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Language of Chemistry

ICSE Class 7 Chemistry • Chapter 4

Chapter Overview

Just like English uses letters to make words and words to make sentences, Chemistry has its own unique language. The letters are Symbols of elements, the words are Chemical Formulae, and the sentences are precise Chemical Equations. This chapter introduces you to this global scientific language.

4.1 Chemical Symbols

A chemical symbol firmly represents exactly one single atom of a specific element. It is a highly abbreviated shorthand notation developed by J.J. Berzelius.

Rule 1: The first letter of the element’s English name is utilized (capitalized). Example: Hydrogen ($H$), Oxygen ($O$), Carbon ($C$).
Rule 2: If two elements start with the exact same letter, a second prominent letter is added (written in small case). Example: Calcium ($Ca$), Cobalt ($Co$), Chlorine ($Cl$).
Latin Names: Many highly common metals derive their symbols directly from their ancient Latin names. Example: Sodium (Natrium $\rightarrow Na$), Iron (Ferrum $\rightarrow Fe$), Potassium (Kalium $\rightarrow K$), Gold (Aurum $\rightarrow Au$).

4.2 Writing a Chemical Formula (Criss-Cross Method)

The chemical formula firmly dictates the exact number of specific atoms of each element present in one molecule of a compound.

We use the famous Criss-Cross Method utilizing the valencies we learned in the previous chapter to write a perfect formula.

Steps for the Criss-Cross Method

Let's logically deduce the formula for Calcium Chloride.

Write the symbol of the basic radical (metal) heavily on the left and the acidic radical (non-metal) directly on the right:
$Ca \quad Cl$
Write their respective exact valencies clearly on top of them:
$Ca^2 \quad Cl^1$
Strictly criss-cross the valency numbers physically transferring them as a subscript to the opposite radical:
$Ca$ gets $1$, $Cl$ gets $2$.
Ignore writing the number $1$. Ensure any common mathematical factors are carefully divided out. The final flawless formula is precisely:
$CaCl_2$

Example 2: Aluminium Sulphate

Symbols: $Al$ and $SO_4$
Valencies: $Al^3$ and $SO_4^2$
Criss-cross: $Al$ gets $2$, the entire $SO_4$ group gets $3$.
Since $SO_4$ is a polyatomic radical receiving a number totally greater than $1$, we must logically enclose it strictly in brackets.
Final Formula: $Al_2(SO_4)_3$

4.3 Chemical Equations

Chemical Equation: A completely symbolic, shorthand scientific representation of an entire chemical reaction using exact formulas and specific symbols.

Reactants: The starting substances undergoing the change (written on the Left Hand Side).
Products: The entirely new substances physically formed during the reaction (written on the Right Hand Side).
The Arrow ($\rightarrow$): Means "yields" or "produces".

Example Equation:

$C + O_2 \rightarrow CO_2$

This simply translates to: One solid atom of Carbon heavily reacts perfectly with one diatomic molecule of Oxygen gas to seamlessly yield precisely one entire molecule of Carbon dioxide gas.

AI Image Prompt: A vibrant visual showing a chemical reaction. On the left (Reactants), show a black lump of coal (Carbon) and floating blue $O_2$ pairs. A glowing yellow energetic arrow physically points to the right (Products), showing stable $CO_2$ molecules floating away (one black sphere sandwiched solidly between two blue spheres).

Practice Zone

Q1. Determine the accurate chemical formula for Sodium Carbonate.

Ans: Symbols: $Na$ and $CO_3$. Valencies: $Na^1$ and $CO_3^2$. Criss-crossing gives $Na$ the $2$ and $CO_3$ the $1$. Making the final solid formula exactly $Na_2CO_3$.