What is the Valency of Carbonate?

In Class 9 Chemistry (Atoms and Molecules), learning how to write chemical formulas using the criss-cross method requires you to memorize the valency of various ions. The Carbonate ion is a very common polyatomic ion you will encounter frequently.

Question (Click to Flip)

What happens when you add an acid to a carbonate?

Answer

Whenever any acid (like HCl) reacts with any metal carbonate (like CaCO3), it causes a brisk effervescence (fizzing) and releases Carbon Dioxide (CO₂) gas.

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Key Facts

Carbonate compounds are generally insoluble in water. The major exceptions to this rule are Sodium Carbonate, Potassium Carbonate, and Ammonium Carbonate, which easily dissolve in water.

The Formula and Charge

Chemical Formula: CO₃ (One Carbon atom bonded to three Oxygen atoms).
Charge: It has an overall ionic charge of -2 (written as CO₃²⁻).

The Valency

The valency of an ion is numerically equal to the magnitude of its charge (ignoring the plus or minus sign). Therefore, the valency of Carbonate is 2.

Why is the charge -2?

To understand this, look at the individual valencies:

Carbon (C) wants to share 4 electrons (Valency +4).
Oxygen (O) wants to gain 2 electrons (Valency -2). Since there are 3 oxygen atoms, the total requirement is -6.
Mathematical total: (+4) + (-6) = -2. The group of atoms is short of 2 electrons to be completely stable, which is why the whole molecule carries a -2 charge.

Writing Formulas using Carbonate

Because Carbonate has a valency of 2, it will combine with two atoms of a metal with valency 1, or one atom of a metal with valency 2.

Sodium Carbonate (Washing Soda): Sodium (Na) has a valency of 1. By cross-multiplying, the formula becomes Na₂CO₃.
Calcium Carbonate (Chalk/Limestone): Calcium (Ca) has a valency of 2. Since both Calcium and Carbonate have a valency of 2, they cancel out. The formula is simply CaCO₃.

Questions and Answers

What happens when you add an acid to a carbonate?+

Whenever any acid (like HCl) reacts with any metal carbonate (like CaCO3), it causes a brisk effervescence (fizzing) and releases **Carbon Dioxide (CO₂) gas**.

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