Physical Properties of Diamond

Structure of Diamond

Before we discuss the physical properties, it is important to understand diamond’s structure. This is because the structure explains why diamond has its unique properties.

In diamond, each carbon atom is bonded to four other carbon atoms. These bonds are arranged in a tetrahedral shape. This pattern repeats throughout the whole structure, forming a giant three-dimensional network.

All the bonds in diamond are strong covalent bonds. There are no weak bonds or gaps in the structure. This arrangement makes diamond extremely strong and gives it most of its special properties.

Detailed Physical Properties

1. Hardness

Diamond is the hardest natural substance known. It has a hardness of 10 on the Mohs scale, which is the highest rating possible. This means diamond can scratch any other natural material, but nothing natural can scratch diamond.

The extreme hardness comes from the strong covalent bonds connecting all carbon atoms in the 3D network. To scratch or break diamond, you must break many of these very strong bonds at once, which requires enormous force.

This hardness makes diamond valuable for cutting and drilling. Diamond-tipped tools are used in Nigerian industries to cut glass, drill through rocks for oil exploration, and shape other hard materials.

2. Appearance and Luster

Pure diamond is transparent and colorless, like clear glass. However, some diamonds contain tiny amounts of other elements that give them colors. For example, diamonds with boron appear blue, while those with nitrogen appear yellow.

When diamond is cut properly, it shows brilliant sparkle called luster. This happens because diamond bends light strongly. Light entering a cut diamond bounces around inside before coming out, creating the famous “diamond sparkle” that makes jewelry attractive.

The brilliant luster only appears when diamond is cut and polished. Raw diamonds straight from the ground look dull and unimpressive, like ordinary stones.

3. Electrical Conductivity

Diamond does not conduct electricity at all. It is an insulator. This is very different from graphite, which is also pure carbon but conducts electricity well.

Diamond cannot conduct electricity because all four outer electrons of each carbon atom are used to form covalent bonds. There are no free electrons to move and carry electric current.

This property is useful in electronics. Diamond can be used as an insulating material in devices that get very hot, because diamond can handle high temperatures without conducting electricity.

4. Density

Diamond has a density of 3.5 g/cm³. This is quite high compared to most substances. For comparison, water has a density of 1.0 g/cm³, and graphite (another carbon allotrope) has a density of only 2.3 g/cm³.

The high density exists because carbon atoms in diamond are packed very tightly together in the 3D structure. There is very little empty space between atoms.

You can use density to identify real diamonds. If you measure the mass and volume of a suspected diamond, you can calculate its density. If the density is not close to 3.5 g/cm³, it is not a real diamond.

5. Crystal Shape

Natural diamond crystals are octahedral in shape. An octahedron looks like two pyramids joined at their bases. It has eight triangular faces, six corners, and twelve edges.

Not all diamonds show perfect octahedral shape. Some are distorted or have irregular shapes due to how they formed. But the basic crystal structure tends toward the octahedral form.

This natural shape is different from the many-sided shapes you see in jewelry. Those shapes are created by cutting and polishing the diamond after mining.

6. Melting and Boiling Points

Diamond has an extremely high melting point of about 3550°C. At normal atmospheric pressure, diamond does not actually melt. Instead, it turns into graphite at temperatures above 1500°C when exposed to air.

To truly melt diamond, you need temperatures above 3550°C and very high pressure. These extreme conditions are rarely found except in laboratories or deep in the Earth.

The high melting point comes from the strong covalent bonds throughout the structure. Breaking down diamond requires breaking countless strong bonds, which needs tremendous heat energy.

7. Thermal Conductivity

Diamond is an excellent conductor of heat. In fact, it conducts heat better than any metal, including copper and silver. This might seem strange since diamond does not conduct electricity.

Diamond conducts heat through vibrations of atoms in the rigid structure. Heat energy passes quickly through the tightly connected network of atoms. This happens even though electrons do not move freely.

This property makes diamond useful in electronics. Diamond heat sinks can remove heat from computer chips better than metal heat sinks.

8. Refractive Index

Diamond has a high refractive index of about 2.42. Refractive index measures how much a material bends light. The higher the refractive index, the more the light bends.

When light enters diamond, it bends strongly and bounces around inside the crystal. This creates the brilliant sparkle when diamond is cut properly. The light breaks into rainbow colors as it exits, creating “fire” in the diamond.

This high refractive index is one reason diamond is so valuable for jewelry. No other colorless natural gem sparkles quite like diamond.

9. Insolubility

Diamond does not dissolve in water, acids, bases, or common solvents. The strong 3D network of covalent bonds cannot be broken by these liquids.

This chemical resistance makes diamond very stable. A diamond ring can last for generations without the stone being damaged by sweat, perfume, cleaning chemicals, or water.

10. Optical Properties

Pure diamond is transparent to visible light, ultraviolet light, and X-rays. This means these types of radiation can pass through diamond without being blocked.

