Those darn formulae!

Jayanthi Tiruvaeipati

Chemistry is the study of chemicals, their formulae and the reactions they undergo. Learning to write the formulae is important in order to understand the chemical reactions they undergo and the products they form. Why must a particular compound be written in a particular way is a question that students often ask. Though students know the formulae of some compounds like water, carbon dioxide etc., they are not taught why a particular compound is written the way it is written in VIII^th class. They write the formula of sodium chloride as NaCl₂ but the formula is NaCl. The students think that the symbol of sodium is Na and formula of chlorine is Cl₂ so the formula of sodium chloride should be NaCl₂ but this formula is incorrect because the valency of Na is one and Cl is one so the formula is NaCl. For some compounds when the valencies of the combining species, anion or cation, are equal they cancel each other. For example Ca is 2 and oxygen is also 2 so the formula of calcium oxide is CaO. Why it is done this way is also explained based on the valencies of the two species in a compound. The following discussion will help the students overcome the difficulty in understanding and writing the formulae of chemical compounds.

The chemical formula of a compound is a symbolic representation of its composition. To write the chemical formula, prior knowledge of symbols and combining capacities of elements is essential. The symbols of elements can be learnt from the periodic table. The knowledge of distribution of electrons in an atom gives an idea of the combining capacity of an atom of one element with the atoms of another element. The combining capacity of an element is known as valency. Ionic and covalent compounds can be written with the knowledge of the valency of elements and their ions (I have given examples of covalent compounds at the end of this article). The valency of an atom of element can be thought of as the hands or arms of that atom. An OCTOPUS has eight arms and HUMANS have two. One octopus can hold four human beings. If the octopus is represented as O and Human is represented as H then the formula should be OH₄. The subscript 4 indicates the number of humans held by the octopus.

A simple teaching aid to teach the students to write the chemical formulae can be prepared as follows:

Materials required: Cardboard, chart paper, scissors, sketch pens

1. Prepare a cardboard with two holders as shown in the picture and label them as cation and anion.



2. Make small placards of the size of the holders on the board.
3. Write the names and symbols of univalent, divalent, and trivalent simple and poly atomic ions on the cards.



4. Stack the cards representing metallic and non-metallic ions separately.
5. Insert the card for a metallic ion (positively charged) in the left holder *NH₄⁺ is a non-metallic cation.
6. Insert the card for a non-metallic ion (negatively charged) in the right holder.



7. Crisscross the charges (valencies) and write the formulae of the compounds.

While writing the formulae of chemical compounds certain rules are to be followed:

1. The valencies or charges on the ions must be balanced.
2. When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first on the left and the non-metal is written on the right.
3. In a compound formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of polyatomic ion is one, the bracket is not required.
4. Complete the below tabular column by using the crisscross method.

Reference: NCERT textbook class IX science

The author is a Chemistry teacher in Jawahar Navodaya Vidyalaya, Navodaya Vidyalaya Samiti in Andhra Pradesh. She can be reached at tiruvaeipati@gmail.com.