Valency and Formulae
Class 9 Science: atoms and molecules (valency and chemical formulae)
Pick a metal ion and a negative ion, then add ions until the plus and minus charges cancel. The smallest numbers that balance give the chemical formula.
Words to know
- Valency
- The combining power of an atom: how many electrons it gives, takes or shares.
- Ion
- An atom (or group) with a charge, because it has lost or gained electrons.
- Cation
- A positive ion, like Na⁺ or Mg²⁺. Metals usually form cations.
- Anion
- A negative ion, like Cl⁻ or O²⁻.
- Chemical formula
- Symbols and numbers that show which atoms are in a compound and how many of each.
Quick check
Answer 3 questions to check what you learned.
Try at home
Read the labels on a salt packet, a baking soda box and an antacid. Find the chemical names (sodium chloride, sodium bicarbonate, magnesium hydroxide) and look up their formulae.