Writing Chemical Formulae

Rules of Writing Chemical Formulae

A chemical formula is used to represent the types of atoms and their numbers in a single molecule of an element or a compound by showing the elements present in a compound along with their relative proportions. The elements(s) or their atoms present in the compound are denoted by one or two or more different alphabets that symbolise the elements. There are specific rules for writing the chemical formula of a compound. Here you will learn about those rules and how to apply them. 

Difference between Chemical Formulae of Elements and Compounds

A molecule of an element is represented by stating the number of a single type of atoms present in it. For example, Chlorine (Cl) has two atoms in its molecule, thus it is represented by the chemical formulae Cl2.

On the other hand, the molecule of a compound is made up of two or more different elements that are arranged in a definite ratio through chemical bonds. For example, a water molecule is made up of two atoms of Hydrogen (H) and one atom of oxygen chemically bonded to each other, having a formula H2O.

Rules of Writing Chemical Formula

The following rules should be adhered to write the chemical formula of a binary compound,

  • One must know the valencies of the two elements or radicals present.

  • In a chemical formula, the sum of the total number of positive and negative valencies present in the compound should be zero. This can be done by finding the lowest common multiple of the two valencies.

  • If present, you must always place the metal at the beginning of the formula.

Write the Chemical Formula of the following

1. Write the chemical formula of calcium carbonate.

Ans: Valencies of elements and radicals:

Ca = 2 (Group II)

CO-23 =  -2

 The lowest common multiple of valencies = 4

Ca = 2 x 2 = 4

CO-23 =  2 x 2 = 4

Now,                  

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Ca2 (CO3)2

CaCO3

2. Write the chemical formula for magnesium chloride.

Valencies of elements and radicals:

Mg = 2 (Group II)

Cl =  -1 (Group VIIa)

The lowest common multiple of valencies = 2

Mg = 2 x 1 = 2

Cl =  1 x 2 = 2

Now,

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MgCl2

Chemical formulae is an important parameter that not only represents the ratios in which the elements combine but also significant while formulating any chemical equation.

FAQs (Frequently Asked Questions)

1. Write the Chemical Equation of Photosynthesis.

Ans: The process of photosynthesis has the following chemical equation: 

6CO2 + 6H2O → C6H12O6 + 6O2

The reactants consist of six carbon dioxide molecules and six water molecules which are converted by sunlight captured by chlorophyll into a glucose molecule and six oxygen molecules as the products.

2. X is an Element from Group 8 which shows Variable Valency. Name the Substance X and write its Formula when it Forms a Reddish-Brown Oxide.

Ans: Since X shows variable valency and forms a reddish-brown oxide, it has to be Iron (Fe) which is a metal of Group 8 (VIIIb) of the periodic table. It is the most-used and cheapest metal.

Thus X is Iron.

Now, the oxide which is reddish-brown in colour is ferric oxide.

Valencies of elements and radicals:

Fe = 3 (Group VIIIb)

O =  -2 (Group IVa)

The lowest common multiple of valencies = 6

Fe = 3 x 2 = 6

Cl =  2 x 3 = 6

Now,  

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The formula of ferric oxide: Fe2O3

3. a. Write the Chemical Formula of Washing Soda.

b. Write the Chemical Formula of Baking Soda.

c. Write the Chemical Formula of Plaster of Paris.

Ans: 

a. The Formula of Washing Soda:

Valencies of elements and radicals:

Na = 1 (Group I)

CO-23 =  -2

The lowest common multiple of valencies = 2

Na = 1 x 2 = 2

CO-23 =  2 x 1 = 2

Now,                                

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Na2CO3 – sodium carbonate


b. Formula of Baking Soda is:

Valencies of elements and radicals:

Na = 1 (Group I)

HCO3-1 =  -1

 Lowest common multiple of valencies = 1

Na = 1 x 1 = 1

HCO3-1 =  1 x 1 = 1

Now,            

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NaHCO3 - sodium bicarbonate


c. Plaster of Paris or PoP : As the material is commonly known as, is calcium sulphate hemihydrate as far as its chemical identity is concerned. The chemical formula of the compound is CaSO4.1/2H2O.