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Target: calculate neutrons using A − Z and distinguish mass number from standard atomic weight. Prepare: Cards B/D, count mat; examples carbon-12 (A 12 Z 6), oxygen-16 (A 16 Z 8), magnesium-24 (A 24 Z 12).
Launch · 2 min. Offer A = protons + neutrons and ask which count must be known before subtraction.
Model · 4 min. Carbon-12: p 6, n 12 − 6 = 6. A is an integer count for this isotope, not a decimal average from a periodic-table tile.
Guided reading · 5 min. Work oxygen-16: n 16 − 8 = 8. Let learners check by adding p + n = 16, then inspect magnesium-24 without supplying its answer.
Practice route · 7 min. Choose Day 3 route; derive all three p/n counts and explain which number a rounded periodic weight cannot substitute for.
Audit · 4 min. Partner proposes n = A + Z; learner tests carbon-12 and repairs with reverse-addition evidence.
Exit · 3 min. “Magnesium-24 with Z 12: neutrons?” Key: 12; explain 12 + 12 = 24.