Atoms and Molecules: Class 9 Study Guide
Atoms combine in fixed ways, which lets us write formulae and calculate amounts.
The law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction. The law of constant proportions says that a compound always contains the same elements in the same ratio by mass. For water, the ratio of hydrogen to oxygen is 1 : 8 by mass.
John Dalton explained these laws by saying that matter is made of atoms, which join in fixed ratios. A molecule is a group of atoms held together, such as H₂O or O₂.
Atomic and molecular mass
The atomic mass unit (u) is a small mass unit used for atoms. Approximate atomic masses are H = 1, C = 12, N = 14, O = 16, Na = 23, Cl = 35.5. The molecular mass is the sum of the atomic masses. Water, H₂O, is 2 × 1 + 16 = 18 u. Carbon dioxide, CO₂, is 12 + 2 × 16 = 44 u.
| Substance | Formula | Molecular mass (u) |
|---|---|---|
| Water | H₂O | 18 |
| Carbon dioxide | CO₂ | 44 |
| Ammonia | NH₃ | 17 |
| Sodium chloride | NaCl | 58.5 |
The mole
A mole is a counting unit. One mole of any substance contains 6.022 × 10²³ particles, which is the Avogadro number. The mass of one mole in grams equals its atomic or molecular mass. So 1 mole of water is 18 g, and 36 g of water is 2 moles.
Add the atomic masses.
In grams.
Count the particles.
g, mol, number.
Writing formulae
The valency is the combining power of an atom. To write a formula, write the symbols, put the valencies below, cross them over and simplify. Magnesium (valency 2) and chlorine (valency 1) give MgCl₂. Aluminium (valency 3) and oxygen (valency 2) give Al₂O₃.
Worked examples and common mistakes
Example: How many moles are in 44 g of CO₂? 44 ÷ 44 = 1 mole. Example: The mass of 0.5 mole of water is 9 g. Common mistakes: writing the number of atoms as a big number in front of the formula, mixing molecular mass (u) with molar mass (g per mole), and forgetting to simplify the crossed-over valencies.
Check your understanding: model answers
1. 12 + 4 = 16 u. 2. 9 ÷ 18 = 0.5 mole. 3. CaCl₂.
✨ Key Takeaways
- Mass is conserved in a chemical reaction.
- A compound has a fixed ratio of elements.
- Molecular mass is the sum of the atomic masses.
- One mole has 6.022 × 10²³ particles.
💬 Questions for Discussion & Reflection
- Find the molecular mass of CH₄ (C = 12, H = 1).
- How many moles are in 9 g of water?
- Write the formula of calcium chloride (Ca valency 2, Cl valency 1).
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