Why is calcium carbide CaC2 if the valency of carbon is 4 and calcium is 2?

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1. Usual valencies

Calcium (Ca) is a metal with a valency of +2, because it easily loses two electrons to form the ion Ca²⁺.

Carbon (C) usually has a valency of 4, meaning it can form four bonds (like in methane, CH₄).

So at first glance, you might expect calcium to combine with two carbons, each using up two valences — but the bonding in calcium carbide (CaC₂) is different from that simple picture.

2. The carbide ion structure

In calcium carbide, the carbon atoms are not separate. Instead, they exist as a carbide ion, written as C₂²⁻.

This ion consists of two carbon atoms triple-bonded together (similar to acetylene, C₂H₂), carrying an overall –2 charge.

So:

C≡C2−

3. How the formula balances

Calcium ion: Ca²⁺

Carbide ion: C₂²⁻

The charges +2 and –2 balance perfectly, giving the neutral compound CaC₂.

So, the formula is not based on the valency of a single carbon atom, but on the combined carbide ion (C₂²⁻) unit.

That’s a great question — and it shows you’re thinking deeply about how valency and bonding really work.

Calcium normally has a valency of +2 because it loses two electrons to form the ion Ca²⁺. Carbon, in most compounds, has a valency of 4 because it can form four bonds, as in methane (CH₄). So, at first glance, you might wonder why calcium carbide has the formula CaC₂ instead of something like CaC₄.

The answer lies in how the carbon atoms are bonded inside calcium carbide. In this compound, the two carbon atoms are not separate. They are joined together to form a special ion called the carbide ion, written as C₂²⁻. This ion consists of two carbon atoms connected by a triple bond (just like in acetylene, C₂H₂) and carrying an overall charge of minus two.

When calcium, which forms a Ca²⁺ ion, combines with the carbide ion C₂²⁻, the +2 charge from calcium balances the –2 charge from the carbide ion. This makes the compound electrically neutral, and its formula becomes CaC₂.

So, the formula is not based on the valency of a single carbon atom, but on the combined unit of two carbons in the carbide ion.

In summary, calcium carbide is CaC₂ because it contains one calcium ion (Ca²⁺) and one carbide ion (C₂²⁻). The charges balance perfectly, and the carbon atoms share a triple bond within the carbide ion.