Valency & Formula Puzzles

• what valency (combining capacity) means and the valencies of common elements.
• how to use the "criss-cross" method to work out the chemical formula of a compound from valencies.
• olympiad-style puzzles that work backwards: given a formula, find the unknown valency.

1. Valency — the combining capacity of an element, e.g. H = 1, O = 2, N = 3, C = 4, Na = 1, Mg = 2, Al = 3.
2. Criss-cross method — the valency of one element becomes the subscript of the other in the formula, then simplify if possible.
3. Working backwards — from a known formula you can read off an unknown element's valency (e.g. MCl2 tells you M has valency 2).
4. Radicals as a unit — groups like SO4 (valency 2), CO3 (valency 2), NH4 (valency 1) combine as a single unit, sometimes needing brackets, e.g. (NH4)2SO4.

Puzzle: Aluminium has valency 3 and oxygen has valency 2. What is the formula of aluminium oxide?

Step 1 — write symbols: Al and O
Step 2 — criss-cross valencies: Al gets subscript 2 (from O's valency), O gets subscript 3 (from Al's valency)
Step 3 — formula becomes Al2O3
Step 4 — check: total positive valency (2×3=6) equals total negative valency (3×2=6)

• Forgetting to simplify a criss-crossed ratio when both valencies share a common factor (e.g. 4 and 2 should simplify to 1 and 2, not stay as 2 and 4).
• Forgetting brackets around a radical when more than one unit is needed, e.g. (NH4)2SO4, not NH42SO4.
• Mixing up which element's valency becomes which subscript in the criss-cross method.

Open the Practice tab for graded questions on Valency & Formula Puzzles.

• Use the platform-native live simulation or PhET-style tool for this topic (number line, Venn, physics playground, molecule builder, sensor dashboard, etc.).
• Mirror / body / home activity: physically do the concept (count objects, measure, role-play) and photograph or describe for portfolio.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.