However, some diamonds contain defects or impurities that absorb certain wavelengths of light. This gives the diamond color. The most valuable diamonds for jewelry are either perfectly colorless or have intense, even coloring.

Comparison with Graphite Properties

It is helpful to compare diamond properties with graphite, since both are pure carbon but have very different properties:

• Hardness: Diamond is the hardest; graphite is soft and slippery
• Electrical conductivity: Diamond does not conduct; graphite conducts well
• Density: Diamond (3.5 g/cm³) is denser than graphite (2.3 g/cm³)
• Structure: Diamond has 3D network; graphite has layers
• Appearance: Diamond is transparent; graphite is black and opaque

These differences all come from different arrangements of carbon atoms in the two allotropes.

Testing Diamond Properties

You cannot do most diamond tests in a school lab because diamonds are too expensive. However, you can understand how scientists test these properties:

Hardness test: Try to scratch diamond with other materials. Nothing natural can scratch it.

Density test: Measure mass on a balance and volume by water displacement. Calculate density = mass ÷ volume. Real diamond gives 3.5 g/cm³.

Electrical conductivity test: Connect diamond to a circuit with a battery and bulb. The bulb will not light because diamond does not conduct electricity.

Thermal conductivity test: Touch diamond to ice. The ice melts faster than with metal or glass because diamond conducts heat well.

Common Exam Mistakes

Practice Questions

Multiple Choice Questions

1. The density of diamond is:
   
   a) 2.3 g/cm³
   
   b) 3.5 g/cm³ ✓
   
   c) 1.8 g/cm³
   
   d) 5.0 g/cm³
2. Diamond does not conduct electricity because:
   
   a) It is transparent
   
   b) It is very hard
   
   c) All electrons are used in covalent bonds with no free electrons ✓
   
   d) It has octahedral shape
3. What is the crystal shape of natural diamond?
   
   a) Cubic
   
   b) Hexagonal
   
   c) Tetrahedral
   
   d) Octahedral ✓
4. Diamond is the hardest natural substance because:
   
   a) It is transparent
   
   b) It has high density
   
   c) Each carbon atom forms four strong covalent bonds in a 3D network ✓
   
   d) It does not conduct electricity
5. Which property is TRUE about diamond?
   
   a) It conducts electricity well
   
   b) It is soft and can be scratched easily
   
   c) It has excellent thermal conductivity ✓
   
   d) It dissolves in acids

Essay Questions

1. a) State five physical properties of diamond. (5 marks)
   
   b) Explain why diamond does not conduct electricity. (3 marks)
   
   c) Why is diamond used for cutting glass? (2 marks)
   
   Examiner’s Tip: For part (a), state properties clearly without explanation. For (b) and (c), you must explain using the structure of diamond.
2. a) Describe the structure of diamond. (4 marks)
   
   b) Explain how the structure of diamond accounts for its hardness. (4 marks)
   
   c) Compare the electrical conductivity of diamond and graphite. (4 marks)
   
   Examiner’s Tip: Draw a diagram for part (a) showing the 3D tetrahedral arrangement. In part (c), explain WHY each allotrope behaves differently.
3. a) What is the density of diamond? (1 mark)
   
   b) Explain why diamond has high refractive index. (3 marks)
   
   c) Describe how you would test whether a substance is diamond or glass. (6 marks)
   
   Examiner’s Tip: For part (c), mention at least three different tests (hardness, density, thermal conductivity). Explain what result you would expect for each material.
4. a) State three physical properties that diamond and graphite have in common. (3 marks)
   
   b) State three physical properties that differ between diamond and graphite. (3 marks)
   
   c) Explain why these two allotropes have different properties despite being pure carbon. (6 marks)
   
   Examiner’s Tip: In part (c), discuss atomic arrangement and bonding in detail. This is worth 6 marks, so write thoroughly.

Memory Aids

To remember diamond does NOT conduct electricity: “Diamond = DEAD to electricity” (all electrons are locked, none free)

To remember diamond properties (HOTD):

• Hard (hardest substance)
• Octahedral (crystal shape)
• Transparent (when pure)
• Dense (3.5 g/cm³)

To remember why diamond is hard: “Diamond is a 3D FORTRESS” (3-Dimensional network with four bonds making a fortress-like structure)

To remember thermal vs electrical conductivity: “Diamond is HOT YES, ELECTRIC NO” (conducts heat yes, electricity no)

Diamond density (3.5): Think “Diamond = 3.5” (same number of letters in “diamond” as first number, same for “three point five”)

Related Topics

• Allotropes of Carbon – Understand different forms of carbon including diamond
• Uses of Diamond – Industrial and decorative applications of diamond
• Graphite – Compare with another carbon allotrope
• Physical Properties of Graphite – See how properties differ from diamond
• Carbon – Learn about the element that forms diamond
• Covalent Bonding – Understand the bonds that make diamond structure