4.1 Bonding in Carbon: the Covalent Bond
Carbon forms millions of compounds, yet it never forms ions. Find out why carbon shares electrons instead of giving or taking them, how shared pairs make single, double and triple bonds, why covalent compounds behave so differently from salts, and how the same carbon atoms make soft graphite, hard diamond and the football-shaped molecule C60.
Look around you: the paper of this book, the cotton of your shirt, the plastic of your pen, the fuel in a scooter, the sugar in your tea, the medicines in a pharmacy, even your own body — all are built on carbon. Chemists know well over ten million carbon compounds, many more than the compounds of all the other elements put together. Yet carbon is a very ordinary element: it makes up only a tiny fraction of the Earth's crust. What is it about the carbon atom that lets it build so much?
The answer starts with a surprise. In Chapter 3 you learned that atoms become stable by giving or taking electrons and so forming ions. Carbon does not do this. In this section you will see why, and what it does instead: it shares electrons, forming a covalent bond. You will draw electron-dot structures of hydrogen, oxygen, nitrogen, water, methane and carbon dioxide, see why covalent compounds melt at low temperatures and do not conduct electricity, and meet the three famous forms of pure carbon — diamond, graphite and the fullerenes.
Why Carbon Shares: the Covalent Bond
Every atom is most stable when its outermost shell is full — two electrons for the first shell (hydrogen and helium), eight for the later ones (Section 3.2). Sodium reaches this by losing its single outer electron to chlorine. Carbon has electrons in the arrangement 2, 4: four electrons in the outer shell, exactly half-way to eight. Look at what each of the two ionic routes would demand.
- Gain four electrons to become $\mathrm{C^{4-}}$: the nucleus has only 6 protons, and it would have to hold 10 electrons. Six positive charges cannot keep four extra electrons against their mutual repulsion.
- Lose four electrons to become $\mathrm{C^{4+}}$: only 2 electrons would be left around 6 protons, and a very large amount of energy is needed to pull four electrons away one after another.
Neither route is practical. The way out is for atoms to share electrons, so that each shared pair is counted as belonging to both atoms and completes the outer shell of each.
A simple example: two hydrogen atoms. Each has one electron and needs a second to fill its first shell. They bring their electrons together and share them. The shared pair is attracted to both nuclei at once and holds the atoms together, and each hydrogen now has the two electrons of helium. A hydrogen molecule $\mathrm{H_{2}}$ is formed. We draw such pictures with dots for the electrons of one atom and crosses for the electrons of the other; the dots and crosses are only labels, since all electrons are identical.
Electron-Dot Structures and Multiple Bonds
To draw the electron-dot structure of a molecule: write the symbols of the atoms side by side; give each atom its outer electrons (as dots or crosses); then pair them up between the atoms so that every atom ends with a full outer shell — 2 electrons for hydrogen, 8 for carbon, nitrogen, oxygen and chlorine. Electrons that are not shared stay on their own atom as lone pairs.
| Molecule | Outer electrons used (total) | Shared pairs | Lone pairs | Structure |
|---|---|---|---|---|
| hydrogen $\mathrm{H_{2}}$ | $1+1=2$ | 1 | 0 | H–H |
| chlorine $\mathrm{Cl_{2}}$ | $7+7=14$ | 1 | 6 | Cl–Cl |
| oxygen $\mathrm{O_{2}}$ | $6+6=12$ | 2 | 4 | O=O |
| nitrogen $\mathrm{N_{2}}$ | $5+5=10$ | 3 | 2 | N≡N |
| water $\mathrm{H_{2}O}$ | $6+1+1=8$ | 2 | 2 | H–O–H |
| methane $\mathrm{CH_{4}}$ | $4+4\times1=8$ | 4 | 0 | C with four C–H |
| carbon dioxide $\mathrm{CO_{2}}$ | $4+6+6=16$ | 4 | 4 | O=C=O |
Properties of Covalent Compounds
In an ionic crystal such as sodium chloride, strong forces hold every ion to its neighbours in all directions, so it takes great heat to melt it. A covalent compound is made of separate molecules. The bonds inside a molecule are strong, but the forces between one molecule and the next are weak. To melt or boil such a substance you only have to pull the molecules apart from each other, not break the bonds within them.
- Low melting and boiling points, because the forces between molecules are weak. Many are gases or liquids at room temperature (methane, ethanol), others are soft solids (candle wax, sugar).
- Poor conductors of electricity, because the electrons are held in bonds and no ions are present to carry a current.
- Usually insoluble in water but soluble in organic solvents such as petrol; exceptions such as sugar and ethanol dissolve in water.
