an octet and each hydrogen a duet. Methane is tetrahedral; the flat cross layout does not show its shape.CHHHH
  • Carbon
  • Hydrogens
Four shared pairs, with no lone pair on carbon. Methane is tetrahedral, not flat.

Carbon dioxide (CO2)

Carbon dioxide dot-and-cross diagramCarbon shares two pairs with each oxygen, making two double bonds. Carbon contributes four dots in total. Each oxygen contributes six crosses: two in the shared pairs and four in two lone pairs. All three atoms have octets.OCO
  • Carbon
  • Oxygens
Two double bonds. Each oxygen has two lone pairs; carbon has none.

Ammonia (NH3)

Ammonia dot-and-cross diagramNitrogen contributes five dots: three in shared pairs and two in one lone pair. Each hydrogen contributes one cross. Nitrogen has an octet and every hydrogen a duet. Ammonia is pyramidal, not flat.NHHH
  • Nitrogen
  • Hydrogens
Deduce three shared pairs and one lone pair from nitrogen's five outer electrons. Ammonia is pyramidal.

Only outer-shell electrons are shown. Shared electrons count towards both bonded atoms' outer shells. Dots and crosses identify the source atom, not different types of electron.

H2, O2, H2O, CH4 and CO2: count every shared pair and every lone pair. These flat electron diagrams do not show three-dimensional molecular shapes.
Five patterns worth recognizing
MoleculeShared pairsLone pairs
H2One pair between the H atoms: a single bond.None.
O2Two pairs between the O atoms: a double bond.Two on each O.
H2OOne pair in each of two O-H bonds.Two on O.
CH4One pair in each of four C-H bonds.None on C.
CO2Two pairs in each C=O bond: O=C=O.Two on each O; none on C.

Worked exampleDeduce an unfamiliar diagram: ammonia

Nitrogen has five outer electrons. How are electrons arranged in NH3?

Try it first. Then show the worked stepsHide the worked steps
  1. Nitrogen needs three more electrons around it; each hydrogen needs one.
  2. Form three shared pairs, one between N and each H. These give three N-H bonds.
  3. Six electrons surround N in bonds. Its remaining two electrons form one lone pair.
  4. Check: eight electrons around N, two around each H, and eight outer electrons in total.
Answer

Three single N-H bonds and one lone pair on nitrogen. Sharing fills shells without transferring electrons to form ions.

Worked exampleDot-and-cross diagram for nitrogen trifluoride

Nitrogen trifluoride, NF3, is a covalent molecule. Nitrogen has 5 outer electrons and fluorine has 7. Draw its dot-and-cross diagram, outer electrons only.

Try it first. Then show the worked stepsHide the worked steps
  1. Each fluorine atom needs one more electron, so it shares one pair with nitrogen. Nitrogen needs three more electrons, so it forms three shared pairs, one with each fluorine.
  2. Check the count. Nitrogen has 3 shared pairs plus 1 lone pair: 8 electrons. Each fluorine has 1 shared pair plus 3 lone pairs: 8 electrons. This is the same pattern as ammonia, with F in place of H.
Answer

3 shared pairs and 1 lone pair, so nitrogen has 8 outer electrons

Check your understanding

In the dot-and-cross diagram of NF3, how many shared pairs and lone pairs does nitrogen have?

  1. 5 shared pairs and no lone pair, so nitrogen has 10 outer electrons
  2. 3 shared pairs and no lone pair, so nitrogen has 6 outer electrons
  3. 3 shared pairs and 1 lone pair, so nitrogen has 8 outer electrons
Show the answer

3 shared pairs and 1 lone pair, so nitrogen has 8 outer electrons

Nitrogen has 5 outer electrons and needs 3 more, so it shares one pair with each fluorine. The two unshared electrons form a lone pair, as in ammonia.

Worked exampleDot-and-cross diagram for the hydroxide ion

The hydroxide ion, OH-, has a covalent bond between O and H and an overall charge of 1-. Draw its dot-and-cross diagram, outer electrons only.

Try it first. Then show the worked stepsHide the worked steps
  1. Oxygen (6 outer electrons) shares one pair with hydrogen. The extra electron that gives the 1- charge sits on the oxygen.
  2. Oxygen now has 8 outer electrons: one shared pair and three lone pairs (5 of its own unshared electrons plus the gained one). Hydrogen has 2. Put the whole ion in square brackets with the charge outside, because the charge belongs to the whole ion.
Answer

On oxygen, which then has one shared pair and three lone pairs

Check your understanding

In a dot-and-cross diagram of the hydroxide ion, OH-, where does the extra electron that gives the 1- charge go?

  1. On hydrogen, which then holds three electrons in its first shell
  2. On oxygen, which then has one shared pair and three lone pairs
  3. Between O and H, which makes a double bond in the ion
Show the answer

On oxygen, which then has one shared pair and three lone pairs

Oxygen shares one pair with H. With its other 5 electrons plus the gained one, it has 8 outer electrons. Bracket the whole ion with the charge outside.

Strong bonds inside; weaker attractions between

Melting a molecular substance separates molecules without breaking them apart.

Methane and iodine have simple molecular structures: many separate molecules. Strong covalent bonds hold atoms together within each molecule. Weaker intermolecular forces attract one molecule to another. These are two different levels of attraction.

Solid iodine

Ordered iodine molecules in a solidSix iodine molecules, each formed from two iodine atoms joined by a solid covalent-bond line. Their positions are ordered. Dashed lines between separate molecules indicate intermolecular attractions. The drawing is schematic and does not show motion.IIIIIIIIIIII
The molecules vibrate around fixed positions.

Liquid iodine

Intact iodine molecules in a liquidSix iodine molecules, each formed from two iodine atoms joined by a solid covalent-bond line. Their arrangement is less regular after melting. Every molecule still has its two atoms joined: the covalent bonds have not broken. The drawing is schematic and does not show motion.IIIIIIIIIIII
The molecules can move past one another. Each I2 molecule stays intact.
  • Covalent bond within a molecule
  • Attraction between molecules

Melting overcomes enough intermolecular attractions to let the molecules move past one another. It does not break the covalent bonds inside them.

Simplified snapshots, not the measured crystal structure or a simulation. Only a few intermolecular attractions are marked; they are still present in the liquid.

On melting, molecules move apart and past one another. The covalent bonds within each molecule remain intact.

Simple molecular substances generally melt and boil at low temperatures compared with ionic lattices: relatively little energy overcomes their intermolecular forces. Methane is a gas at room temperature, while iodine is a solid. Simple molecular does not mean always gaseous; intermolecular attraction varies between substances.

Their pure solids and liquids generally do not conduct electricity: neutral molecules have no mobile ions or delocalised electrons to carry charge. A solution needs separate consideration: a molecular substance may form ions by reacting with water.

Watch out for this

A student says: "Iodine melts easily because its covalent bonds are weak." What is wrong with this?

Show the answer

A low melting point indicates weak attractions between molecules. It does not show that the covalent bonds inside the molecules are weak. Melting and breaking a molecule chemically are different changes.

Check your understanding

Iodine turns into a vapour on gentle warming. Why is so little energy needed?

  1. The covalent bond inside each I2 molecule is weak and breaks
  2. The ions in solid iodine are only weakly attracted to each other
  3. Only weak forces between the I2 molecules have to be overcome
  4. Iodine has a giant covalent structure with only a few bonds
Show the answer

Only weak forces between the I2 molecules have to be overcome

Iodine is simple molecular. Warming separates the molecules; the covalent I-I bonds stay intact, so the vapour is still I2.