The combining capacity of elements or the radicals with the other elements or radicals to form the molecule of an element or compound is called valency. Natural numbers such as 1, 2, 3, 4, 5, and 6 represents the valency.

The outermost orbit or shell of an atom is called valence shell and the number of electrons present in the valence shell of an atom is called valence electrons.

The molecular formula of an element is defined as the symbolic representation which shows the actual number of atoms in one molecule of the element.

The molecular formula of a compound is defined as the symbolic representation which shows the actual number of an atom of different elements present in one molecule of the compound.

The chemical reaction which is expressed in terms of full names of reactant and product molecules is called word equation. For example, Hydrogen reacts with oxygen to produce water. In word equation, we have

Hydrogen + oxygen→Water

The chemical equation which is written in terms of symbols by writing their molecular formula is called formula equation.
Heat Potassium chlorate → Potassium chloride + oxygen
KClO$$\overset{Heat}{\rightarrow}$$ KCl +O2

Following are the ways to write chemical equation:

When potassium chlorate is heated potassium chloride and oxygen is formed. Writing in chemical equation, we have

Step 1: Write the word equation.

Potassium chlorate  $$\overset{Heat}{\rightarrow}$$Potassium chloride + oxygen

Step 2: Convert the word equation into formula equation.

KClO3  $$\overset{Heat}{\rightarrow}$$KCl +O2

Step 3: Count the number of atoms, ions, etc of the reactant and product and make them equal by using the suitable number in front of them.

2KClO3  $$\overset{Heat}{\rightarrow}$$ 2KCl +3O2