Carbon and its Compounds · Lesson 1 of 13
Bonding In Carbon – The Covalent Bond
“Carbon skips the ion drama and shares electrons instead.”
• Explain why carbon shares electrons instead of forming C⁴⁺ or C⁴⁻ ions. • Explain single, double and triple covalent bonds using simple molecules. • Draw and interpret electron-dot structures for hydrogen, oxygen, nitrogen, water, ammonia and methane. • Explain the typical melting, boiling and electrical behaviour of covalent compounds. • Compare diamond, graphite and fullerene as allotropes of carbon.
Carbon appears in food, medicines, fuels, fibres and living structures, yet the amount of carbon in the earth's crust and atmosphere is comparatively small. Its importance comes from the way carbon atoms bond. Before exploring the huge variety of carbon compounds, we need to understand why carbon usually forms covalent bonds.
Why Carbon Does Not Form Simple Ions
Carbon has atomic number six, so its electrons are arranged as two in the first shell and four in the outer shell. A completely filled outer shell is especially stable. Carbon therefore needs four additional electrons to complete an octet, or it would have to lose four electrons and leave only the first shell filled.
Neither route is favourable. If carbon gained four electrons, the resulting nucleus with six protons would have to hold ten electrons, including four extra electrons. If carbon lost four electrons, an enormous amount of energy would be needed to remove them. Carbon avoids both difficulties by sharing electrons with other atoms.
A Covalent Bond is a bond formed when atoms share one or more pairs of electrons.
A shared pair belongs to the outer shells of both bonded atoms. Electron sharing allows atoms to move toward a stable outer-shell arrangement without producing large, highly charged ions.
Formation Of Simple Covalent Molecules
Hydrogen Molecule
Each hydrogen atom has one electron and needs one more to fill its first shell. Two hydrogen atoms share one pair of electrons. Each atom then counts the shared pair as part of its outer shell.
Chlorine Molecule
A chlorine atom has seven valence electrons and needs one additional electron for a complete octet. Two chlorine atoms each contribute one electron to a shared pair, producing a single covalent bond in Cl₂.
Oxygen Molecule
Each oxygen atom has six valence electrons and needs two more. Two oxygen atoms therefore share two pairs of electrons. Two shared pairs form a double covalent bond.
Water Molecule
Oxygen needs two electrons and each hydrogen needs one. Oxygen shares one electron pair with each of two hydrogen atoms, giving two single O–H bonds. This satisfies the valency of oxygen and hydrogen.
Nitrogen Molecule
A nitrogen atom has five valence electrons and needs three more to complete an octet. Two nitrogen atoms share three pairs of electrons, producing a triple bond.
Ammonia Molecule
In ammonia, NH₃, nitrogen shares one electron pair with each of three hydrogen atoms. The three N–H single bonds allow the hydrogen atoms to complete their first shell while nitrogen completes its octet.
Methane And Carbon Tetravalency
Methane, CH₄, is one of the simplest carbon compounds and an important fuel. Carbon has four valence electrons and shares one electron with each of four hydrogen atoms. Four C–H single covalent bonds are formed. This demonstrates the tetravalency of carbon: carbon commonly forms four covalent bonds.
Tetravalency is the ability of carbon to form four covalent bonds because it has four valence electrons.
Properties Of Covalent Compounds
The bonds holding atoms together inside a covalent molecule can be strong, but the forces of attraction between separate molecules are usually much weaker. Consequently, many carbon compounds have lower melting and boiling points than ionic compounds.
Electron sharing also means that ordinary covalent compounds do not contain freely moving ions. As a result, they are generally poor conductors of electricity.
| Feature | Typical Covalent Compound Behaviour |
|---|---|
| Bond inside molecule | Strong covalent bond |
| Force between molecules | Comparatively weak |
| Melting and boiling points | Generally low compared with ionic compounds |
| Charged particles | No ions formed by ordinary covalent bonding |
| Electrical conduction | Generally poor |
Allotropes Of Carbon
An Allotrope is one of the different structural forms in which the same element can exist.
Diamond
In diamond, each carbon atom is bonded to four other carbon atoms, creating a rigid three-dimensional arrangement. This strong network makes diamond extremely hard.
Graphite
In graphite, each carbon atom is bonded to three other carbon atoms in planar hexagonal arrangements. These layers are stacked one above another. Graphite is smooth and slippery and, unlike most non-metals, it conducts electricity.
Fullerenes
Fullerenes are another class of carbon allotropes. The first identified fullerene, C₆₀, contains carbon atoms arranged in a cage resembling a football. It is called buckminsterfullerene.
Synthetic Diamonds
Diamonds can also be synthesised by subjecting pure carbon to very high pressure and temperature. These synthetic diamonds are small but otherwise closely resemble natural diamonds.
Quiz
Why does carbon generally form covalent bonds instead of C⁴⁺ ions?
How many shared electron pairs make a double covalent bond?
Which molecule contains a triple covalent bond?
Why are many covalent compounds poor conductors of electricity?
Which statement correctly compares diamond and graphite?
Practice Problems
- Explain step by step why forming C⁴⁺ and C⁴⁻ ions is difficult for carbon.
- Draw electron-dot structures for H₂, O₂ and N₂ and identify the number of shared pairs in each.
- Explain how the structure of methane demonstrates carbon's tetravalency.
- Draw or describe the bonding in water and ammonia using shared electron pairs.
- Explain why covalent compounds generally have lower melting and boiling points than ionic compounds.
- Compare diamond and graphite in terms of carbon bonding, structure and physical properties.
- Explain what makes C₆₀ a fullerene.
Key Takeaways
• Carbon usually achieves a stable outer shell by sharing electrons rather than gaining or losing four electrons. • A single, double and triple covalent bond contains one, two and three shared electron pairs respectively. • Methane demonstrates carbon's tetravalency through four C–H covalent bonds. • Covalent compounds are generally poor electrical conductors because ordinary covalent bonding does not create mobile ions. • Diamond, graphite and fullerene are allotropes of carbon with different structures and therefore different physical properties.
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Versatile Nature Of Carbon