Carbon and its Compounds • Topic 1 of 3

Covalent Bonding & Allotropes of Carbon

Carbon is the element that makes life possible. Its atomic number is 6, so its electronic configuration is 2, 4 — it has four electrons in its outermost shell. To complete its octet, carbon could in principle either gain four electrons (to become C4-) or lose four (to become C4+). Both are extremely difficult: gaining four electrons would force six protons to hold ten electrons, and losing four would need a huge amount of energy. So carbon does neither — instead it shares its four electrons with other atoms.

Covalent bonding

A bond formed by the sharing of a pair of electrons between two atoms is called a covalent bond. Each shared pair counts towards the octet of both atoms. In methane, CH4, one carbon atom shares one electron with each of four hydrogen atoms, forming four single covalent bonds, so carbon gets eight electrons and each hydrogen gets two. Covalent compounds usually have low melting and boiling points (the molecules are held by weak forces), are poor conductors of electricity (no free ions or electrons) and are often insoluble in water.

Tetravalency and catenation

Because carbon needs four more electrons, it forms four bonds — this is its tetravalency. Carbon can also bond with other carbon atoms to form long chains, branched chains and rings. This unique ability to link with itself is called catenation, and it is far stronger in carbon than in any other element because the carbon–carbon bond is very strong and stable. Together, tetravalency and catenation explain why carbon forms millions of compounds — more than all other elements combined.

Single, double and triple bonds

  • A single bond shares one pair of electrons, e.g. C−C in ethane (C2H6).
  • A double bond shares two pairs, e.g. C=C in ethene (C2H4).
  • A triple bond shares three pairs, e.g. C≡C in ethyne (C2H2).

Allotropes of carbon

The same element existing in different physical forms with different properties is shown by allotropy. Carbon has several allotropes. In diamond each carbon is bonded to four others in a rigid three-dimensional network, making it the hardest natural substance. In graphite each carbon bonds to only three others in flat hexagonal layers; the layers slide over one another, so graphite is soft, slippery and (because of free electrons) a good conductor of electricity. Fullerenes such as buckminsterfullerene C60 are cage-like molecules shaped like a football. All three are made only of carbon, yet their very different structures give them very different properties.

Covalent sharing in methane CH4 and the differing structures of diamond and graphiteMethane CH₄CHHHH4 shared pairs (tetravalency)Diamond (3-D)each C bonded to 4Graphite (layers)each C bonded to 3Allotropes: same element, different structure and properties
Solved Examples
1
Worked Example
Why does carbon form covalent bonds rather than ionic bonds?
Solution
  1. Carbon has 4 valence electrons; to form an ion it must either gain 4 electrons (giving C4-) or lose 4 (giving C4+).
  2. Gaining 4 electrons is hard because 6 protons cannot hold 10 electrons stably; losing 4 needs enormous energy.
  3. So carbon completes its octet by sharing four electron pairs — that is, by covalent bonding.

Answer: Because gaining or losing 4 electrons is energetically impossible, carbon shares electrons and forms covalent bonds.

2
Worked Example
Draw (describe) the electron-dot structure of methane CH4 and state how many electrons each atom has after bonding.
Solution
  1. Carbon (2, 4) places its 4 valence electrons around it; each of the 4 hydrogen atoms brings 1 electron.
  2. Each H shares one electron pair with C, forming four C−H single bonds.
  3. Carbon now has 8 shared electrons (octet); each H has 2 shared electrons (duplet).

Answer: Four C−H shared pairs; carbon gets 8 electrons, each hydrogen gets 2.

3
Worked Example
Define catenation and explain why carbon shows it more than other elements.
Solution
  1. Catenation is the self-linking of atoms of the same element to form chains, branches and rings.
  2. Carbon–carbon bonds are short and very strong, so the chains are stable.
  3. This strong, stable C−C bond lets carbon form chains of almost any length, far longer than other elements such as silicon can.

Answer: Catenation is self-linking of atoms; carbon shows it most because the C−C bond is exceptionally strong and stable.

4
Worked Example
How many shared electron pairs are present in a single, a double and a triple bond? Give one example of each.
Solution
  1. A single bond shares 1 pair of electrons, e.g. C−C in ethane (C2H6).
  2. A double bond shares 2 pairs, e.g. C=C in ethene (C2H4).
  3. A triple bond shares 3 pairs, e.g. C≡C in ethyne (C2H2).

Answer: Single = 1 pair (ethane), double = 2 pairs (ethene), triple = 3 pairs (ethyne).

5
Worked Example
Diamond is very hard but graphite is soft and slippery, although both are made only of carbon. Explain.
Solution
  1. In diamond each carbon is bonded to four others in a rigid 3-D network, so it is extremely hard.
  2. In graphite each carbon is bonded to only three others, forming flat layers held together by weak forces.
  3. The graphite layers slide over one another easily, making it soft and slippery.

Answer: The difference in structure (3-D network vs sliding layers) gives diamond hardness and graphite softness, even though both are pure carbon.

6
Worked Example
Why is graphite a good conductor of electricity while diamond is not?
Solution
  1. In diamond all four valence electrons of each carbon are used in bonding, so there are no free electrons to carry charge.
  2. In graphite each carbon uses only three electrons in bonding; the fourth electron is free to move within the layers.
  3. These mobile (delocalised) electrons let graphite conduct electricity.

Answer: Graphite has one free electron per carbon that can move and conduct; diamond has none, so it is a non-conductor.

Key Points

  • Carbon (atomic number 6, configuration 2,4) completes its octet by sharing four electron pairs, forming covalent bonds.
  • Tetravalency (four bonds) plus catenation (self-linking into chains, branches and rings) let carbon form millions of compounds.
  • Covalent compounds have low melting/boiling points, are poor conductors and are often insoluble in water.
  • Bonds can share one pair (single, C-C), two pairs (double, C=C) or three pairs (triple, C-C triple).
  • Allotropes of carbon: diamond (rigid 3-D network, hardest), graphite (sliding layers, soft, conducts) and fullerenes (cage-like, e.g. C60).
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The number of electrons in the outermost shell of a carbon atom is:
Explanation: Carbon has configuration 2, 4, so it has 4 valence electrons.
Q2.A covalent bond is formed by:
Explanation: A covalent bond is the sharing of a pair of electrons between two atoms.
Q3.The self-linking of carbon atoms to form long chains is called:
Explanation: Catenation is the ability of an element to bond with its own atoms forming chains and rings.
Q4.Which allotrope of carbon conducts electricity?
Explanation: Graphite has one free electron per carbon, so it conducts electricity; diamond does not.
Practice more MCQs → 📝 Assignment · 30 marks