Writing Of Chemical Formulae

A chemical formula can be described as a set of chemical symbols that display the elements in any compound as well as their proportion. Before we delve into formulating chemical formulae, we must familiarise ourselves with their fundamentals. 

Difference Between Chemical Formulae of Elements and Compounds

The molecule of an element can be represented using a chemical formula by stating the symbol of the single type of atom present in it, along with its count. For instance, the molecule of chlorine (Cl) has two atoms. Therefore, it is represented by the chemical formula Cl2.

The molecule of a compound is composed of two or more elements that are combined in a certain ratio using chemical bonds. For instance, a molecule of the water compound is made by the chemical combination of two atoms of hydrogen (H) and one atom of oxygen (O). Thus, it is represented by the chemical formula H2O.

Atoms and Molecules

Atoms combine to make the materials and objects in our surroundings. Matter that has only one type of atom is known as an element. Atoms can also form molecules with complicated shapes. For example, carbon atoms join to form spherical molecules made of hexagonal and pentagonal units.

The following points indicate the need for using formulae to represent chemical compounds.

• Chemists have to write countless chemical equations every day and it would take them too much time to write the names of compounds and read them.

• Therefore, chemists make use of symbols as mathematicians do. Every element is represented by a unique symbol. There’s a specific method to describe what’s in each molecule.

• The chemical formula of a molecule has symbols for all the elements present in it. It often uses a number at the left to represent the count of atoms that comprise the element. For instance, the chemical formula for water is H2O. It indicates that one molecule of water is composed of two hydrogen atoms and one oxygen atom.

Writing Chemical Formulae

As mentioned earlier, the chemical formula is an expression of the number and types of atoms that exist in a molecule. Each element has its own symbol. For example, the symbol for hydrogen is H. In a chemical formula, the number of atoms of each type is indicated in the form of a subscript on the right-hand side of the symbol. For example, in the chemical formula for water, H2O, the subscript 2 indicates that the molecule has two atoms of hydrogen.

Methods for Writing Chemical Formulae

Step 1: Decide on the nature of the bond present in the molecule.

If prefixes are used, the bond is covalent. If prefixes are absent, the bond is ionic. 

Step 2: Identify the symbols of the atoms present in the molecule.

Step 3: Add a subscript in case a prefix is used. It will help balance the charge.

Notations for Chemical Equations

A chemical equation is composed of chemical formulas. It has reactants on the left and products on the right side of an arrow symbol (→). The arrow is usually read out as ‘yields’. In an equation, all chemical formulae on one side are separated by a plus sign. The physical state of each molecule is also indicated in parentheses as subscripts. The gaseous state of a compound is indicated as (g); the solid state is indicated as (s); the liquid state is indicated as (l), and the aqueous state is indicated as (aq). The aqueous state is used for bases, acids and other ionic substances when they are dissolved in water. 

For instance, the combustion of methane can be represented in the form of a chemical equation as shown below.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Common Symbols

Symbols can be used to differentiate between different kinds of reactions. Different types of arrows indicate different types of reactions. 

The following types of arrows are used in chemical equations:

• The forward arrow (→) indicates that the reaction is taking place in the positive direction. In this case, the reaction gives a higher yield of products.

• The backward arrow (←) indicates that the reaction is taking place in the backward direction. In this case, the reaction gives a higher yield of reactants.

• The bidirectional arrow (↔ or ⇋) refers to an equilibrium system. 

When the process requires power, it’s usually highlighted over the Arrow. The capitalised Greek alphabet delta (Δ) is added to the reaction arrow in order to show it is the case that heat or energy has been added to the reaction. The letter hv indicates that the energy is by way of light.

Rules for Writing Chemical Formulas of Binary Compounds with Valency

There are certain rules for writing chemical formulas. A chemical formula indicates the masses and mass proportions of the various elements that make up a compound.

A binary compound is composed of two distinct elements. The formulae of such compounds are written using valencies. Both kinds of atoms making up a binary compound either lose, gain, or share an equal number of electrons.

• To write the chemical formula of a binary compound, calculate the valencies of the two elements involved.

• Write the chemical formula in such a way that the sums of valencies of both elements are equal. Then, find the common factor of the two valencies.

• If there is a metal present, it must be placed at the top of the list in formula.

Conclusion

In a chemical equation, the reactants are written on the left side, while the products are written on the right side of the arrow. The coefficients of the formulae indicate the moles of a substance produced or utilised during the reaction. Chemical equations also include the physical state of reactants and products—whether they are aqueous (dissolved in water), solid, liquid, or gaseous.