Covalent bonding and simple molecules Cambridge IGCSE Chemistry revision
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In plain words
Two non-metal atoms both want more electrons, so neither will give any away. Instead they share. A shared pair of electrons holds the two atoms together: a covalent bond. A few atoms joined this way make a small molecule, such as water or methane.
Three things to know
- A covalent bond is a pair of electrons shared between two non-metal atoms. Sharing gives each atom a full outer shell.
- Simple molecules such as H₂, Cl₂, H₂O, CH₄, NH₃ and CO₂ have low melting and boiling points, because the forces between the molecules are weak. The covalent bonds themselves are strong.
- They do not conduct electricity: there are no ions and no free electrons. O₂ and CO₂ have double bonds; N₂ has a triple bond.
Tips and tricks
- Low melting point? Blame the weak forces between the molecules. The covalent bonds inside each molecule are strong and don't break when it melts.
- In a dot-and-cross diagram, count the electrons round each atom at the end: 8 in the outer shell (2 for hydrogen).
It lands in your notebook with its questions as flashcards.
Covalent bonding and simple molecules: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
How is a covalent bond formed?
Each atom counts the shared pair towards its outer shell.
Which substance has covalent bonds?
Water is made of non-metals only.
Why do simple molecular substances have low boiling points?
Boiling separates the molecules; it does not break the bonds inside them.
How many pairs of electrons are shared in a nitrogen molecule, N₂?
Nitrogen atoms are joined by a triple bond.
Why do simple covalent substances not conduct electricity?
Something charged has to be able to move.
How many covalent bonds does the carbon atom form in methane, CH₄?
Carbon has four outer electrons and shares each of them.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Covalent bonding and simple molecules
Cambridge IGCSE Chemistry 0620 · 8 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by a covalent bond.[2]
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A shared pair of electrons between two atoms.
Explain why methane has a low boiling point.[2]
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There are only weak forces between the methane molecules, so little energy is needed to separate them. (The covalent bonds are not broken.)
State how many covalent bonds there are in a molecule of ammonia, NH₃, and how many electrons are shared in total.[2]
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3 bonds; 6 electrons.
Describe the bonding in a molecule of oxygen, O₂.[2]
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A double covalent bond: the two atoms share two pairs of electrons.
Answers: Covalent bonding and simple molecules
- 1. A shared pair of electrons between two atoms.
- 2. There are only weak forces between the methane molecules, so little energy is needed to separate them. (The covalent bonds are not broken.)
- 3. 3 bonds; 6 electrons.
- 4. A double covalent bond: the two atoms share two pairs of electrons.