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Covalent bondingIB MYP Chemistry: Revision notes

Section 1

Sharing electrons

Covalent bonding happens between non-metal atoms. Instead of transferring electrons, the atoms share pairs of electrons so that each atom has a full outer shell.

A covalent bond is a shared pair of electrons. It is a strong bond because the shared pair is attracted to the nuclei of both atoms at the same time.

Covalent bonding forms molecules: small groups of atoms, such as H₂O.

Key termscovalent bondmolecule
Common mistake

Covalent bonds are not formed by transferring electrons. Transfer is ionic bonding.

Section 2

Single, double and triple bonds

The number of electrons shared depends on how many each atom needs.

  • Single bond: one shared pair (2 electrons), shown as a single line, e.g. H–H.
  • Double bond: two shared pairs (4 electrons), shown as a double line, e.g. O=C=O.
  • Triple bond: three shared pairs (6 electrons), shown as a triple line, e.g. N≡N.

A hydrogen atom needs only one more electron, so it always forms one bond. An atom of carbon needs four, nitrogen three and oxygen two to have a full outer shell. You can work this out as 8 minus the number of outer electrons.

Key termssingle bonddouble bondtriple bond

Section 3

Dot-and-cross diagrams

A dot-and-cross diagram shows only the outer electrons. Use dots for one atom's electrons and crosses for the other's. Overlapping circles show the shared pairs. Electrons that are not shared are called non-bonding pairs (lone pairs).

  • H₂: one shared pair; each hydrogen has two electrons.
  • Cl₂: one shared pair; each chlorine has three non-bonding pairs.
  • H₂O: oxygen shares one pair with each hydrogen; oxygen has two non-bonding pairs.
  • NH₃: nitrogen shares one pair with each of three hydrogens; it has one non-bonding pair.
  • CH₄: carbon shares one pair with each of four hydrogens; no non-bonding pairs.
  • CO₂: carbon shares two pairs with each oxygen (two double bonds); each oxygen has two non-bonding pairs.
Key termsnon-bonding pairdot-and-cross diagram
Exam tip

Count the electrons around each atom at the end. Every atom except hydrogen should have eight, and hydrogen should have two.

Section 4

Worked example: ammonia

Nitrogen has five outer electrons and each hydrogen has one.

  1. Nitrogen needs three more electrons to have eight, so it forms three bonds.
  2. Each hydrogen shares one pair with nitrogen, so the formula is NH₃.
  3. Three of nitrogen's outer electrons are used in bonds. That leaves two, which form one non-bonding pair.
  4. Check: nitrogen has 3 shared pairs + 1 non-bonding pair = 8 electrons. Each hydrogen has 2.

Must know

  • Covalent bonding is between non-metals: atoms share pairs of electrons.
  • A single bond is one shared pair, a double bond is two, and a triple bond is three.
  • Atoms share until they have a full outer shell.
  • Know the dot-and-cross diagrams for H₂, Cl₂, H₂O, NH₃, CH₄ and CO₂.
  • Non-bonding pairs are outer electrons not used in bonds.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Covalent bonding

  1. A biogas plant in Kerala produces methane, CH₄, from rotting plant waste. The methane is burned for cooking. Carbon atoms have four electrons in their outer shell and hydrogen atoms have one.
    Describe how a covalent bond forms between a carbon atom and a hydrogen atom.2 marks
  2. A scientist studying greenhouse gases examines carbon dioxide, CO₂, which forms when fossil fuels burn. Carbon atoms have four electrons in their outer shell and oxygen atoms have six.
    Explain how the atoms in a molecule of carbon dioxide each end up with a full outer shell.2 marks
  3. A student uses a molecular model kit to compare molecules of methane (CH₄), ammonia (NH₃) and water (H₂O). Carbon atoms have four outer electrons, nitrogen atoms five and oxygen atoms six. She records how many hydrogen atoms attach to the central atom of each molecule.
    Describe the pattern in her results and use it to predict the number of hydrogen atoms that attach to a fluorine atom, which has seven outer electrons.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).