Meet Carbon
Carbon (C)
Carbon is a non-metal element, symbol C, atomic number 6. Its four outer electrons give it valency 4. It is abundant in rocks, fuels and all living things.
Question: Why can’t carbon simply lose or gain four electrons?
Answer: Removing or adding four electrons needs huge energy; sharing electrons in covalent bonds is easier.
Sharing Electrons
Understanding Covalent Bonds
What is a Covalent Bond?
A covalent bond forms when two atoms share a pair of electrons.
In H₂, each hydrogen supplies one electron; sharing lets both feel a complete set of two, locking them together.
Key Points:
- Covalent = shared electron pair.
- Sharing fills hydrogen outer shell (2 e⁻).
Single vs Double Bonds
O₂ molecule showing a double covalent bond
Bond order and strength
A single bond shares one electron pair between atoms.
Adding more shared pairs forms double and triple bonds, which pull atoms closer and increase bond strength.
Key Points:
- Single bond (C–H): longest & weakest.
- Double bond (C=C): medium length, stronger.
- Triple bond (C≡C): shortest & strongest.
- More shared pairs = higher bond energy; differentiate bonds by one, two, or three lines.
Methane Example
Electron-dot diagram of methane
Electron-Dot Structure of CH₄
Carbon is tetravalent; its four valence electrons seek four more to complete an octet.
In methane, carbon shares one electron with each hydrogen, making four C–H single bonds and giving every atom a full outer shell.
Key Points:
- 4 shared pairs = 4 covalent C–H bonds.
- Carbon achieves octet; each hydrogen attains a duplet.
- Dot diagram places C at centre with H atoms around it.
Diamond—A Carbon Gem
3-D covalent lattice of diamond
Allotrope & 3-D Network
Allotrope = same element, different atomic arrangement and properties.
Diamond is an allotrope where each carbon bonds to four others in a giant 3-D network.
Key Points:
- Strong C–C bonds in all directions make diamond Earth's hardest natural material.
- No free electrons, so diamond does not conduct heat or electricity.
Diamond vs Graphite
Diamond
Graphite
Key Similarities
Saturated Hydrocarbon — Ethane (C₂H₆)
Ethane structure with single C–C bond
How to identify a saturated compound
Ethane has two carbon atoms joined by a single bond.
Each carbon is fully bonded to hydrogens, so no extra atom can attach.
Key Points:
- Saturated = only single bonds (C–C, C–H).
- Molecular formula: C₂H₆.
- Breaking a bond is required to add another atom.
Unsaturated Hydrocarbon: Ethene (C₂H₄)
Structural formula of ethene showing the C=C double bond
Spot the C=C double bond
Ethene’s structure: two carbon atoms linked by a double bond, each carbon bearing two hydrogens.
The C=C bond makes the molecule unsaturated and highly reactive; atoms can add across it to form polyethene plastic.
Key Points:
- C=C double bond = unsaturation
- Double bond shows two parallel lines in the diagram
- Reactivity enables addition reactions, e.g., making plastic
Isomers of Butane
Skeletons of n-butane and iso-butane
Same Formula, Two Structures
Isomerism means one molecular formula can give different structural arrangements.
For butane \(C_4H_{10}\), the carbon backbone can be straight or branched, producing two isomers.
Key Points:
- Both share formula \(C_4H_{10}\).
- n-Butane: straight-chain of four carbons.
- iso-Butane: branched chain with one carbon side group.
- Different structures give slightly different boiling points.