2.5 BETA Verified

Simple molecules and covalent bonds

5 learning objectives • 3 core • 2 extended

Topic 2.5: Simple Molecules and Covalent Bonds

1. Overview

Covalent bonding is the primary way that non-metal atoms achieve stability by sharing electrons. Understanding this topic is essential for explaining the behavior of gases, liquids with low boiling points, and the fundamental structure of organic chemistry and biological molecules.

Key Definitions

  • Covalent Bond: An electrostatic attraction between a shared pair of electrons and the nuclei of the atoms involved.
  • Molecule: A group of two or more atoms held together by covalent bonds.
  • Noble Gas Configuration: A stable electronic structure where an atom has a full outer shell (usually 8 electrons, or 2 for Helium).
  • Intermolecular Forces: Weak forces of attraction existing between individual molecules.
  • Lone Pair: A pair of outer-shell electrons that are not involved in bonding.

Core Content

Formation of Covalent Bonds

A covalent bond forms when two non-metal atoms share a pair of electrons. This allows both atoms to achieve a full outer shell (noble gas configuration).

  • One shared pair = Single Covalent Bond
  • Two shared pairs = Double Covalent Bond
  • Three shared pairs = Triple Covalent Bond

Dot-and-Cross Diagrams

These diagrams show the arrangement of outer-shell electrons. Electrons from one atom are shown as dots (•), and from the other as crosses (x).

  • Hydrogen (H₂): Two H atoms share their only electron to get 2 in the outer shell.
    • 📊Two overlapping circles for H, with one 'x' and one '•' in the overlap.
  • Chlorine (Cl₂): Two Cl atoms (Group VII) share one pair to get 8 in the outer shell.
    • 📊Overlapping circles, one 'x' and one '•' in the overlap, 6 non-bonding electrons on each Cl.
  • Water (H₂O): Oxygen shares one electron with each of the two Hydrogen atoms.
    • 📊Oxygen in center with 2 lone pairs; two overlaps with H atoms containing 'x•'.
  • Methane (CH₄): Carbon (Group IV) shares its 4 electrons with four Hydrogen atoms.
    • 📊Carbon in center, four overlaps with H atoms, each containing 'x•'.
  • Ammonia (NH₃): Nitrogen (Group V) shares 3 electrons with three Hydrogen atoms, leaving one lone pair.
    • 📊Nitrogen in center with one lone pair; three overlaps with H atoms containing 'x•'.
  • Hydrogen Chloride (HCl): H shares its electron with one Cl atom.
    • 📊Overlap between H and Cl with 'x•'; Cl has 6 remaining non-bonding electrons.

Properties of Simple Molecular Compounds

  1. Low Melting and Boiling Points: Simple molecules have low MP/BP and are often gases or liquids at room temperature because the forces between the molecules are easy to overcome.
  2. Poor Electrical Conductivity: They do not conduct electricity in any state (solid, liquid, or gas) because they do not have delocalized electrons or mobile ions to carry a charge.

Extended Content (Extended curriculum only)

Complex Covalent Molecules

In some molecules, atoms must share more than one pair of electrons to reach a full outer shell.

  • Oxygen (O₂): Double bond (two shared pairs).
    • O = O (Word equation: Oxygen + Oxygen → Oxygen)
    • O(g) + O(g) → O₂(g)
    • 📊Two O atoms overlapping with two 'x's and two '•'s in the overlap.
  • Nitrogen (N₂): Triple bond (three shared pairs).
    • N ≡ N (Word equation: Nitrogen + Nitrogen → Nitrogen)
    • N(g) + N(g) → N₂(g)
    • 📊Two N atoms overlapping with three 'x's and three '•'s in the overlap.
  • Carbon Dioxide (CO₂): Two double bonds.
    • O = C = O
    • C(s) + O₂(g) → CO₂(g)
    • 📊Carbon in center, overlapping with two Oxygen atoms; each overlap contains 4 electrons.
  • Ethene (C₂H₄): Contains a C=C double bond.
    • 📊Two Carbons double-bonded to each other, each Carbon single-bonded to two Hydrogens.
  • Methanol (CH₃OH):
    • 📊Carbon bonded to 3 H atoms and 1 O atom; the O atom is also bonded to an H atom.

Explaining Properties via Intermolecular Forces

  • Melting/Boiling Points: While the covalent bonds inside the molecule are very strong, the intermolecular forces between the molecules are very weak. Very little thermal energy is required to overcome these weak forces and separate the molecules.
  • Conductivity: Covalent compounds consist of neutral molecules. Because there are no free-moving ions or electrons (delocalized electrons), no current can flow.

Key Equations

Process Word Equation Symbol Equation
Formation of Water Hydrogen + Oxygen → Water 2H₂(g) + O₂(g) → 2H₂O(l)
Formation of Methane Carbon + Hydrogen → Methane C(s) + 2H₂(g) → CH₄(g)
Formation of Ammonia Nitrogen + Hydrogen → Ammonia N₂(g) + 3H₂(g) → 2NH₃(g)

Common Mistakes to Avoid

  • ❌ Wrong: Saying "covalent bonds are weak" to explain low boiling points.
  • ✓ Right: Covalent bonds are strong; it is the intermolecular forces between molecules that are weak.
  • ❌ Wrong: Drawing outer electrons for Hydrogen as 8.
  • ✓ Right: Hydrogen only needs 2 electrons to have a full outer shell.
  • ❌ Wrong: Forgetting to draw the non-bonding (lone) electrons in dot-and-cross diagrams.
  • ✓ Right: Always ensure atoms like Chlorine and Oxygen show their full set of 8 valence electrons, including those not in the bond.

Exam Tips

  • Command Word "Describe": If asked to describe a covalent bond, always mention "shared pair of electrons."
  • Command Word "Explain": If asked why CO₂ is a gas, you must mention "weak intermolecular forces" and "little energy needed to break them."
  • Diagram Precision: In dot-and-cross diagrams, ensure the circles overlap and the shared electrons are clearly placed inside the overlap area.
  • Check the Valency: Carbon always forms 4 bonds, Nitrogen 3, Oxygen 2, and Hydrogen/Halogens 1. Use this "4-3-2-1" rule to check your structures.
  • State Symbols: Always include (g) for H₂, N₂, O₂, F₂, Cl₂, CH₄, NH₃, and CO₂ at room temperature. H₂O is usually (l).