Meet Carbon
Model of a carbon atom
Why does carbon build countless compounds?
In the periodic table, carbon is in Group 14, Period 2.
With four valence electrons, it is tetravalent and forms four strong covalent bonds.
Strong C–C bonds allow extensive self-linking, called catenation, creating chains, rings and networks.
Quick check: How many covalent bonds can a single carbon atom form?
Key Points:
- Position: Group 14, Period 2.
- Tetravalency → 4 covalent bonds.
- Catenation forms long chains and rings.
- These traits explain carbon’s vast number of compounds.
Covalent Bonding
Electron sharing in a hydrogen molecule (H₂)
H₂: A Model Covalent Bond
Atoms that cannot easily lose or gain electrons become stable by sharing them.
In H₂, each hydrogen shares one electron; the pair forms a covalent bond and completes the 2-electron duplet for both atoms.
Key Points:
- Electron sharing binds the two atoms.
- Each H achieves a stable duplet (2 electrons).
- A covalent bond is a shared pair of electrons.
Single vs Double …and Triple Bonds
Counting Bond Pairs
Atoms share electrons as discrete bond pairs.
The number of shared pairs decides the bond order.
More pairs make bonds shorter and stronger.
Key Points:
- Single bond: 1 pair; longest, weakest. Example \( \text{H}_2 \).
- Double bond: 2 pairs; shorter, stronger. Example \( \text{O}_2 \).
- Triple bond: 3 pairs; shortest, strongest. Example \( \text{N}_2 \).
Example: Methane
Electron-dot diagram of methane (CH₄)
How the dots add up
Carbon shares its four valence electrons with four hydrogens in a tetrahedral sharing pattern.
The molecule is a saturated hydrocarbon; each C–H bond is single and every atom reaches stability.
Key Points:
- Place C in the centre; set 4 H at the corners of a tetrahedron.
- Draw one shared electron pair between C and each H.
- Check: C now has 8 electrons, every H has 2.
Carbon Allotropes
Diamond
Graphite
Key Similarities
Saturated vs Unsaturated
Ethene (C₂H₄) shows a carbon–carbon double bond.
How to spot saturation
Alkanes are saturated: each carbon forms four single bonds.
Alkenes contain at least one C=C double bond, making them unsaturated and more reactive.
Key Points:
- Single bonds only → saturated alkane.
- A C=C or C≡C means the molecule is unsaturated.
- Spot the double bond in ethene to classify it as an alkene.
Isomer Magic
Straight-chain (n-butane) vs branched (iso-butane)
Structural Isomers of Butane
Butane \( \mathrm{C_4H_{10}} \) exists as two structural isomers.
n-butane forms a straight carbon chain, while iso-butane branches at the central carbon.
Same formula, new layout—this change alters boiling point and other properties.
Key Points:
- Same molecular formula: \( \mathrm{C_4H_{10}} \)
- Different layout: chain vs branched
- Leads to different physical properties