Writing Chemical Formula: Steps, Examples

A chemical formula is a set of chemical symbols used to show the elements contained in a compound and their proportions. There are certain ways and rules of writing chemical formulas. Before learning how to write chemical formulas, let us explore chemical symbols and valency.

We can name simple compounds given the chemical formula if we know the names of the elements and a few fundamental guidelines. Carbon and copper both start with the letter CC Hydrogen and Helium start with the letter HH, and there are so many elements that start with the same letter. Still, every element has a specific symbol to represent it. Thus, symbols play an important role in identifying the elements. Let us learn how to write chemical formulas and the rules to follow. 

Chemical Symbols

A symbol is a short form that stands for the atom of a specific element. Each element is denoted by a symbol, usually the first letter of its name in English or Latin, written in capital.

Example: Sulphur, an element, is denoted by the symbol ‘\({\rm{S}}\)’ Similarly, hydrogen is denoted by the symbol ‘\({\rm{H}}\)’.

However, when the first letter of more than one element is the same, the elements are denoted by two letters. The first letter is written in capital, while the second one is written in small.

Example: Carbon, cobalt, and copper are elements whose first letter is\({\rm{C}}\) Therefore, only carbon is denoted by the symbol \({\rm{C}}\), Cobalt is denoted by two letters ‘\({\rm{Co}}\)’ Copper is denoted by the two letters ‘\({\rm{Cu}}\)’ that is taken from its Latin name cuprum.

Symbols of some common elements are given below:

Significance of Symbol

It represents  

1. Name of the element
2. One atom of the element
3. Quantity of the element equal in mass to its atomic mass or gram atomic mass.

Valency

Valency is the combining capacity of an atom or a radical.

An element or a radical’s valency is the number of hydrogen atoms that will combine with or displace one atom of that element or radical.

Since no other element has a combining capacity less than that of hydrogen, its valency is taken to be one and is considered a standard.

Modern Definition of Valency

The number of electrons that an atom can lose/accept during a chemical reaction is called its valency.

Elements with one, two or three electrons in their outermost shell are usually metals. Electrons in the outermost shell are known as valence electrons. To obtain a stable electronic configuration, these atoms lose their valence electrons and form positive ions- cations.

Elements with five, six or seven electrons in their outermost shell are normally non-metals. To attain stability, these atoms should have eight electrons in their outermost orbit, so they gain electrons and form negative ions- anions.

Below given is the table that shows the valency for valence electrons of elements:

Variable Valency

Certain elements exhibit more than one valency, i.e., they show variable valency.

If an element exhibits two different positive valencies, then the suffix “ous” for the lower valency and the suffix “ic” for the higher valency is used.

For example,

Radical

A radical is an atom or a group of atoms of the same or of different elements that behave as a single unit with a positive or negative charge.

For example, in the compound ammonium carbonate \({\left( {{\rm{N}}{{\rm{H}}_4}} \right)_2}{\rm{C}}{{\rm{O}}_3}\), ammonium \({\rm{NH}}_4^ +\) is a basic radical with combining power of \(1\) and carbonate \(\left( {{\rm{CO}}_3^{2 – }} \right)\) is an acid radical with a combining power of \(2\).

Writing Chemical Formula

The chemical formula of a compound is the systematic representation of the actual number of each type of atom in its molecule. The following methods are used to write the formula of a compound:

1. Mass ratio method
2. Valency method

Mass Ratio Method

The rules of writing formula from the mass ratio

i. Find the mass ratio of each element in the compound.
ii. Find the atomic ratio of each element.
iii. Change the atomic ratio into a whole number to get a simple ratio.

Let us see this method with the help of an example-

Example- To determine the molecular formula of water.
In an experiment, it was found that on electrolysis, \(90\,{\rm{g}}\) of water produced \(10\,{\rm{g}}\) of hydrogen and \(80\,{\rm{g}}\) of oxygen. Use this value to calculate (i) the mass ratio of hydrogen and oxygen in water (ii) the ratio of hydrogen and oxygen atom in water.

Solution:

Step I. Calculation of Mass Ratio

Mass ratio of hydrogen \({\rm{ = }}\,\frac{{{\rm{Mass}}\,{\rm{of}}\,{\rm{hydrogen}}}}{{{\rm{Mass}}\,{\rm{of}}\,{\rm{Water}}}}{\rm{ = }}\frac{{{\rm{10}}\,{\rm{g}}}}{{{\rm{90}}\,{\rm{g}}}} = \,\frac{1}{9}\)

Mas ratio of oxyzen \({\rm{ = }}\,\frac{{{\rm{Mass}}\,{\rm{of}}\,{\rm{hydrogen}}}}{{{\rm{Mass}}\,{\rm{of}}\,{\rm{Water}}}}{\rm{ = }}\frac{{{\rm{80}}\,{\rm{g}}}}{{{\rm{90}}\,{\rm{g}}}} = \,\frac{8}{9}\)

