The Atomic Average Mystery
Topic 2.3.2: Calculating Relative Atomic Mass (Aᵣ)
📖 The Weighted Scale Story
Imagine you're at a particle playground. You have two types of chlorine atoms playing on a seesaw. 75 lightweight atoms (³⁵Cl) jump on the left side, while 25 heavier atoms (³⁷Cl) sit on the right.
Where does the balance needle point? Not at 36 (the middle), but at 35.5 — because there are MORE light atoms pulling the average DOWN!
🧮 Aᵣ = The Weighted Average of All Isotopes
⚖️ The Weighted Scale Simulator
Isotope A (Light)
Isotope B (Heavy)
🎯 Practice Mode: Calculate Magnesium's Aᵣ
Magnesium has three naturally occurring isotopes:
- • ²⁴Mg: 79% abundance
- • ²⁵Mg: 10% abundance
- • ²⁶Mg: 11% abundance
Calculate the relative atomic mass of magnesium.
🤔 Concept Check: Why is Chlorine 35.5?
A student says: "Chlorine's Aᵣ is 35.5 because each chlorine atom has 17.75 protons."
Is this correct? Explain why or why not.
📚 Key Takeaways
- •Aᵣ = Weighted average of isotope masses
- •Formula: ((Mass₁ × %₁) + (Mass₂ × %₂) + ...) ÷ 100
- •The number on the Periodic Table is an AVERAGE, not a single atom's mass
- •More abundant isotopes have a greater influence on Aᵣ
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