Chemical formula is written with the help of the symbolic representation of the element and its carrying capacity that is valency.

Symbols are given by the International Union of Pure and Applied Chemistry and are generally the first two letters of the compounds or the Latin Names of the compounds. While valency or the carrying capacity is the combining power of the element. It is the measure of how many a particular element say for example Sodium combined with other elements say chlorine.

Simply put, valency is like human hands: the number of hands we have we can hold that amount of material similarly in the case of the valency of the element.

Some basic concepts for writing formula are -:

• Valency – The carrying capacity of the element.
• Ion – charged species are known as ions based whether they are positively charged or negatively charged they are classified into two categories. These are as follows -Cations (these are positively charged ions), Anions (these are negatively charged ions). While writing a chemical formula the cations are written followed by the Anions.
• Atomicity- It is defined as the number of atoms constituting a molecule. For example, Oxygen – O2- Diatomic, Phosphorus – P4 is tetra-atomic, Polyatomic Sulphur – S8 

Rules of writing a chemical formula

Some of the basic rules while writing a chemical Formula are as follows-:

1. The valences of charges on the ion should be balanced.
2. When a compound is written than it is a rule that a metal is written followed by a nonmetal .So the order is name of metal followed by that of Non – metal .For example if we need write the formula for Sodium chloride that is common salt it is written as the symbol of sodium ( Na) followed by the symbol of Chloride ( Cl ) .Metal to the written on the left while the nonmetal on the right.
3. While writing the chemical formula for a polyatomic ion, the number of ions present in the compound are depicted by enclosing it in a circular bracket. For example – Mg (OH)2.

Methodology of writing the chemical formula 

The simplest compounds are binary compounds. While writing the Chemical formula the compounds are written followed by the valences and then the valences are criss-crossed to get the formula after simplifying the powers.

For example, to write the formula for Sodium chloride we have to write symbols first 

This is followed by writing the valency as follows -:

The number of Sodium atoms becomes 1 post cross and Chloride is also one so the chemical formula is – NaCl

Example -2 

Hydrogen sulphide 

Hydrogen – H   Sulphide – S

Valency -1      Valency – 2 

Criss cross 

The number of hydrogen atoms comes to be 2 and that of sulphide as 1 so representing the formula as – H2S as the number of hydrogens comes out to be 2 after crossing.

Formula for Baking soda 

Baking Soda which is used in baking for getting fluffy and spongy cakes and in other foods like pakoras etc.

The chemical Name for Baking soda is Sodium Hydrogen carbonate or sodium bicarbonate.

 The valency of Sodium is 1 and that of Hydrogen carbonate is also 1.

Sodium is the metal so it takes the left position while Hydrogen carbonate is nonmetal so it gets the right position. 

Symbol of sodium is Na 

While that of Hydrogen carbonate is HCO3  –

So, the formula is represented as follows

Sodium      Hydrogen carbonate            

Na               HCO3

1                  1

Post criss cross 

The number of atoms of both are 1 

Chemical formula – NaHCO3

Formula for Rust 

Rusting is a process that occurs when iron comes in contact with water or moisture and over a period of time a reddish-brown chemical compound named Rust is deposited on the surface. Rust is nothing but hydrated Ferric oxide 

. Iron has a chemical symbol of Fe and Oxygen as O and Hydrogen as H. As the ferric ion is used it means that the valency of Iron is 3 and of oxygen is 2 and Hydrogen 1.

Iron oxide formula 

Iron   oxide 

Fe     O

3      2      

 So, post criss crosses the number of atoms if Iron becomes 2 and that of oxygen as 3. hence the chemical formula is as follows-:

Fe2O3 – Iron oxide ——————(1)

But rust is hydrated iron oxide so it requires water molecules. These are as follows -Water contains Hydrogen symbol – H and oxygen symbol O with the valency of 1 and 2 respectively.

The formula of Water is as follows -:

Hydrogen   Oxygen

H                O

1                 2

Post criss cross

The formula is -H2O – Water————(2)

And combining 1 and 2 we get

Fe2O3.x H2O this is the formula for rust 

The chemical reaction for rusting is as follows -:

Fe + O2+xH20 ————–Fe2O3.xH20

Conclusion 

The Chemical formula is a symbolic representation of the chemical compounds. It is used in various chemical reactions. For writing a chemical formula there are certain prerequisites like the valency, atomicity and the symbols of the elements should be known. The valency is the combining power of the element that is the number of atoms of the other element it can hold followed by the symbol which is the abbreviated form of the element and then comes Atomicity which is the number of atoms of atoms that constitute a given molecule.