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Edexcel Separate Sciences · Chemistry · Papers 1 & 2

SC6 · Covalent bondingTopic 1 — Key concepts in chemistry

Shared electrons and molecules

Revise the key ideas

Sharing electrons

  • A covalent bond is a shared pair of electrons between atoms, usually non-metal atoms. Attraction between the shared electrons and both positive nuclei holds the atoms together.
  • Sharing can give atoms complete outer shells: hydrogen needs two electrons in its first shell, while the common carbon, nitrogen, oxygen and chlorine examples reach eight outer electrons.
  • A single bond shares one pair, a double bond two pairs, and a triple bond three pairs. The electrons remain shared rather than transferring to form opposite ions.
  • Dot-and-cross diagrams show which atom supplied each electron. Dots and crosses are a drawing convention: the electrons have the same physical properties.
  • Show outer electrons, including unshared or lone pairs, and count each shared pair towards both atoms' outer shells. Inner shells can be omitted in outer-shell diagrams.
  • A molecular formula gives numbers of atoms in one molecule. A displayed formula shows bonds as lines; each ordinary single line represents one shared electron pair.

Single-bond molecules

  • Hydrogen, H₂, has one shared pair between two hydrogen atoms. Each atom counts the pair as a complete first shell of two electrons.
    Hydrogen dot and crossOuter-shell electron diagram for Hydrogen. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.HHו and × distinguish origins; outer electrons only
    H–H: one shared pair; two electrons count for each hydrogen.
  • Chlorine, Cl₂, has one shared pair and three lone pairs on each chlorine atom. Each chlorine contributes one electron to the bond and has eight around it.
    Chlorine dot and crossOuter-shell electron diagram for Chlorine. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.ClCl××××××ו and × distinguish origins; outer electrons only
    Cl–Cl: one shared pair and three lone pairs around each Cl; dots belong to one chlorine and crosses to the other.
  • Water, H₂O, has two O–H single bonds and two lone pairs on oxygen. Each hydrogen has two electrons; oxygen has eight counted around it.
    Water dot and crossOuter-shell electron diagram for Water. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.OHH×ו and × distinguish origins; outer electrons only
    Two shared pairs and two oxygen lone pairs; the sketch is not to scale.
  • Ammonia, NH₃, has three N–H single bonds and one lone pair on nitrogen. Nitrogen contributes one electron to each bond.
    Ammonia dot and crossOuter-shell electron diagram for Ammonia. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.NHHH××ו and × distinguish origins; outer electrons only
    Three shared pairs and one nitrogen lone pair.
  • Methane, CH₄, has four C–H single bonds and no lone pairs on carbon. Carbon's four outer electrons contribute to four shared pairs.
    Methane dot and crossOuter-shell electron diagram for Methane. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.CHHHH×××ו and × distinguish origins; outer electrons only
    Four shared pairs; actual methane has a three-dimensional tetrahedral arrangement.
  • Flat diagrams communicate bonding and electron counts but not all three-dimensional shapes. Methane is not a flat cross; water is not a straight line.
  • Hydrogen chloride, HCl, has one shared pair between hydrogen and chlorine and three lone pairs on chlorine. Hydrogen counts two outer electrons and chlorine eight.
    Hydrogen chloride dot and crossOuter-shell electron diagram for Hydrogen chloride. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.HCl××××××ו and × distinguish origins; outer electrons only
    H–Cl: one shared pair; three lone pairs belong to chlorine. Dots indicate hydrogen and crosses chlorine electrons.
  • Typical atoms and small molecules are roughly 10⁻¹⁰ m across. Diagrams enlarge them greatly; do not measure a diagram to find an atom’s size unless a scale is provided.

Double bonds and carbon dioxide

  • Oxygen, O₂, has two shared pairs forming a double bond. Each oxygen also has two lone pairs, giving eight outer electrons around each atom.
    Oxygen dot and crossOuter-shell electron diagram for Oxygen. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.OO×××××ו and × distinguish origins; outer electrons only
    O=O: two shared pairs and two lone pairs around each oxygen.
  • Carbon dioxide, CO₂, has structure O=C=O: two C=O double bonds. Four pairs are shared in the whole molecule, and each oxygen has two lone pairs.
    Carbon dioxide dot and crossOuter-shell electron diagram for Carbon dioxide. Each dot-cross pair is a covalent bond; dot-dot pairs are lone pairs. Flat schematic, not molecular geometry.OCO×××ו and × distinguish origins; outer electrons only
    O=C=O: four shared pairs in total; carbon electrons are crosses and oxygen electrons dots. Each oxygen has two lone pairs.
  • In CO₂ carbon contributes four outer electrons, two to each double bond. Each oxygen contributes two to its double bond and retains four in lone pairs.
  • The number of shared pairs differs from the number of atoms: CO₂ contains three atoms but four shared pairs. Count lines or electron pairs carefully.
  • Molecular oxygen is an element because both atoms are oxygen; carbon dioxide is a compound because carbon and oxygen are different elements.
  • Strong covalent bonds need considerable energy to break. A change of state of a simple molecular substance mainly overcomes attractions between molecules, not its covalent bonds.

Simple molecular substances

  • Simple molecular substances are made of separate small molecules. They often have low melting and boiling points because the attractions between molecules (intermolecular forces) are relatively weak and need little energy to overcome.
  • Low boiling point does not mean the bonds inside each molecule are weak. During ordinary boiling, intact molecules separate rather than decomposing into individual atoms.
    Within and between moleculesSolid lines represent strong covalent bonds within two hydrogen molecules; a dashed line indicates an intermolecular attraction between the molecules.Strong bond within each moleculeWeaker attraction between molecules
    Boiling separates molecules; it does not normally break H–H covalent bonds.
  • Simple molecular substances usually do not conduct electricity because they have no charged particles that can move freely: no mobile ions or delocalised electrons. Moving neutral molecules cannot carry an electric current.
  • Solubility varies: some molecular substances dissolve in water and others do not. Avoid claiming that every covalent substance is insoluble.
  • Some molecules react or ionise in water, giving ions in solution: aqueous hydrogen chloride conducts even though molecular HCl is covalent. Structure and conditions matter.
  • Giant covalent structures such as diamond are exceptions to simple molecular melting-point patterns and are covered in CC7.

Polymers and bonding models

  • Some small molecules, called monomers, can chemically join into long-chain molecules called polymers. Not every small molecule is a monomer.
  • Ethene can join by addition polymerisation to form poly(ethene). The C=C bond is converted into single bonds as repeating units link; the polymer still contains covalent bonds.
  • Polymers have strong covalent bonds within long molecules; attractions between chains influence their properties. They are not giant covalent lattices like diamond.
  • Ball-and-stick models show how atoms join and their three-dimensional arrangement, but exaggerate the gaps between them. The sticks represent bonds. Space-filling models show the space atoms occupy but can hide the bonds.
  • Dot-and-cross diagrams reveal shared and lone pairs but do not show actual atom sizes, bond lengths or full three-dimensional shape. Choose a model for the information required.

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