Why Atoms Seek Octets

Octet Rule

Atoms are most stable when their valence shell holds 8 electrons. They reach this octet by losing, gaining, or sharing electrons.

This drive for an octet underlies ionic and covalent bonding in carbon compounds.

Why Not Ionic for Carbon?

Lose 4 e⁻ → C⁴⁺

Four ionisations need ≈ 5 000 kJ mol⁻¹.
Such immense energy is unavailable in normal reactions.
C⁴⁺ would quickly pull electrons back, so it is unstable.

Gain 4 e⁻ → C⁴⁻

Carbon must hold 10 electrons with only +6 charge.
Extra electron–electron repulsion makes C⁴⁻ highly unstable.
Energy released on gaining 4 e⁻ is far too small.

Key Takeaway

Both ionic routes are energetically prohibitive.
Carbon therefore shares electrons to complete its octet – covalent bonding.

Covalent Bond Defined

Covalent Bond

A covalent bond is a mutual sharing of one or more electron pairs between atoms, giving each a stable configuration.

Key Characteristics:

  • Formed by a shared electron pair
  • Shown as a single dash (—) in structural notation
  • Ensures each atom reaches octet or duplet stability

Example:

H–H represents the single covalent bond in a hydrogen molecule.

Single Bonds: Methane Example

Lewis dot diagram of CH4

Lewis dot diagram of CH₄

Visualising four shared pairs in CH₄

In the Lewis dot–cross diagram of methane, carbon places one dot on each side.

Each hydrogen adds one cross, pairing with a carbon dot to form a single covalent bond.

All atoms reach stability: carbon’s octet, hydrogen’s duet.

Key Points:

  • 4 C–H single bonds; four shared pairs.
  • Dots (●) from carbon, crosses (×) from hydrogen.
  • Diagram shows carbon’s tetravalency and octet completion.

Double Bonds: Ethene

Electron dot and line structure of ethene

Electron dot and line structure of ethene

Carbon–Carbon Double Bond in \(C_2H_4\)

In ethene, two electron pairs are shared between the carbon atoms.

One pair makes a strong σ bond; the other forms a π bond, so we draw C=C.

Key Points:

  • Two shared pairs → double bond (C=C).
  • Double bond is shorter and stronger than a single C–C bond.

Triple Bonds: Ethyne

Electron dot and line structure of ethyne

Electron dot and line structures of ethyne (C₂H₂)

Representing the C≡C bond

In ethyne, each carbon shares three electron pairs with the other carbon, forming a triple bond.

This bond is the shortest and strongest among carbon–carbon links.

Key Points:

  • Three shared pairs = triple bond (≡).
  • Draw as C≡C or with three lines in electron-dot model.
  • Shortest C–C bond length gives maximum bond strength.

Single vs Double vs Triple

As shared pairs increase, bonds become shorter and stronger.

Bond Type Bond Order Average Length (Å) Relative Strength
Single (C–C) 1 1.54 Weakest
Double (C=C) 2 1.34 Stronger
Triple (C≡C) 3 1.20 Strongest

Carbon Chains: Catenation

1

Self-linking ability

Each carbon forms strong covalent bonds with another carbon, allowing endless C–C connections.

2

Variety of structures

Repeated bonding builds long chains, branches, and rings—basis of organic diversity.

Pro Tip:

Covalent bonding lets carbon create stable, long chains—so you can easily outline catenation now!

Match Molecule to Bond Type

Drag each molecule to the matching bond column to prove you can classify single, double, and triple C–C bonds.

Draggable Items

CH₄
C₂H₄
C₂H₂
C₆H₆

Drop Zones

Single Bond

Double Bond

Triple Bond

Tip:

Count the shared electron pairs between the carbons—1 = single, 2 = double, 3 = triple.

Key Takeaways on Covalent Carbon

Tetravalent Sharing

Carbon shares four electrons, completing its octet by covalent bonding.

Varied Bond Types

Single, double, and triple covalent bonds give flexibility in structure and reactivity.

Endless Catenation

Carbon atoms link to themselves, forming long chains, rings, and branches.

Compound Diversity

These features create millions of organic compounds, proving carbon’s unmatched versatility.