Topic: Stoichiometry – Relative Masses of Atoms and Molecules
Lesson 1: Relative Atomic Mass (Ar)
| Section | Details |
|---|---|
| Objective | Describe relative atomic mass (Ar) as the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C. |
| Activities | – Starter (5 mins): Show a carbon atom model and explain its significance as the reference for atomic masses. – Main (25 mins): 1. Define relative atomic mass (Ar) and explain the concept of using isotopes to calculate Ar. 2. Demonstrate calculation of Ar using a simple example (e.g., chlorine with isotopes ³⁵Cl and ³⁷Cl). – Plenary (10 mins): Students solve questions to calculate Ar for elements given isotope data and relative abundances. |
| Resources | Atomic model diagrams, periodic table, worksheets with isotope data. |
| Time | 40 minutes total: 5 mins (Starter) + 25 mins (Main) + 10 mins (Plenary). |
| Homework | Calculate the Ar for oxygen given its isotopes: ¹⁶O (99.76%) and ¹⁷O (0.24%). |
| Assessment | Worksheet: Define and calculate relative atomic masses for elements with two isotopes. |
| Past Paper Practice | IGCSE Chemistry 0620/41/M/J/20 Q2(a) – Calculating relative atomic mass from isotope data. |
Lesson 2: Relative Molecular Mass (Mr)
| Section | Details |
|---|---|
| Objective | Define relative molecular mass (Mr) as the sum of the relative atomic masses and apply this to ionic compounds using relative formula mass (Mr). |
| Activities | – Starter (5 mins): Ask: How do we calculate the mass of a molecule? Introduce water (H₂O) as an example. – Main (25 mins): 1. Define relative molecular mass (Mr) as the sum of the Ar values of all atoms in a molecule. 2. Practice calculating Mr for simple molecules (e.g., H₂O, CH₄, CO₂). 3. Extend the concept to ionic compounds, defining relative formula mass (Mr) and calculating it for compounds like NaCl and CaCO₃. – Plenary (10 mins): Students calculate Mr for molecules and ionic compounds given their formulas. |
| Resources | Periodic table, worksheets with molecular formulas, whiteboard for demonstrations. |
| Time | 40 minutes total: 5 mins (Starter) + 25 mins (Main) + 10 mins (Plenary). |
| Homework | Calculate the Mr for NH₃, H₂SO₄, and Na₂CO₃. |
| Assessment | Worksheet: Calculate relative molecular and formula masses for a variety of molecules and compounds. |
| Past Paper Practice | IGCSE Chemistry 0620/41/O/N/21 Q4(a) – Calculating Mr for molecules and ionic compounds. |
Notes for Teachers:
- Emphasize the importance of the periodic table in providing relative atomic masses (Ar values).
- Use isotopic data to explain how Ar values are averages, making connections to isotopes.
- Provide practice with a variety of molecules and ionic compounds to reinforce the concept of Mr.
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