Chemical Formula | How to Write Chemical Formula

In chemistry, a chemical formula is a shorthand way of describing a chemical compound, or a molecule with the help of symbols and valency of elements. It is one of the fundamental aspects of chemistry.

In other words, a chemical formula is the shortest way to represent a molecule or compound that consists of a symbol or a group of symbols with proper subscripts.

A chemical formula of a compound is a symbolic representation of its constituent elements and the number of atoms of each combining element. For example, the chemical formula for water is H2O, which tells that the molecule consists of two hydrogen atoms and one oxygen atom.

Importance of Chemical Formula in Chemistry

There is very importance of chemical formula in chemistry as it provides a quick and easy way to represent a complex molecule and compound. It represents the proportions in which the constituent elements join to form the compound.

A chemical formula provides the information about the types and numbers of atoms in a molecule or compound. This information is crucial for understanding the properties and behavior of chemicals.

We also use chemical formulas in chemical reactions. During a chemical reaction, atoms are rearranged, and new molecules are formed. The chemical formula allows us to predict the products of a reaction and balance the chemical equation to ensure the conservation of matter.

Types of Chemical Formulas

There are various types of chemical formulas used in chemistry. The most common types are molecular formula, empirical formula, and structural formula.

Molecular Formula:

A molecular formula is a symbolic notation that tells the types and numbers of atoms present in a molecule. It represents the actual number of atoms of each element in a molecule. In molecular formula, we denote elements by their respective symbols (as in the periodic table). We write the number of atoms of each element in the molecule as a subscript.

For example, the molecular formula for glucose is C6H12O6, telling that the molecule contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.

Empirical Formula:

An empirical formula is a symbolic notation that represents the simplest ratio of the atoms of each element in a molecule or compound. It is simply a molecular formula reduced to its lowest whole number ratio.

For example, the empirical formula for glucose is CH2O. We usually obtain empirical formula based on the analysis of experimental data. We can also derive the empirical formula from the molecular formula.

Structural Formula

A structural formula is a notation that represents the arrangement of atoms in a molecule. It exhibits the connections between atoms and bonds that hold them together. For example, the structural formula for glucose is:

   H                  H
   |                  |
H--C--OH       H--C--OH
   |                  |
H--C--OH  ->   H--C--OH
   |                  |
   H                  H

How to Write Chemical Formula in Chemistry

Writing a chemical formula is a straightforward once you understand the symbols and rules for combining elements. In order to write a chemical formula, you need to know the symbols for each element present in the compound, formula of the radicals, and the valency of the elements in that compound.. The valence is the number of electrons an atom has to lose, gain, or share to achieve a stable electronic configuration.

In chemistry, most of the compounds are binary compounds that consists of only two different elements. However, compounds having more than two elements are also known. We can write formulae of binary compounds using valencies as both types of atom forming a compound lose, gain, or share the same number of electrons.

There are the following key points that you should keep in the mind while writing a chemical formula of a simple compound or binary compound using valency.

Let’s take an example of aluminium oxide, for which we will write a correct chemical formula using the following rules discussed below.

(1) Write the chemical symbols of constituent elements. By the convention, write the symbol of cation first followed by the symbol of anion, shown in the below figure.

How to write a chemical formula of a simple compound

(2) Above each symbol, write the proper valence number as shown in the above figure.

(3) If the metal is present, always put it first (i.e. on the left side) in the chemical formula. In the above example, Al is a metal and oxygen is non-metal. Always write the non-metal on the right side.

(4) Then, obtain the proper formula by “crisscrossing” the valence numbers, dropping all positive or negative signs.

(5) Now if the subscripts are numerically equal or equal to 1, then drop them. For example: Na1Cl1 = NaCl, Ca2O2 = CaO

(6) Write the chemical formula such that the positive and negative charges must balance each other and the overall structure must be neutral.

(7) If a radical is present more than once, enclose it in parentheses and place the subscript outside the parentheses, as shown in the below figure. A radical is a group of covalently bonded atoms that consider as a single unit.

(8) Al last, rewrite the formula, dropping arrows and valencies written above. The correct formula of aluminium oxide is Al2O3, as shown in the above figure.

We check the neutrality of the formula by multiplying the valence of cation times its subscript to find the total positive valence. Repeat the same process for the anion to find the total negative valence. These two numbers must be equal to get a valid formula. Look at the below example to check the neutrality of the chemical formula Al2O3.

Al = +3 * 2 = +6
O = -2 * 3 = -6
So, overall charge on the compound = +6 – 6 = 0

You must take care to look up the valences of metal ions as many of these can have more than one valence.


In this chapter, we have discussed what is a chemical formula in chemistry and how to write it. Hope that you will have understood the basic concepts of writing the chemical formula and practiced all examples.
Thanks for reading!!!

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