Descriptive Chemistry
What to know
Descriptive chemistry tests recognition of elements, compounds, and trends rather than heavy calculation. Know the periodic table layout: groups (columns) share valence electrons and properties; periods are rows. Key families: alkali metals (very reactive, form +1 ions), alkaline earth metals (+2), halogens (form −1 ions, diatomic), noble gases (inert), and transition metals (variable oxidation states, colored compounds, catalytic activity).
Recognize common compounds and their properties: oxides of nonmetals are acidic (CO2, SO2), oxides of metals are basic (Na2O); amphoteric oxides like Al2O3 react with both acids and bases. Know common gases (O2, H2, CO2, NH3, Cl2), their tests, and everyday reactions (combustion, rusting, photosynthesis, respiration). Understand trends in reactivity, metallic character, and how position predicts ion charge.
Traps
- Assuming all metal oxides are basic-amphoteric oxides are exceptions.
- Forgetting that halogens are diatomic (Cl2, not Cl).
- Predicting a fixed charge for transition metals (they vary; iron is +2 or +3).
- Confusing a physical property (color, density) with a chemical one (reactivity).
Worked example
Predict the formula of the compound between aluminum and oxygen. Al forms Al³⁺, O forms O²⁻. Cross charges: Al2O3. Rule of thumb: for main-group ionic compounds, swap the magnitude of the charges as subscripts, then simplify.