- Exceptions: diamond has a very high melting point, and graphite conducts electricity — both are explained by their structure (next subsection).
| Substance | Type | Melting point | Conducts when molten? |
|---|---|---|---|
| sodium chloride $\mathrm{NaCl}$ | ionic | $1074\ \mathrm{K}$ ($801\ ^{\circ}\mathrm{C}$) | yes |
| methane $\mathrm{CH_{4}}$ | covalent | $91\ \mathrm{K}$ ($-182\ ^{\circ}\mathrm{C}$) | no |
| water $\mathrm{H_{2}O}$ | covalent | $273\ \mathrm{K}$ ($0\ ^{\circ}\mathrm{C}$) | very poorly |
Allotropes of Carbon: Diamond, Graphite and Fullerene
Pure carbon exists in several forms that look and behave completely differently — one is the hardest natural substance, another is so soft that it leaves a mark on paper. They differ only in how the same carbon atoms are joined.
- Diamond: each carbon is bonded to four others (single bonds) in a rigid three-dimensional network. It is the hardest natural substance, has a very high melting point, is transparent and does not conduct electricity, since every outer electron is used in a bond. Uses: jewellery, cutting and drilling tools.
- Graphite: each carbon is bonded to three others, forming flat hexagonal layers. The layers are held together only weakly and slide over each other, so graphite is soft and slippery; the fourth electron of each atom is free to move, so graphite conducts electricity. Uses: pencil leads, lubricant, electrodes.
- Fullerene: a molecule of carbon atoms arranged in a hollow cage. The first one found, $\mathrm{C_{60}}$ (1985), has 60 carbon atoms in 20 hexagons and 12 pentagons, like a football, and is named after the architect Buckminster Fuller, whose domes it resembles. Each carbon is bonded to three others.
1. “Carbon gives away or takes four electrons.” It does neither; it shares.
2. Forgetting lone pairs. Oxygen in water has two lone pairs and nitrogen in $\mathrm{N_{2}}$ has one each; they count towards the octet.
3. “Covalent bonds are weak, so covalent compounds melt easily.” The bonds within molecules are strong; it is the forces between molecules that are weak. In diamond the whole crystal is one giant molecule, which is why it is the exception.
4. “Graphite conducts because it is a metal.” It is a non-metal that conducts because one electron per atom is free.
5. Counting a shared pair once. A shared pair counts for both atoms: in $\mathrm{CH_{4}}$ carbon has four shared pairs, 8 electrons.
Exercises
- 1 What is a covalent bond? In which kind of elements does it occur?
- 2 Write the electron arrangement of C, H, N, O and Cl, and the number of electrons each needs to complete its outer shell.
- 3 Explain why carbon does not form $\mathrm{C^{4+}}$ or $\mathrm{C^{4-}}$ ions.
- 4 How many electrons does a carbon atom share in forming methane, and with how many atoms?
- 5 State the number of shared pairs in $\mathrm{H_{2}}$, $\mathrm{O_{2}}$ and $\mathrm{N_{2}}$ and name the type of bond in each.
- 6 What is meant by a lone pair? How many lone pairs has each oxygen atom in $\mathrm{O_{2}}$?
- 7 Why does a shared pair of electrons hold two atoms together?
- 8 Chlorine atoms have 7 outer electrons. How does the molecule $\mathrm{Cl_{2}}$ form? How many lone pairs does it have?
- 1 Draw (describe) the electron-dot structure of $\mathrm{H_{2}O}$ and give the number of lone pairs on oxygen.
- 2 Do the same for $\mathrm{CH_{4}}$ and $\mathrm{CO_{2}}$.
- 3 How many shared pairs and lone pairs does ammonia, $\mathrm{NH_{3}}$, have?
- 4 Hydrogen chloride, HCl: give the structural formula and the number of lone pairs.
- 5 Carbon tetrachloride, $\mathrm{CCl_{4}}$: how many shared pairs and how many lone pairs altogether?
- 6 Add up the outer electrons and check your drawing of $\mathrm{CO_{2}}$.
- 7 Explain why a double bond is drawn between the atoms in $\mathrm{O_{2}}$ but a single bond in $\mathrm{H_{2}}$.
- 8 Challenging. Hydrogen peroxide has the structure H–O–O–H. Count the shared pairs and lone pairs, and check with the total outer electrons.
- 1 Why do covalent compounds generally have low melting and boiling points?
- 2 Why are covalent compounds poor conductors of electricity?
- 3 A candle melts easily on a flame but a piece of rock salt does not. Explain.
- 4 Name one covalent solid that is very hard and one that conducts electricity.
