Question

How to write the formula of calcium carbonate using the crisscross method.

Answer 1

Hint: The crisscross method is a method used for balancing the charge of an ionic compound by interchanging the charge numbers of reacting ions with the number of ions needed to form a neutral compound.

Complete answer:
Calcium carbonate is an ionic compound consisting of calcium ions and carbonate ions. Its molecular formula can be determined by the crisscross method of balancing charge.
Calcium is an alkaline earth metal belonging to the second group of the periodic table. This s-block atom has two unpaired electrons in its outermost valence shell and will be divalent in nature. Moreover it is highly electropositive in nature and will therefore lose two electrons to form a dipositive cation.
Carbonate is a divalent anion that accepts two electrons from the metal it is attached to.
In order to balance the charges, we need to make sure that the product of the number of cations in a molecule and its charge is equal to the product of the number of anions in the molecule and its charge.
 $\text{number of cations}\times \text{charge on cation}=\text{number of anions}\times \text{charge on anion}$
Therefore in the crisscross method, the charge on anion determines the number of cations in the molecule and the charge on cation decides the number of anions in the molecule and then the numbers are reduced to the simplest ratio possible.

Therefore the formula of calcium carbonate is $\text{CaC}{\text{O}}_3$ .

Note:
Ionic compounds consist of ions but the overall compound has to be neutral, therefore this crisscross charge balancing method is used to ensure the overall neutrality of the ionic compound. To use this method effectively the valencies of commonly used elements must be memorized or one should have a prior knowledge of the electronic configurations of frequently used atoms.