General Chemistry: Atomic Structure & Periodic Trends
Free MCAT study guide — Chemical and Physical Foundations of Biological Systems
Atomic Structure
Atoms consist of protons (p⁺), neutrons (n⁰), and electrons (e⁻). The atomic number (Z) = number of protons defines the element. Mass number (A) = protons + neutrons.
Quantum Numbers
| Quantum Number | Symbol | Values | Describes |
|---|---|---|---|
| Principal | n | 1, 2, 3... | Shell / energy level |
| Angular momentum | l | 0 to n−1 | Subshell shape (s,p,d,f) |
| Magnetic | mₗ | −l to +l | Orbital orientation |
| Spin | mₛ | +½ or −½ | Electron spin |
Key Point: Aufbau: fill lowest energy first. Hund's rule: fill singly before pairing. Pauli: max 2e⁻ per orbital.
Periodic Trends
| Property | Across Period (→) | Down Group (↓) | Reason |
|---|---|---|---|
| Atomic radius | Decreases | Increases | More protons (→), more shells (↓) |
| Ionization energy | Increases | Decreases | Harder to remove e⁻ from smaller atoms |
| Electronegativity | Increases | Decreases | Smaller atoms attract e⁻ more strongly |
| Electron affinity | More negative | Less negative | Smaller atoms gain e⁻ more easily |
Key Point: Exceptions: IE drops at B (2s→2p) and O (paired 2p e⁻). Noble gases have very high IE and near-zero EA.
Nuclear Chemistry
| Decay Type | Particle | ΔZ | ΔA | Example |
|---|---|---|---|---|
| Alpha (α) | ⁴₂He | −2 | −4 | ²³⁸U → ²³⁴Th + α |
| Beta⁻ (β⁻) | ⁰₋₁e | +1 | 0 | ¹⁴C → ¹⁴N + β⁻ |
| Beta⁺ (β⁺) | ⁰₊₁e | −1 | 0 | ¹¹C → ¹¹B + β⁺ |
| Gamma (γ) | photon | 0 | 0 | Excited → ground state |
| Electron capture | e⁻ captured | −1 | 0 | ⁵⁷Co + e⁻ → ⁵⁷Fe |
Half-life: N = N₀ × (½)t/t½
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