The complete lecture — but every concept comes alive in the live panel on the right as you read. Scroll down; the animation keeps pace.
1 — Sub-atomic particles & the electron
The atom is not solid — it is built from three particles. Discharge-tube experiments revealed cathode rays, deflected toward the positive plate, independent of the gas: the electron.
- Electron — charge −1.602 × 10⁻¹⁹ C (relative −1), mass ≈ 1/1836 a.m.u.
2 — The proton & the neutron
- Proton — positive (canal) rays; charge +1, mass 1.0073 a.m.u.
- Neutron — Chadwick, 1932; neutral, mass 1.0087 a.m.u.
Nucleons = protons + neutrons, packed in the nucleus.
3 — Rutherford's model
α-particles fired at gold foil: most passed straight, a few deflected, ~1 in 20,000 bounced back. So the atom is mostly empty space with a tiny, dense, positive nucleus.
Defect: a revolving electron should radiate energy and spiral in — the atom would collapse.
4 — Atomic number, mass number & isotopes
DefinitionsZ = protons · A = protons + neutrons
neutrons = A − Z
- Isotopes — same Z, different A (e.g. ³⁵Cl, ³⁷Cl).
relative atomic mass
75% ³⁵Cl + 25% ³⁷Cl → (75×35+25×37)/100 = 35.5
5 — Electromagnetic radiation & Planck
Key relationsc = ν λ
E = h ν = h c / λ (h = 6.63 × 10⁻³⁴ J·s)
6 — Bohr's atomic model
- Postulate — fixed circular orbits; energy emitted/absorbed only on a jump, E₂ − E₁ = hν.
Hydrogenrₙ = 0.529 n² Å · Eₙ = −1312/n² kJ·mol⁻¹
7 — The hydrogen emission spectrum
| Series | Falls to | Region |
|---|
| Lyman | n=1 | UV |
| Balmer | n=2 | Visible |
| Paschen | n=3 | IR |
Rydberg1/λ = R_H(1/n₁² − 1/n₂²)
8 — Defects of Bohr & the wave picture
- de Broglie — λ = h/mv (dual nature).
- Heisenberg — Δx·Δp ≥ h/4π.
- Orbital — region of maximum probability of finding the electron.
9 — Quantum numbers
| QN | Tells | Values |
|---|
| n | shell / energy | 1,2,3… |
| ℓ | subshell shape | 0..n−1 (s,p,d,f) |
| m | orientation | −ℓ..+ℓ |
| s | spin | ±½ |
10 — Atomic orbitals & shapes
- s — spherical (2 e⁻).
- p — dumb-bell, pₓ p_y p_z (6 e⁻).
- d — cloverleaf (10 e⁻).
11 — Aufbau, Pauli & Hund
Order of filling1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p…
- Pauli — an orbital holds 2 electrons of opposite spin.
- Hund — fill singly first, then pair.
12 — Electronic configurations
Electrons fill orbitals in energy order, two per box (opposite spins), singly first within a subshell. Move the slider in the live panel to choose an element (Z = 1–20) and watch its configuration build.
Anomalies: Cr = [Ar]3d⁵4s¹ and Cu = [Ar]3d¹⁰4s¹ — half/fully-filled d is extra stable.
13 — Worked numericals
de Broglie
λ = h/mv = (6.63e−34)/[(9.1e−31)(2.2e6)] = 3.31 × 10⁻¹⁰ m
Bohr E₂
E₂ = −1312/2² = −328 kJ·mol⁻¹
Balmer line n=3→2
1/λ = R_H(1/4 − 1/9) → λ = 656 nm (red)
14 — Exam recap
- Three particles: charge, mass, discoverer.
- Rutherford's experiment, conclusions & defects.
- Z, A, neutrons = A−Z; isotopes; relative atomic mass.
- Planck E = hν; Bohr postulates & formulas.
- Hydrogen spectrum series; Rydberg.
- Quantum numbers, orbital shapes, Aufbau/Pauli/Hund, configurations.