Atoms

Definition: A pure substance that cannot be broken down into any other substances, made from one type of atom.
Examples:
Existence: Some elements exist as individual atoms while others exist as molecules.
Fact: Most elements are metals:
- Aluminium (Al): Used for bike frames (lightweight) and kitchen foil.
- Copper (Cu): Used for electrical wires (good conductor of electricity).
Historical Note: Ancient Greeks believed in four elements (earth, air, fire, water).
Current Understanding: There are actually 118 chemical elements known today.
Natural States: Some elements can be found in pure form in nature:
- Graphite and diamond (forms of carbon)
- Gold (metal, found in rocks)
Air Composition: Mostly nitrogen (N) and oxygen (O).
Atomic Uniformity: Atoms in a specific material (e.g., copper wire) are identical to each other, different from those in another material (e.g., aluminium foil).

Definition: A pure substance made from more than one element, with elements chemically bonded together.
Chemical Formula: Represents the fixed ratio of elements in a compound. Examples:
- Water: $H₂O$ (made of hydrogen and oxygen)
- Carbon Dioxide: $CO₂$
Separation of Components:
- Requires a chemical reaction (e.g., electrolysis, thermal decomposition) to break apart compounds.

Definition: Formed when two or more elements or compounds are present without being chemically bonded.
Characteristics:
- No specific amounts or ratios in a mixture.
- Examples:
- A mixture of sand and water can have varying amounts of each.
Separation:
- Mixtures can be separated using physical processes (no chemical reactions).
- Examples of separation methods:
- Filtration: Separating sand and water.
- Crystallization or Distillation: Separating salt from water.
- Magnetism: Separating iron filings from sulfur powder.