Covalent bonding and structure

What is a covalent bond?

A covalent bond is the electrostatic attraction between the nuclei of two bonding atoms and a shared pair of electrons in their outer shells.

Which elements form covalent bonds?

Covalent bonding occurs between non-metal atoms.

How does covalent bonding achieve stability?

Each atom’s positive nucleus attracts the shared electrons, allowing atoms to achieve a stable noble gas configuration without transferring electrons.

What are the types of covalent bonds and how many electrons are shared?

Bond type	Representation	Shared electrons
Single	C–C	2
Double	C=C	4
Triple	C≡C	6

What is bond energy?

Bond energy is the energy required to break one mole of covalent bonds in the gaseous state.

• Units: kJ mol⁻¹

• Higher bond energy = stronger covalent bond.

What is average bond enthalpy?

The mean energy needed to break a bond in a gaseous molecule, averaged across many different compounds.

• Used as a measure of bond strength.

• Multiple bonds (double/triple) generally have higher enthalpies due to greater electron density.

How does bond length relate to bond strength?

In general:

• Shorter bonds are stronger bonds.

• Triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds.

What are exceptions to the octet rule?

• Expanded octet (period 3+)

  • Some central atoms can accommodate more than 8 outer electrons.

  • Examples:

    • SO₂ → expanded bonding around S

    • PCl₅ → 10 electrons around P

• Electron deficiency

  • Some central atoms have less than 8 electrons.

  • Example: BCl₃ → boron has only 6 outer electrons.

What is a dative covalent (coordinate) bond?

A covalent bond in which both electrons in the shared pair come from the same atom.

• Represented by an arrow (→) from the lone pair donor to the electron-deficient atom.

Examples of dative covalent bonding?

• Ammonium ion (NH₄⁺):

  • N in NH₃ donates a lone pair to H⁺.

  • Forms NH₄⁺ with a dative bond.

• Al₂Cl₆ dimer:

  • In AlCl₃, Al is electron deficient.

  • Two Cl atoms donate lone pairs to Al.

  • Forms two dative bonds → Al₂Cl₆.

Cl₂ is what type of bond

single

HCl is what type of bond

single

NH₃ is what type of bond

single bonds and lone pair

O₂ is what type of bond

double

CO₂ is what type of bond

two double bonds

C₂H₄ is what type of bond

double bond between carbons

N₂ is what type of bond

triple