Mass of hydrogen : Mass of oxygen \({\rm{ = }}\,\frac{1}{9}:\frac{8}{9}\,\, = \,1\,:\,8\)

Step II. Calculation of the Atomic Ratio

Atomic ratio of hydrogen \({\rm{ = }}\,\frac{{{\rm{Mass}}\,{\rm{ratio}}\,\,{\rm{of}}\,{\rm{hydrogen}}}}{{{\rm{Relative}}\,{\rm{atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen}}}} = \frac{1}{1} = 1\)

Atomic ratio of oxygen \({\rm{ = }}\,\frac{{{\rm{Mass}}\,{\rm{ratio}}\,\,{\rm{of}}\,{\rm{oxygen}}}}{{{\rm{Relative}}\,{\rm{atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{oxygen}}}} = \frac{8}{{16}} = \frac{1}{2}\)

Atomic ratio \( = \,{\rm{H}}\,{\rm{:}}\,{\rm{O}}\,{\rm{ = }}\,{\rm{1:}}\frac{1}{2}\)

Step III. Change of Fractional Ratio into Whole Numbers

On multiplying the atomic ratio by \(2\) we get

Atomic ratio \( = \,{\rm{H}}\,{\rm{:}}\,{\rm{O}}\,{\rm{ = }}\,{\rm{1}}\,\, \times \,2\,:\,\frac{1}{2} \times \, = \,2\,:\,1\)

Therefore, the molecular formula of water is \({{\rm{H}}_2}{\rm{O}}\).

Valency Method

A compound’s chemical formula can also be written using the symbols and valencies of the atoms that constitute it.

Rules for writing the formulae of molecular compounds by valence method

i. The atomic symbol of each element is written side by side. The symbol of metal or a cation is written first, then that of the non-metal or an anion.
ii. The valency of the element is written below its symbol
iii. The valency of one atom is transferred to the right side bottom of the other atom.

That is, the valencies are criss-crossed. This also means that the valency of one element is made the subscript of the other element.

Let us see an example of how to write molecular formulas by the valency method.

Example. Write the formula of a molecule that is formed by the combination of carbon and chlorine atoms.

Solution: Step I. Write the symbol side by side.

Symbol\( \to \) \({\rm{C}}\) \({\rm{Cl}}\)

Step II. Write the valency below the respective symbol-

Symbol \( \to \) \({\rm{C}}\) \({\rm{Cl}}\)

Valency \( \to \) \({\rm{4}}\) \({\rm{1}}\)

Step III. Criss-cross the valencies

Thus, the formula of the molecule will be \({\rm{CC}}{{\rm{l}}_4}\) and the name of the molecule is carbon tetrachloride.

Below given are the formula of some molecules by valency method:

Summary

In this article, we studied the symbols and valency of chemical elements in detail. We also studied writing the chemical formula using two methods- mass ratio and valency.

1. Chemical formulae provide information about a compound’s chemical makeup.
2. They also show how the basic parts interact to generate the compound.
3. When representing a compound in a chemical equation, its chemical formula is critical.
4. Ions, radicals, and other chemical species can also be represented using chemical formulae.

Now, we know that the molecular formula depends on the element’s symbol and its valency.

FAQs

Q.1. How do you write chemical formulas?
Ans: A chemical formula represents a molecule that uses chemical symbols to represent the different types of atoms in the molecule, followed by subscripts to denote the number of each type of atom in the molecule. (When more than one atom of a given type is present, a subscript is used.)

Q.2. What is rust? Write its chemical formula?
Ans: Rust is a reddish-brown layer that forms on the surface of the iron as a result of the oxidation of the iron metal. The chemical formula of rust is \({\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}\)

Q.3. How to write the chemical formula of ionic compounds?
Ans: Simple ionic compounds are of binary nature, and the symbols of the cations and the anion are written side by side with their valencies at their bottom, and the criss-cross method is applied to transfer the valency of one atom to the other atom, and the chemical formula is obtained.

Q.4. What are the steps involved in writing the chemical formula using the valency method?
Ans: The steps involved in writing the chemical formula using the valency method are as follows:
i. The atomic symbol of each element is written side by side. The symbol of metal or a cation is written first, then that of the non-metal or an anion.
ii. The valency of the element is written below its symbol
iii. The valency of one atom is transferred to the right side bottom of the other atom.

Q.5. Define valency?
Ans: Valency is the combining capacity of an atom or a radical.
OR
The number of electrons that an atom can lose/accept during a chemical reaction is called its valency.

Q.6. What does the chemical symbol signify?
Ans: The significance of the chemical symbol is:
i. Name of the element
ii. One atom of the element
iii. Quantity of the element equal in mass to its atomic mass or gram atomic mass.

We hope this detailed article on writing chemical formulas is helpful to you. If you have any queries on this page or in general about chemical formula writing, ping us through the comment box below and we will get back to you as soon as possible.