Elements & Compounds

• Pure substance — single type of particle; fixed composition.
• Element — cannot be broken into simpler substances by chemical means (e.g. O₂, Fe, Au).
• Compound — two or more elements chemically combined in fixed ratio (e.g. H₂O, NaCl).
• Properties of compounds differ from constituent elements.

1. Element — ~118 known; metals, non-metals, metalloids.
2. Compound — fixed composition; law of constant proportions applies.
3. H₂O — always 2:1 H:O by atoms; properties unlike H or O alone.
4. Separation of elements in compound needs chemical reaction (electrolysis of water).
5. Diagram (text) — element: one kind of atom; compound: atoms bonded in fixed ratio.
6. NCERT Ch. 2 — gold, oxygen, water, carbon dioxide examples.
7. Real world — iron rust (compound Fe₂O₃·xH₂O); table salt NaCl.
8. Symbols — Dalton's atomic symbols (NCERT historical).

Classifying gold, air, distilled water, and brass

Step 1 — Gold (Au) → **element** (pure single type of atom).
Step 2 — Distilled water (H₂O) → **compound** (fixed 2:1 ratio, chemical formula).
Step 3 — Air → **mixture** (N₂, O₂, Ar, CO₂ — variable composition).
Step 4 — Brass (Cu + Zn) → **mixture** (alloy; not fixed ratio like compound).
Conclusion: elements and compounds are pure substances; air and brass are mixtures.

• Calling oxygen in air an element sample (air is mixture of elements/compounds).
• Misconception: compounds can have variable composition (fixed by law of constant proportions).
• Confusing atom with molecule (O atom vs O₂ molecule).
• Thinking alloys are compounds (alloys are homogeneous mixtures).
• Using physical methods to separate elements in a compound (needs chemical change).

• Distinguish element and compound with two examples each.
• Is water an element or compound? Why?
• What is a pure substance?
• How do properties of a compound differ from its elements?

Open the Practice tab for graded questions on Elements & Compounds.