Chemistry: Atomic Structure, Bonding and Mole Concept
Chemistry in AP POLYCET tests Class 10 atomic structure, periodic classification, chemical bonding, acids, bases and salts, metallurgy, and carbon compounds. Most questions reward correct valency, balanced equations and mole ratios.
Core rules
- Mole concept: one mole contains 6.022 × 10²³ particles; n = mass/molar mass.
- Periodic trends: across a period, atomic radius decreases and electronegativity increases; down a group, atomic radius increases.
- Bonding: ionic bonds form by electron transfer between metals and non-metals; covalent bonds form by sharing between non-metals.
- Acids and bases: pH below 7 is acidic, above 7 is basic; acid + base → salt + water.
- Carbon compounds: members of a homologous series differ by CH₂; saturated compounds mostly undergo substitution, unsaturated ones addition.
- Metallurgy: roasting heats sulphide ores in air; calcination heats carbonate ores in limited air.
Worked example: calculate the number of molecules in 11 g of CO₂. Molar mass = 12 + 2(16) = 44 g/mol. n = 11/44 = 0.25 mol. Number of molecules = 0.25 × 6.022 × 10²³ = 1.5055 × 10²³. The same mole ratio gives gas volume at STP: 0.25 × 22.4 L = 5.6 L.
Traps
- Forgetting to balance an equation before using mole ratios.
- Confusing roasting with calcination.
- Writing formulas without checking valency, such as MgCl instead of MgCl₂.
- Treating pH as linear; pH 3 is 100 times more acidic than pH 5.
- Ignoring coefficients when comparing moles of reactants and products.