- 5 Naphthalene balls disappear slowly at room temperature. What does this tell you about the forces between naphthalene molecules?
- 6 Substance P melts at $1273\ \mathrm{K}$ and conducts when dissolved in water. Substance Q melts at $195\ \mathrm{K}$, is a gas at room temperature and does not conduct. Which is ionic and which covalent? Express both melting points in °C.
- 7 Convert $91\ \mathrm{K}$ to °C and $-95\ ^{\circ}\mathrm{C}$ to K (use $T/\mathrm{K}=t/^{\circ}\mathrm{C}+273$).
- 8 Name three covalent compounds that are used as fuels.
- 1 What is allotropy? Name three allotropes of carbon.
- 2 Why is diamond very hard and a non-conductor of electricity?
- 3 Why does graphite conduct electricity and feel slippery?
- 4 How many other carbon atoms is each carbon bonded to in diamond, graphite and $\mathrm{C_{60}}$?
- 5 Describe the shape of a $\mathrm{C_{60}}$ molecule, and find its molecular mass.
- 6 Why can a pencil write on paper?
- 7 What mass of carbon dioxide is formed on burning $6\ \mathrm{g}$ of graphite in excess oxygen? ($\mathrm{C}=12$, $\mathrm{O}=16\ \mathrm{u}$.)
- 8 Applied. Which allotrope would you choose for (a) a glass cutter, (b) the electrode of a dry cell, (c) a lubricant for the moving parts of a hot machine, (d) a ring? Give a reason for each.
- 9 Challenging. Every atom of $\mathrm{C_{60}}$ has three bonds. How many bonds (lines) are there in the molecule? (Each bond joins two atoms.)
E1. Board · 1 mark The number of electrons shared between the two atoms in a nitrogen molecule is (A) 2 (B) 4 (C) 6 (D) 10
E2. Board · A–R Assertion (A): Graphite conducts electricity but diamond does not. Reason (R): In graphite each carbon atom is bonded to three others, leaving one electron per atom free to move. Choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
E3. Board · 3 marks Why do carbon compounds have low melting and boiling points and do not conduct electricity? Why is carbon unable to form ionic compounds easily?
E4. Board · Case-based A pencil “lead” is made of graphite, while the drill tip used by a dentist has tiny diamonds. A student learns that both are pure carbon, and that a third form, $\mathrm{C_{60}}$, has 60 atoms. (i) What name is given to these different forms of carbon? (1 mark) (ii) Explain why graphite is soft but diamond is hard. (2 marks) (iii) What is the molecular mass of $\mathrm{C_{60}}$? ($\mathrm{C}=12\ \mathrm{u}$) (1 mark)
E5. NEET Foundation. The total number of lone pairs of electrons in one molecule of carbon dioxide is (A) 2 (B) 4 (C) 6 (D) 8
E6. Board · 2 marks State the number of shared pairs and lone pairs in (a) $\mathrm{H_{2}O}$; (b) $\mathrm{NH_{3}}$; (c) $\mathrm{CH_{4}}$.
Research question. Can you tell, with only a battery, a bulb and a heat source, whether common kitchen and stationery substances are made of molecules (covalent) or of ions (ionic)?
Why it matters. Materials scientists choose materials by how their atoms are bonded: graphite for battery electrodes and pencils, diamond-coated tools for cutting stone, fullerenes and graphene for new electronics and medicines. The simplest tests — melting behaviour, conduction, solubility — are the first things a chemist does with an unknown solid.
What to do.
- Choose six safe samples: table salt, sugar, candle wax, a graphite pencil lead, a pinch of baking soda and a little tap water.
- Make a simple tester: a torch bulb, a 3 V battery and two wires with bare ends. Test each solid dry, then each dissolved in water, noting whether the bulb glows (never test mains-powered devices).
- Warm a small piece of wax and a pinch of sugar on a metal spoon over a candle flame (an adult present) and note which melts easily.
- Classify each sample as ionic, covalent molecular, or “giant covalent”, and explain your evidence.
Tools: torch bulb, 3 V battery, wires, spoon, candle, a notebook.
Hand in: A table of sample, melting behaviour, dry conduction, solution conduction and your classification, with two sentences on any surprise.
Going further. Why does the bulb glow when the pencil lead is connected but not when a diamond-like crystal is? What does that tell you about the electrons?
🧪 Self-check quiz
Q1. Carbon forms covalent bonds rather than ionic bonds because
Q2. How many shared pairs of electrons are there in one molecule of oxygen, O₂?
Q3. Which statement about covalent compounds is generally true?
Q4. Graphite conducts electricity because
Q5. The number of carbon atoms in a molecule of the fullerene named buckminsterfullerene is