Why does pure diamond not conduct electricity even though it is made only of carbon atoms?

Introduction / Context:
Diamond is a famous form of pure carbon that is extremely hard and widely used in jewellery and cutting tools. A common conceptual question in basic physics and chemistry is why this form of carbon does not conduct electricity, whereas other forms such as graphite can conduct. Understanding this contrast helps students connect ideas about bonding, electron movement, and electrical conduction in solids.

Given Data / Assumptions:

• We are considering pure crystalline diamond at normal temperature and pressure.
• Diamond is made only of carbon atoms arranged in a three dimensional tetrahedral lattice.
• We are interested in the ability of diamond to conduct electric current when a potential difference is applied.
• Conduction in solids requires mobile charge carriers, usually free electrons or ions.
• No impurities or defects are assumed in the crystal to provide extra charge carriers.

Concept / Approach:
The basic idea is that electrical conduction in a solid requires electrons that are not tightly bound and can move freely through the material when an electric field is applied. In metals, outer electrons are delocalised and form an electron sea. In insulators, valence electrons are tightly bound in covalent bonds and there is a large energy gap to the empty conduction band. Diamond is a classic example of a covalent network solid where each carbon atom forms strong sigma bonds with four neighbours, leaving no free electrons.

Step-by-Step Solution:
Step 1: In diamond, each carbon atom uses its four valence electrons to form four strong covalent bonds in a tetrahedral arrangement.Step 2: Because all valence electrons are tied up in these sigma bonds, there are no loosely bound or delocalised electrons that can move under an electric field.Step 3: The energy gap between the filled valence band and the empty conduction band in diamond is very large, so thermal energy at ordinary temperatures cannot excite electrons into the conduction band.Step 4: With no mobile charge carriers, an applied voltage cannot produce a sustained electric current, so diamond behaves as an electrical insulator and does not conduct electricity.

Verification / Alternative check:
We can verify this explanation by comparing diamond with graphite, another allotrope of carbon. In graphite, each carbon atom forms only three sigma bonds and has one electron in a delocalised pi system that can move along the layers, so graphite conducts electricity. The difference in conduction is therefore not due to the presence of carbon itself, but due to the bonding arrangement and the presence or absence of free electrons. Experimental measurements of resistivity also show that diamond has extremely high resistance, confirming its insulating character.

Why Other Options Are Wrong:
Option a is incorrect because a compact structure alone does not prevent conduction; many metals are compact yet conduct well. Option b is wrong because being crystalline does not automatically imply insulating behaviour; metals are also crystalline. Option c is incorrect because the presence of only carbon atoms does not stop conduction, as shown by graphite. Option e is wrong because diamond does not contain ions; it is a neutral covalent network with no positive or negative ions that could cancel out.

Common Pitfalls:
Students sometimes think that any solid made of a single element should behave like a metal, which is not true. Others confuse the idea of hardness with electrical insulation and assume that a hard material cannot conduct. Another common misconception is to imagine that electrons are always free to move inside a solid, without considering how strongly they may be bound in covalent bonds. It is important to link conduction to the presence of free charge carriers and to band structure, not just to the element name or physical hardness. Remember that in diamond the key reason for non conduction is the lack of free electrons in its covalent lattice.

Final Answer:
No free electrons are present in its covalent crystal lattice.