Chemistry Basics – Comprehensive Study Notes
For Manipur PSC – General Awareness & Current Affairs
(Rigorous, IIT‑JEE style depth, derivations, key formulae, tricky points & a hard practice question)
1. Atomic Structure
| Concept | Key Points | Formulae / Derivations |
|---|---|---|
| Nucleus & Electron Shells | • Proton (+1 e), Neutron (0 e). • Electron cloud described by quantum numbers (n, l, m<sub>l</sub>, m<sub>s</sub>). |
Bohr radius: (a_0 = \frac{4\pi\varepsilon_0\hbar^2}{me^2}) Rydberg energy: (E_R = \frac{1}{2} \frac{m e^4}{(4\pi\varepsilon_0)^2 \hbar^2}) |
| Isotopes & Ions | • Same Z, different N. • Ion charge = (Z - N_e). |
(m_{atom} \approx Z m_p + N m_n - E_b/c^2) |
| Magnetic Moment | • Spin‑orbit coupling gives total (J = L+S). • Landé g‑factor: (g_J = 1 + \frac{J(J+1)+S(S+1)-L(L+1)}{2J(J+1)}). |
|
| Tricky Point | Distinguish between nuclear mass and atomic mass – atomic mass includes binding energy contribution (mass defect). |
2. Periodic Table Trends
| Trend | Explanation | Practical Implication |
|---|---|---|
| Atomic Radius | Decreases left‑to‑right (increased nuclear charge) & increases down a group (extra shells). | Helps predict reactivity of alkali metals vs. halogens. |
| Ionization Energy (IE) | First IE ↑ left‑to‑right, ↓ down. | Determines tendency to lose electrons. |
| Electron Affinity (EA) | More negative (larger magnitude) for halogens. | Drives formation of anions. |
| Electronegativity (Pauling) | ↑ left‑to‑right, ↓ down. | Influences bond polarity. |
| Tricky Point | Metalloids (e.g., Si, Ge) behave like both metals and non‑metals; check their position to predict semiconductor behaviour. |
3. Chemical Bonding
3.1 Lewis (Valence‑Bond) Theory
- Bond order (= \frac{1}{2}(\text{total valence electrons} - \text{core electrons})).
- Derivation: For a diatomic molecule XY, (n_\text{val} = Z_X + Z_Y).
- Predicting geometries by VSEPR: count lone pairs & bond pairs.
3.2 Molecular Orbital (MO) Theory
- σ bonds: constructive interference; π bonds: side‑by‑side overlap.
- MO diagram: energy levels split into bonding/antibonding orbitals.
- Bond order = (\frac{(N_b - N_a)}{2}).
- O₂ spin‑triplet vs. singlet: 2 unpaired electrons → paramagnetic.
3.3 Ionic vs Covalent vs Metallic
| Characteristic | Ionic | Covalent | Metallic |
|---|---|---|---|
| Electron transfer | ✔ | ✖ | Shared delocalized |
| Conductivity | ✔ (melt/solution) | ✖ | ✔ (solid) |
| Molar mass trend | Low | Varied | Usually high |
Tricky Point
- Covalent vs. ionic: Use Fajan’s rules – small, highly charged cation + large, polarizable anion → covalent character.
4. Stoichiometry & Mole Concept
| Concept | Formulae |
|---|---|
| Avogadro’s number | (N_A = 6.022\times10^{23}) mol(^{-1}) |
| Molar mass | (M = \frac{\text{grams}}{\text{moles}}) |
| Mole–mass relation | (n = \frac{m}{M}) |
| Ideal Gas Law | (PV = nRT) |
| Partial Pressure | (P_i = y_i P_\text{total}) |
Derivation (Ideal Gas)
- From kinetic theory: (PV = \frac{1}{3}Nm\overline{v^2}).
- Using ( \overline{v^2} = \frac{3kT}{