Deducing Formulae from Diagrams
Master the art of counting atoms and determining chemical ratios
1The Hook: Chemistry is Like LEGO Counting
Imagine you have a LEGO creation in front of you. To rebuild it, you need to know: "How many red bricks? How many blue bricks?" That's exactly what we do with molecules. We count the atoms to write the molecular formula.
The Rule: A molecular formula tells you the number and type of atoms in ONE molecule.
2Interactive: Molecule Counter (Glucose)
Rotating Glucose Molecule
Rotate to count from all angles
Your Task: Count the Atoms
3The Zoom: Ionic Compounds (Giant Lattices)
Ionic compounds don't exist as single molecules. They form giant lattices—like a 3D crystal of alternating positive and negative ions. To find the formula, we look at the ratio of ions in the structure.
Key Principle: Count how many of each ion surround a central ion. If an ion is shared between multiple units, divide by the number of sharers!
Silicon Dioxide (SiO₂) Lattice Slice
Legend:
Interactive Ratio Finder
Step 1:
Click on ONE Silicon atom (gray sphere) in the diagram
4Challenge: Correct the Equation from Diagrams
A student drew this equation using particle diagrams. But the atom count is WRONG! The diagram shows 2 O₂ molecules + 1 C atom making 2 CO₂ molecules. This violates the Law of Conservation of Mass! Count the particles and fix it.
O₂
O₂
C
CO₂
CO₂
Count the Atoms in the Diagram
Reactants (Left Side)
Products (Right Side)
❌ PROBLEM: 1 C atom in → 2 C atoms out!
Atoms cannot be created. We need more carbon on the left!
Fix the Equation
To balance this equation, we need equal atoms on BOTH sides. How many of each reactant do we need to make 2 CO₂ molecules?
⚠Examiner's Trap Alert
TRAP: Forgetting Shared Atoms
In lattice structures, corner/edge atoms are SHARED. Always divide by the number of units sharing them.
TRAP: Molecular vs Empirical Formula
Molecular (C₆H₁₂O₆) shows ACTUAL atoms in one molecule. Empirical (CH₂O) shows SIMPLEST ratio. For ionic compounds, we ALWAYS write empirical formulas.
TRAP: Not Balancing Equations
Diagrams show PARTICLES, not coefficients. Count ALL atoms on both sides. Atoms cannot be created or destroyed!
Summary: The Formula Detective Checklist
For Molecular Compounds:
Count atoms of each element. Write as: Element₁ᵃElement₂ᵇElement₃ᶜ
For Ionic Lattices:
Find ratio of ions. Account for sharing. Simplify to lowest whole numbers.
For Equations from Diagrams:
Count particles, not just types. Balance ATOMS on both sides (Law of Conservation of Mass).
🔑 Golden Rule:
The diagram is your evidence. Count carefully. Rotate if needed. Check twice, write once!
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