2.5 Simple molecules and covalent bonds

2026 Syllabus Objectives

Core:

  1. State that a covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations
  2. Describe the formation of covalent bonds in simple molecules, including H2\text{H}_2, Cl2\text{Cl}_2, H2O\text{H}_2\text{O}, CH4\text{CH}_4, NH3\text{NH}_3 and HCl\text{HCl}. Use dot-and-cross diagrams to show the electronic configurations in these and similar molecules
  3. Describe in terms of structure and bonding the properties of simple molecular compounds: (a) low melting points and boiling points (b) poor electrical conductivity

Supplement: 4. Describe the formation of covalent bonds in simple molecules, including CH3OH\text{CH}_3\text{OH}, C2H4\text{C}_2\text{H}_4, O2\text{O}_2, CO2\text{CO}_2 and N2\text{N}_2. Use dot-and-cross diagrams to show the electronic configurations in these and similar molecules 5. Explain in terms of structure and bonding the properties of simple molecular compounds: (a) low melting points and boiling points in terms of weak intermolecular forces (specific types of intermolecular forces are not required) (b) poor electrical conductivity

What is Covalent Bonding? 🔑

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Covalent bonding is the process by which non-metal atoms share pairs of electrons to achieve stable electronic configurations.

Key Features of Covalent Bonding

  • The bond is formed by the sharing of a pair of electrons between two atoms
  • This sharing leads to noble gas electronic configurations
  • Each atom contributes one electron to each bond
  • Molecules are formed from atoms linked together by covalent bonds

Important: The shared electrons are more likely to be found between the two nuclei. The forces of attraction between the shared electrons and the nuclei are greater than any repulsive forces, holding the molecule together.

Why Do Atoms Form Covalent Bonds?

  • Hydrogen atoms share electrons to gain a share in 2 electrons (like helium, the nearest noble gas)
  • Other non-metal atoms share electrons to gain a share in 8 electrons in their outer shells (like the noble gases neon, argon, etc.)

Formation of Simple Diatomic Molecules ⚡

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Hydrogen Molecule (H2\text{H}_2)

When two hydrogen atoms come together, each contributes one electron to form a shared pair of electrons.

Electronic structure before bonding: Each hydrogen atom has 1 electron

Electronic structure after bonding: Each hydrogen atom has a share in 2 electrons (stable configuration like helium)

Displayed formula: H-H\text{H-H}

The dot-and-cross diagram shows the outer electrons only, with electrons from different atoms represented by dots and crosses to show their origin.

Chlorine Molecule (Cl2\text{Cl}_2)

Chlorine atoms belong to Group VII (the halogens) and have 7 electrons in their outer shell (electronic configuration: 2,8,7).

Electronic structure before bonding: Each chlorine atom has 7 outer electrons

Electronic structure after bonding: Each chlorine atom has a share in 8 outer electrons (2,8,8) - stable configuration

Displayed formula: Cl-Cl\text{Cl-Cl}

Note: Other halogens like bromine (Br2\text{Br}_2) and iodine (I2\text{I}_2) form molecules in the same way. They are larger because the original atoms are bigger.

Dot-and-Cross Diagrams 📌

Dot-and-cross diagrams are used to represent the bonding in molecules:

  • Usually, only outer electrons are shown (inner electrons are not involved in bonding)
  • Electrons are represented by dots or crosses depending on which atom they are from
  • The electron shells of each atom overlap in the diagram
  • The shared electrons are shown in the area of overlap

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