Atomic structure (chemistry perspective)
WJEC Chemistry expects fluent use of atomic-structure ideas, especially when calculating relative atomic masses from isotopic abundances.
Subatomic particles
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | ~1/1836 | -1 | Shells |
Atomic number, mass number, isotopes
- Atomic (proton) number, Z — number of protons; defines the element.
- Mass number, A — protons + neutrons in the nucleus.
- Isotopes — same Z, different A (different neutron count).
Example: chlorine has two stable isotopes:
- Cl-35: 17 protons, 18 neutrons (~75% abundance)
- Cl-37: 17 protons, 20 neutrons (~25% abundance)
Relative atomic mass (Ar)
Ar is the weighted mean mass of an element's atoms, taking into account isotopic abundances.
Formula: Ar = sum of (mass × abundance) / total abundance
For chlorine: Ar = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 35.5
That is why chlorine appears as 35.5 in the periodic table — most students initially expect a whole number.
Development of the atomic model
The chemistry-emphasis history:
- Dalton (1803) — atoms are indivisible spheres unique to each element.
- Thomson (1897) — electron discovery via cathode rays; "plum pudding" model.
- Rutherford (1909) — alpha-scattering experiment; small dense positive nucleus.
- Bohr (1913) — electron shells / fixed energy levels (explains line spectra).
- Chadwick (1932) — neutron discovered, model essentially complete.
WJEC exam tip
When calculating Ar from abundance data, watch the units. If percentages are given, divide by 100 at the end. If raw numbers (e.g. 75 atoms, 25 atoms), divide by the sum (100). Always show the working step (sum / total) — examiners award method marks for it.
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