Chemistry Basics – Comprehensive Study Notes
(For Nagaland Patwari / Lekhpal – General Awareness & Current Affairs; exam‑oriented style with depth, derivations, and a hard practice problem)
1. Introduction
Chemistry is the science of matter, its structure, properties, changes, and the energy involved.
For competitive exams, a firm grasp of conceptual foundations, important formulae, and application problems is essential.
These notes cover the core topics, key derivations, and tricky points that often appear on exams.
2. Atomic & Molecular Fundamentals
| Concept | Key Points |
|---|---|
| Atomic Structure | • Nucleus: protons (positive) + neutrons (neutral). • Electrons (negative) in shells. |
| Atomic Number (Z) | Number of protons = number of electrons in a neutral atom. |
| Mass Number (A) | (A = Z + N) (N = neutrons). |
| Isotopes | Same Z, different A → same chemical behaviour, different physical properties. |
| Bohr Model (simplified) | Energy levels (E_n = -\displaystyle\frac{Z^2 R_{\infty}}{n^2}); (R_{\infty}=13.6) eV. |
| Quantum Numbers | Principal (n), azimuthal (l), magnetic (m_l), spin (s). |
| Valence Electrons | Determine reactivity; located in the outermost shell. |
3. Periodic Table & Trends
| Trend | Observation | Underlying Reason |
|---|---|---|
| Atomic Radius | ↑ down a group; ↓ across a period | Increased shielding + added shells vs. + nuclear charge |
| Ionization Energy (IE) | ↑ down; ↓ across | Same reasons as radius |
| Electron Affinity (EA) | ↓ down; ↑ across (except for inert gases) | Stability of added electron |
| Electronegativity | ↑ down a group; ↓ across (Pauling scale) | Effective nuclear charge & radius |
| Metal‑Nonmetal Boundary | Transition from metallic to non‑metallic behaviour | Balance of IE and EA |
Tricky Point – Effective nuclear charge (Z_{\text{eff}}) is not simply (Z); it is reduced by inner‑shell shielding.
4. Chemical Bonding
4.1 Ionic Bond
- Electrostatic attraction between (\ce{M^+}) and (\ce{X^-}).
- Occurs when ( \Delta \chi > 1.7) (Pauling).
- Example: (\ce{NaCl}).
4.2 Covalent Bond
- Shared electron pair(s).
- Polar vs non‑polar depends on (\Delta \chi).
- Multiple bonds: double/triple (π bonds).
4.3 Metallic Bond
- Delocalised “sea” of electrons over a lattice of cations.
4.4 Bond Polarity Formula
[ \chi_{\text{avg}} = \frac{\sum \chi_i}{n} ] If (\chi_{\text{avg}} < 1.7) → covalent; (>1.7) → ionic.
Tricky Point – Partial ionic character exists in many covalent bonds; no bond is purely ionic or covalent.