Atomic structure, periodic table trends and separation techniques
Atomic structure
An atom consists of a central nucleus (containing protons and neutrons) surrounded by electrons arranged in shells (energy levels).
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1840 (≈0) | −1 | Shells around nucleus |
Atomic number (proton number, Z): the number of protons. This defines which element an atom is. Mass number A: the total number of protons + neutrons (nucleons). Number of neutrons = mass number − atomic number.
In a neutral atom: number of electrons = number of protons.
Isotopes
Isotopes are atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons). They have identical chemical properties (same electron arrangement) but different physical properties (different mass).
Example: Carbon-12 (⁶¹²C, 6 neutrons) and Carbon-14 (⁶¹⁴C, 8 neutrons).
Relative atomic mass (Aᵣ) is the weighted mean mass of all naturally occurring isotopes of an element, relative to 1/12 the mass of carbon-12. It is not always a whole number because of isotopic abundances.
Electron configuration
Electrons occupy shells in order of increasing energy:
- Shell 1: maximum 2 electrons
- Shell 2: maximum 8 electrons
- Shell 3: maximum 8 electrons (at GCSE level)
To write electron configuration: fill shells in order.
- Sodium (Z=11): 2, 8, 1
- Chlorine (Z=17): 2, 8, 7
- Calcium (Z=20): 2, 8, 8, 2
The number of outer-shell electrons determines an element's group in the periodic table and governs its chemical reactivity.
The periodic table
The periodic table arranges elements in order of increasing atomic number. Elements in the same group (column) have the same number of outer-shell electrons and thus similar chemical properties. Elements in the same period (row) have the same number of electron shells.
Group 1 — the Alkali Metals
- Soft metals with low melting points; density increases down the group (Li, Na, K, Rb, Cs, Fr).
- React with cold water to produce a metal hydroxide + hydrogen: 2M + 2H₂O → 2MOH + H₂
- Reactivity increases down the group: outer electron is further from nucleus / less nuclear attraction / more shielding → more easily lost.
- Stored under oil to prevent reaction with oxygen and water.
Group 7 — the Halogens
- Non-metals that exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).
- Reactivity decreases down the group: outer electrons are further from nucleus / more shielding → harder to gain an electron.
- Displacement reactions: a more reactive halogen displaces a less reactive one from its salt solution.
- Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromine — chlorine is more reactive).
- Physical state at room temperature: Cl₂ = pale green gas; Br₂ = orange-brown liquid; I₂ = grey solid.
Group 0 — Noble gases
- Full outer shells → chemically inert (do not react under normal conditions).
- Monatomic gases; boiling points increase down the group (larger atoms, stronger London dispersion forces).
Periodic trends
- Atomic radius increases down a group (more shells); decreases across a period (same number of shells but more protons pull electrons closer).
- Metallic character decreases across a period (left → right); increases down a group.
Separation techniques
Separation techniques exploit differences in physical properties:
| Technique | Separates | Principle |
|---|---|---|
| Filtration | Insoluble solid from liquid | Particle size — solid too large to pass through filter paper |
| Evaporation | Soluble solid from solution | Solvent evaporates on heating; solid remains |
| Crystallisation | Soluble solid from solution | Controlled cooling; pure crystals form |
| Simple distillation | Solvent from solution; liquids with widely different boiling points | Liquid with lower b.p. vaporises first, condenses in condenser |
| Fractional distillation | Liquids with similar boiling points (e.g. ethanol/water) | Fractionating column creates many mini-distillations |
| Chromatography | Mixture of dissolved substances | Different affinities for stationary phase vs mobile phase |
Rf value in chromatography: Rf = distance moved by substance ÷ distance moved by solvent. A pure substance gives a single spot with a consistent Rf. Two substances are the same if their Rf values match under the same conditions.
OCR PAG C5 (Distillation) and PAG C1 (Chromatography) are the most common practical contexts for J258 questions on separation.
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