Chapter 4 · Question 4

Describe the three allotropes of carbon — diamond, graphite, and buckminsterfullerene — with their structures and one key property of each. Why do they have different physical properties despite being made of the same element?

Answer Revealed

Direct Answer:

The three allotropes are different structural forms of elemental carbon with the same chemical properties but different physical properties. (i) Diamond: each carbon atom is bonded to four other carbon atoms in a rigid 3D tetrahedral network. It is the hardest known natural substance. (ii) Graphite: each carbon atom is bonded to three others in a hexagonal planar array, forming layers stacked one above the other. It is soft, slippery, and a good conductor of electricity (unlike most non-metals). (iii) Buckminsterfullerene (

\text{C}_{60}

): carbon atoms arranged in a football-like spherical structure, named after architect Buckminster Fuller. Their physical properties differ because the arrangement and bonding of carbon atoms are different in each allotrope.

Simple Explanation

Diamond, graphite, and buckminsterfullerene are three forms of the same element — pure carbon — just arranged differently. In diamond, every carbon atom makes four bonds in a tight 3D network, making it the hardest substance. In graphite, carbon atoms form flat sheets like stacked paper — each atom makes only three bonds, so the fourth electron can move freely, making graphite conduct electricity. Buckminsterfullerene (

\text{C}_{60}

) looks like a molecular soccer ball made of 60 carbon atoms. Same element, different arrangement → different properties.

Exam-Ready Structure

Allotropes are different structural modifications of the same element in the same physical state. Carbon exhibits three well-known allotropes with strikingly different physical properties: 1. Diamond: (a) Structure — each carbon atom is covalently bonded to four other carbon atoms in a rigid three-dimensional tetrahedral network. (b) Key property — it is the hardest naturally occurring substance known. (c) Synthesis — synthetic diamonds can be made by subjecting pure carbon to very high pressure and temperature; they are small but otherwise identical to natural diamonds. 2. Graphite: (a) Structure — each carbon atom is bonded to three other carbon atoms in the same plane, forming a hexagonal (honeycomb) array. One of these bonds is a double bond. The hexagonal sheets are stacked in layers one above the other with weak forces between layers. (b) Key property — graphite is smooth, slippery, and an excellent conductor of electricity, which is unusual for a non-metal. The conductivity arises because the fourth valence electron of each carbon is delocalised between the layers. (c) Uses — pencil lead, lubricants, electrodes. 3. Buckminsterfullerene (

\text{C}_{60}

): (a) Structure — 60 carbon atoms arranged in a spherical shape resembling a football (soccer ball), with a pattern of pentagonal and hexagonal rings. (b) Naming — named fullerene because the structure resembles the geodesic dome designed by architect Buckminster Fuller. (c) Key property — it is the first identified member of the fullerene family of carbon allotropes and has unique molecular properties investigated in nanotechnology research. 4. Reason for different physical properties: Although all are composed only of carbon atoms, the way the atoms are bonded and arranged (crystal structure, bonding pattern, dimensionality) is different in each allotrope. These structural differences directly affect properties such as hardness, electrical conductivity, and appearance, while chemical properties (combustion to

\text{CO}_2

) remain the same.

Key Points

Allotropes: different structural forms of the same element in the same physical state
Diamond: 3D tetrahedral network, each C bonded to 4 C atoms — hardest natural substance
Graphite: hexagonal layers, each C bonded to 3 C atoms — slippery, good electrical conductor
Buckminsterfullerene ( $\text{C}_{60}$ ): football-shaped sphere of 60 carbon atoms
Different physical properties arise from different atomic arrangements/bonding patterns

Describe the three allotropes of carbon — diamond, graphite, and buckminsterfullerene — with their structures and one key property of each. Why do they have different physical properties despite being made of the same element?