Atomic Structure: MDCAT Notes and MCQs
Positive rays → proton
- Goldstein (1886) used a perforated cathode in a discharge tube and observed canal rays / positive rays moving opposite to cathode rays.
- Produced when cathode rays knock electrons off gas molecules. e/m depends on the gas in the tube; it is maximum for hydrogen (lightest positive particle = proton, named by Rutherford).
- Deflected by electric & magnetic fields (less than electrons), travel in straight lines, cause ionization. Proton mass = 1.6726 × 10−27 kg (≈ 1836 × electron).
Planck's quantum theory (1900)
Energy is emitted or absorbed in discrete packets — quanta; a quantum of light = photon. E = hν = hc/λ, h = 6.626 × 10−34 J s. Energy ∝ frequency, ∝ 1/wavelength.
Quantum numbers
| Number | Describes | Values |
|---|---|---|
| Principal, n | Shell (energy level, size) | 1, 2, 3 …; max electrons 2n2 |
| Azimuthal, l | Sub-shell (shape) | 0 to n−1 (s=0, p=1, d=2, f=3) |
| Magnetic, m | Orientation of orbital | −l … 0 … +l (2l+1 orbitals) |
| Spin, s | Electron spin | +½ or −½ |
Orbital = region of space with maximum probability of finding an electron; holds max 2 electrons. s holds 2, p 6, d 10, f 14 electrons.
Shapes
s — spherical (non-directional). p — dumbbell (px, py, pz along axes, nodal plane at nucleus). d — mostly four-lobed / cloverleaf (dz² is a dumbbell with a ring).
Hydrogen spectrum
- Line spectrum — electron jumps from higher to lower orbit emit a photon of ΔE = hν (Bohr). En = −13.6/n2 eV; rn = 0.529 n2 Å.
- Series (by lower level n1): Lyman (1) — UV; Balmer (2) — visible; Paschen (3), Brackett (4), Pfund (5) — infrared.
Electronic configuration
- Aufbau: fill in increasing energy — (n + l) rule: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d …
- Pauli exclusion: no two electrons in an atom have the same four quantum numbers → max 2 per orbital with opposite spins.
- Hund's rule: degenerate orbitals are singly occupied (parallel spins) before pairing. N: 1s2 2s2 2px12py12pz1.
- Exceptions: Cr = [Ar] 3d5 4s1, Cu = [Ar] 3d10 4s1 (half-filled/filled d stability).
PMDC learning outcomes for this unit
- Discovery of Proton / Planck's Quantum Theory: 2.1 Describe discovery and properties of proton (positive rays)
- Discovery of Proton / Planck's Quantum Theory: 2.2 Define photon as a unit of radiation energy
- Quantum Number: 2.3 Describe the concept of orbitals.
- Quantum Number: 2.4 Distinguish among principal energy level, energy sub-level and atomic orbitals
- Shapes of orbitals: 2.5 Describe the general shapes of s, p and d orbitals.
- Spectrum of Hydrogen: 2.6 Describe hydrogen atom using the quantum theory
- Electronic Configuration: 2.7 Use the Aufbau principle, the Pauli Exclusion Principle and Hund's Rule to write the electronic configuration of atoms.
- Electronic Configuration: 2.8 Write electronic configuration of atom