Covalent bonding in carbon
Carbon has 4 outer electrons. It cannot easily gain or lose four, so it shares electrons, forming covalent bonds. Covalent compounds have low melting points and do not conduct.
Why carbon shares electrons
Carbon (2, 4) needs four more electrons, or must lose four, to get a full outer shell. Gaining four would give C⁴⁻, but a nucleus with six protons cannot hold ten electrons. Losing four would give C⁴⁺, but that needs a very large amount of energy. So carbon shares its four outer electrons with other atoms. A bond formed by sharing a pair of electrons is a covalent bond.
Single, double and triple bonds
| Molecule | Pairs shared | Bond | Line structure |
|---|---|---|---|
| Hydrogen, H₂ | 1 | Single | H-H |
| Chlorine, Cl₂ | 1 | Single | Cl-Cl |
| Oxygen, O₂ | 2 | Double | O=O |
| Nitrogen, N₂ | 3 | Triple | N≡N |
| Methane, CH₄ | 4 (one with each H) | Four single bonds | C in the centre, joined to four H atoms |
How to draw electron dot structures
- Write the symbols of the atoms side by side.
- Show only the outer-shell electrons, as dots (or as dots and crosses for two different atoms).
- Put each shared pair between the two atoms that share it. You may draw overlapping circles for the outer shells.
- Check: each atom should now have 8 electrons around it (hydrogen: 2).
- Methane: C in the centre with four shared pairs, one to each H. Then draw the line structure with four single bonds.
- Oxygen: two shared pairs between the O atoms and two lone pairs on each O. Nitrogen: three shared pairs and one lone pair on each N.
Why covalent compounds behave as they do
| Property | Reason |
|---|---|
| Low melting and boiling points (methane boils at 111 K, ethanol at 351 K) | The forces between molecules are weak |
| Poor conductors of electricity | No ions are formed |
| Often liquids or gases at room temperature | The weak forces between molecules are easily overcome |
Allotropes of carbon
- Allotropes are different forms of the same element. Your textbook names three for carbon: diamond, graphite and the fullerenes.
- Diamond: each carbon atom is bonded to four other carbon atoms in a rigid three-dimensional structure. It is the hardest natural substance.
- Graphite: each carbon atom is bonded to three others in the same plane, in hexagonal layers stacked one above another. It is smooth and slippery, and a good conductor of electricity.
- Fullerenes: the first one found, C-60, has 60 carbon atoms arranged in the shape of a football.
- Their physical properties are very different, but their chemical properties are the same: each burns in oxygen to give carbon dioxide.
- Your textbook puts this in a box, and boxed material is not tested in the board exam. The hardness of diamond and the conductivity of graphite are also in the chapter on metals and non-metals, so learn those two facts.
The LPG in your kitchen (mainly butane) and the CNG in autos and buses (mainly methane) are covalent compounds. They are gases because the forces between their molecules are weak.
2 marks: “Why does carbon form covalent bonds?” Carbon has 4 outer electrons. Gaining 4 (C⁴⁻) or losing 4 (C⁴⁺) is very hard, so it shares electrons to complete its outer shell.
2 marks: “Why are most carbon compounds poor conductors of electricity?” They are covalent and do not form ions, so there are no charged particles to carry the current.
3 marks: “Draw the electron dot structures of (i) methane, (ii) oxygen, (iii) nitrogen.” CH₄: four shared pairs between C and four H. O₂: two shared pairs (double bond) and two lone pairs on each O. N₂: three shared pairs (triple bond) and one lone pair on each N.
C has 4 and shares 4. One shared pair is a single bond, two is double, three is triple: H-H, O=O, N≡N.