Simple Model of the Atom (C1.1)
Atomic structure
An atom consists of a tiny central nucleus (containing protons and neutrons) surrounded by electrons in shells (energy levels).
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | 1/1836 (≈0) | Shells/orbits |
An atom is neutral overall — equal numbers of protons and electrons.
Atomic number (Z): number of protons (= number of electrons in a neutral atom).
Mass number A: total number of protons + neutrons.
Number of neutrons = mass number − atomic number.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers).
Example: Carbon-12 (⁶₁₂C) and Carbon-14 (⁶₁₄C) — both have 6 protons, but C-12 has 6 neutrons and C-14 has 8 neutrons.
Isotopes have identical chemical properties (same electron configuration) but different physical properties (mass, radioactive stability).
Electron shells (electronic configuration)
Electrons fill shells in order, innermost first:
- Shell 1: maximum 2 electrons
- Shell 2: maximum 8 electrons
- Shell 3: maximum 8 electrons (at GCSE level)
Example: Sodium (Na, Z=11) → 2, 8, 1
Electronic configuration determines chemical behaviour (reactivity, bonding).
Elements, compounds and mixtures
- Element: pure substance containing only one type of atom (e.g. O₂, Fe, Cu).
- Compound: substance containing two or more elements chemically bonded together in fixed ratios (e.g. H₂O, NaCl). Properties differ from constituent elements.
- Mixture: two or more substances not chemically bonded — can be separated by physical means; components retain their own properties.
Separation techniques
| Mixture type | Technique | Principle |
|---|---|---|
| Solid + liquid | Filtration | Particle size |
| Solution (salt from water) | Crystallisation/evaporation | Solubility |
| Liquids with different boiling points | Simple/fractional distillation | Boiling point |
| Pigments in solution | Chromatography | Solubility in solvent |
Simple distillation: separates a solvent from a solution (e.g. water from salt water). Liquid boils → vapour condenses → collected.
Fractional distillation: separates a mixture of liquids with different boiling points (e.g. crude oil fractionation). Different fractions collected at different temperatures.
Chromatography: mobile phase (solvent) moves through stationary phase (paper), carrying components at different rates. Compounds with higher solubility in solvent travel further. Rf value:
Rf = distance moved by substance ÷ distance moved by solvent front
Common exam errors
- Saying electrons have significant mass — they don't (≈1/1836 of a proton).
- Confusing atomic number (protons) with mass number (protons + neutrons).
- Saying isotopes have different chemical properties — they have the same (same electron structure).
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