Chemical Formulae

1) Hook — A Fun Real-Life Story

Imagine you are at a bustling Indian sweet shop in Delhi. You see colorful packets labeled “NaCl” and “C12H22O11” — but what do these strange codes mean? These are chemical formulae, the secret language of chemistry that tells us exactly what atoms make up a substance. Just like your favorite mithai has a recipe, every chemical has a formula that reveals its ingredients!

2) Core Concepts — Understanding Chemical Formulae

A chemical formula is a symbolic way to represent the elements in a compound and the number of atoms of each element.

Example: Water is written as H₂O, which means 2 atoms of Hydrogen and 1 atom of Oxygen combine to form water.

How to read a chemical formula?

• Each element is represented by its chemical symbol (e.g., Na for Sodium, O for Oxygen).
• The small number (subscript) after the symbol tells how many atoms of that element are present.
• If there is no number, it means only one atom of that element.

3) Key Formulas / Rules

4) Did You Know?

Salt (NaCl) was once so valuable in India that it was used as currency! The word “salary” comes from the Latin word for salt, since Roman soldiers were paid with salt. Today, chemical formulae help us understand and produce salt and many other compounds efficiently.

5) Exam Tips — Avoid These Common Mistakes

• Do not confuse subscripts with coefficients: In 2H₂O, “2” is a coefficient (2 molecules), “2” in H₂O is subscript (2 atoms of H per molecule).
• Remember the correct order of elements: For ionic compounds, positive ion first (e.g., NaCl, not ClNa).
• Use parentheses carefully: For polyatomic ions, e.g., Mg(OH)₂, the subscript applies to the entire OH group.
• Practice writing formulae from names and vice versa: Board exams often ask to write formulae of common compounds or name given formulae.
• Learn common ions and their charges: This helps in writing correct formulae (e.g., Na⁺, SO₄²⁻).