Covalent Bonding and Coordinate (Dative Covalent) Bonding

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Define and describe covalent bonding:

    • Define covalent bonding as electrostatic attraction between nuclei and shared electrons
    • Describe covalent bonding in: H₂, O₂, N₂, Cl₂, HCl, CO₂, NH₃, CH₄, C₂H₆, C₂H₄
    • Understand that Period 3 elements can expand their octet (SO₂, PCl₅, SF₆)
    • Describe coordinate (dative covalent) bonding in NH₄⁺ and Al₂Cl₆

  2. Understand orbital overlap:

    • Describe σ (sigma) and π (pi) bonds in terms of orbital overlap
    • Describe σ and π bonds in: H₂, C₂H₆, C₂H₄, HCN, N₂
    • Use hybridisation to describe sp, sp², and sp³ orbitals

  3. Understand bond properties:

    • Define bond energy and bond length
    • Use bond energy and bond length to compare reactivity

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1. What is Covalent Bonding?

Covalent bonding is the electrostatic attraction between the nuclei (the positive centers) of two atoms and a shared pair of electrons.

Let's break this down:

  • Electrostatic attraction means the pulling force between opposite charges (positive and negative)
  • The nuclei are positively charged (they contain protons)
  • A shared pair of electrons means both atoms contribute electrons that sit between them
  • These shared electrons are negatively charged, so they attract both positive nuclei, holding the atoms together

Key point: Covalent bonding happens between non-metal atoms. No electrons are transferred from one atom to another (that would be ionic bonding). Instead, electrons are shared.

Why do atoms form covalent bonds? By sharing electrons, atoms can achieve a stable electron configuration similar to noble gases. This makes them more stable and lower in energy.

2. Types of Covalent Bonds

Atoms can share different numbers of electron pairs:

Bond TypeElectrons SharedExample
Single bond2 electrons (1 pair)C–C
Double bond4 electrons (2 pairs)C=C
Triple bond6 electrons (3 pairs)C≡C or N≡N

3. Dot-and-Cross Diagrams

We use dot-and-cross diagrams to show covalent bonding. These diagrams:

  • Show only the outer shell electrons (valence electrons)
  • Use dots for electrons from one atom
  • Use crosses for electrons from the other atom
  • Show electrons in pairs

This helps us see which electrons are shared and which are not.

Single Covalent Bonds

Hydrogen, H₂

  • Each hydrogen atom has 1 outer electron
  • They share their electrons to form a single covalent bond
  • Each hydrogen now has 2 electrons (like helium, a noble gas)
H · + × H  →  H : H  (or H–H)

Chlorine, Cl₂

  • Each chlorine atom has 7 outer electrons
  • They each have 6 electrons already paired, and 1 unpaired
  • They share the unpaired electrons to form a single bond
  • Each chlorine now has 8 electrons in its outer shell (like argon)

Hydrogen chloride, HCl

  • Hydrogen has 1 outer electron
  • Chlorine has 7 outer electrons (1 unpaired)
  • They share one pair of electrons
  • Hydrogen gets 2 electrons, chlorine gets 8

Ammonia, NH₃

  • Nitrogen has 5 outer electrons (1 lone pair + 3 unpaired)
  • Each of 3 hydrogen atoms has 1 electron
  • Nitrogen shares its 3 unpaired electrons with the 3 hydrogens
  • Result: 3 single N–H bonds
  • Important: Nitrogen still has one lone pair (a pair of electrons not involved in bonding)

Methane, CH₄

  • Carbon has 4 outer electrons (all unpaired)
  • Each of 4 hydrogen atoms has 1 electron
  • Carbon shares each of its electrons with a hydrogen
  • Result: 4 single C–H bonds

Ethane, C₂H₆

  • Each carbon has 4 outer electrons
  • The two carbons share one pair of electrons with each other (C–C bond)
  • Each carbon then shares its remaining 3 electrons with 3 hydrogen atoms
  • Result: 6 C–H bonds and 1 C–C bond (all single bonds)

Double Covalent Bonds

Oxygen, O₂

  • Each oxygen has 6 outer electrons
  • They share 2 pairs of electrons (4 electrons total)
  • Result: One double bond (O=O)
  • Each oxygen still has 2 lone pairs

Carbon dioxide, CO₂

  • Carbon has 4 outer electrons
  • Each oxygen has 6 outer electrons
  • Carbon shares 2 electrons with each oxygen
  • Result: Two double bonds (O=C=O)
  • Linear structure

Ethene, C₂H₄

  • Each carbon shares 2 electrons with the other carbon (C=C double bond)
  • Each carbon shares its remaining 2 electrons with 2 hydrogens
  • Result: 4 C–H single bonds and 1 C=C double bond

Triple Covalent Bonds

Nitrogen, N₂

  • Each nitrogen has 5 outer electrons
  • They share 3 pairs of electrons (6 electrons total)
  • Result: One triple bond (N≡N)
  • Each nitrogen still has 1 lone pair
  • This triple bond is very strong, which is why nitrogen gas (N₂) is very unreactive

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