Atomic Structure
The atom
All matter is made of atoms. An atom consists of a tiny, dense nucleus surrounded by electrons arranged in shells (energy levels). The nucleus contains protons (positive charge, relative mass 1) and neutrons (no charge, relative mass 1). Electrons have a negligible mass (1/1836 of a proton) and carry a negative charge.
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | ~0 | Shells |
An atom is electrically neutral overall: the number of protons equals the number of electrons.
Atomic number and mass number
- Atomic number (Z): the number of protons in the nucleus. This uniquely identifies the element.
- Mass number A: the total number of protons + neutrons in the nucleus.
- Number of neutrons = A − Z.
Standard notation: ᴬ_Z X, e.g. ²³_₁₁Na means sodium with 11 protons and 12 neutrons.
Isotopes
Isotopes are atoms of the same element (same atomic number Z) that have different mass numbers — they contain different numbers of neutrons. Their chemical properties are identical (same electron configuration) but their physical properties differ slightly (e.g. density, boiling point).
Examples:
- Carbon-12 (¹²C) and Carbon-14 (¹⁴C): both have 6 protons; ¹²C has 6 neutrons, ¹⁴C has 8 neutrons. ¹⁴C is radioactive.
- Chlorine has two stable isotopes: ³⁵Cl (75%) and ³⁷Cl (25%), giving chlorine a relative atomic mass of ~35.5.
Relative atomic mass (Aᵣ): the weighted mean mass of an atom of the element relative to 1/12 the mass of carbon-12.
Aᵣ = Σ (% abundance × mass number) ÷ 100
Example (Cl): Aᵣ = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.
Electron configuration (shells)
Electrons fill shells outward from the nucleus. Shell capacities:
- Shell 1 (closest): maximum 2 electrons
- Shell 2: maximum 8 electrons
- Shell 3: maximum 8 electrons (at GCSE level)
Write configurations as a sequence of numbers, e.g.:
- 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 chemical behaviour and the group in the periodic table.
Ions
When atoms gain or lose electrons they become ions — charged particles.
- Cations (positive ions): formed when atoms lose electrons. Metals typically form cations. Na → Na⁺ + e⁻.
- Anions (negative ions): formed when atoms gain electrons. Non-metals typically form anions. Cl + e⁻ → Cl⁻.
An ion has the same atomic number as the parent atom (same protons) but a different number of electrons.
Example: Na⁺ has 11 protons and 10 electrons (lost 1 electron). O²⁻ has 8 protons and 10 electrons (gained 2 electrons).
CCEA practical context
CCEA controlled assessment may involve flame tests (identifying metal ions by colour) — linking the colours to electronic transitions. Students should know: Li⁺ = crimson, Na⁺ = yellow, K⁺ = lilac, Ca²⁺ = brick-red, Cu²⁺ = green/blue.
⚠Common mistakes
- Confusing atomic number (protons only) with mass number (protons + neutrons).
- Stating isotopes have different chemical properties — they do NOT; chemical behaviour depends on electron configuration which is the same for all isotopes of an element.
- Forgetting that ions have the same number of protons as the neutral atom — only the electron count changes.
- Shell 3 capacity: at GCSE say max 8 (the 3d sub-shell detail is A-level).
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