AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Covalent bonding: draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane
Start by working out the outer-shell electrons for each atom: hydrogen has one, chlorine has seven, oxygen has six, nitrogen has five, and carbon has four.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Start by working out the outer-shell electrons for each atom: hydrogen has one, chlorine has seven, oxygen has six, nitrogen has five, and carbon has four. Then pair electrons so that each atom reaches a full outer shell, using dots for one atom and crosses for the other. Hydrogen forms a single bond with one shared pair, chlorine forms a single bond with three lone pairs on each atom, oxygen forms a double bond with two lone pairs on each atom, and nitrogen forms a triple bond with one lone pair on each atom. Hydrogen chloride has a single bond and three lone pairs on chlorine. Water has two single bonds and two lone pairs on oxygen. Ammonia has three single bonds and one lone pair on nitrogen. Methane has four single bonds and no lone pairs on carbon.
What earns the marks
- Draw hydrogen as H–H with one shared pair and no lone pairs.
- Draw chlorine as Cl–Cl with one shared pair and three lone pairs on each chlorine atom.
- Draw oxygen as O=O with two shared pairs and two lone pairs on each oxygen atom.
- Draw nitrogen as N≡N with three shared pairs and one lone pair on each nitrogen atom.
- Draw hydrogen chloride as H–Cl with one shared pair and three lone pairs on chlorine.
- Draw water as two O–H bonds with two lone pairs on oxygen.
- Draw ammonia as three N–H bonds with one lone pair on nitrogen.
- Draw methane as four C–H bonds with no lone pairs on carbon.
Common mistakes and how to put them right
Forgetting lone pairs on chlorine, oxygen or nitrogen.
Put it right by counting all outer-shell electrons and showing those not used in bonding.
Drawing oxygen with a single bond.
Put it right by showing a double bond so each oxygen has eight outer-shell electrons.
Drawing nitrogen with a double bond.
Put it right by showing a triple bond so each nitrogen has eight outer-shell electrons.
Drawing methane with lone pairs on carbon.
Put it right by showing four single bonds and no lone pairs on carbon.
Exam tips
- Use a key or label to show which atom provides the dots and which provides the crosses.
- Draw the outer shell only; do not draw inner shells unless the question asks for them.
- Check that each atom in the completed diagram has a full outer shell of electrons (two for hydrogen, and eight for carbon, nitrogen, oxygen and chlorine).
- For water and ammonia, place lone pairs on the central atom and ensure bond angles are shown clearly enough to count the bonds.
Try the lesson
The start of the Covalent bonding: draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane lesson, made from checked teaching notes for this specification point.
Have a go first
In a dot and cross diagram of a hydrogen molecule, H₂, how many electrons are shown in the overlap region between the two hydrogen atoms?
Listen
Start by working out the outer-shell electrons for each atom: hydrogen has one, chlorine has seven, oxygen has six, nitrogen has five, and carbon has four. Then pair electrons so that each atom reaches a full outer shell, using dots for one atom and crosses for the other. Hydrogen forms a single bond with one shared pair, chlorine forms a single bond with three lone pairs on each atom, oxygen forms a double bond with two lone pairs on each atom, and nitrogen forms a triple bond with one lone pair on each atom. Hydrogen chloride has a single bond and three lone pairs on chlorine. Water has two single bonds and two lone pairs on oxygen. Ammonia has three single bonds and one lone pair on nitrogen. Methane has four single bonds and no lone pairs on carbon.
Key words
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Dot and cross diagrams
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Lone pairs
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Single, double and triple covalent bonds
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Full outer shells
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See every definition
- Dot and cross diagrams
- Diagrams in which dots are used for one atom and crosses for the other, pairing electrons so that each atom reaches a full outer shell.
- Lone pairs
- Outer-shell electrons not used in bonding, such as the three lone pairs on each chlorine atom or the two lone pairs on oxygen.
- Single, double and triple covalent bonds
- Bonds formed from one shared pair, two shared pairs or three shared pairs, as in hydrogen, oxygen and nitrogen respectively.
- Full outer shells
- The arrangement each atom reaches by pairing electrons, with two outer-shell electrons for hydrogen and eight for carbon, nitrogen, oxygen and chlorine.
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