Modern Periodic Table — Lecture Notes (Class 11, SaitechAI Edition)
1. Historical Development
- Dobereiner’s Triads (1829): Elements were grouped in triads with similar properties. The atomic mass of the middle element was approximately the mean of the other two.
Example: Li (7), Na (23), K (39). - Newlands’ Law of Octaves (1865): Every eighth element showed similar properties when arranged by increasing atomic mass.
- Mendeleev’s Periodic Law (1869): The properties of elements are periodic functions of their atomic masses.
Limitations: Position of isotopes, anomalous pairs (Co–Ni, Te–I). - Modern Periodic Law (Moseley, 1913): “The physical and chemical properties of elements are periodic functions of their atomic numbers.”
2. Structure of the Modern Periodic Table
- Basis: Atomic number (Z)
- Total elements: 118 (known till 2025)
- Periods: 7 horizontal rows (number of energy shells)
- Groups: 18 vertical columns (number of valence electrons)
- Block classification:
- s-block: Groups 1 & 2
- p-block: Groups 13–18
- d-block: Transition elements (Groups 3–12)
- f-block: Inner transition elements (Lanthanides & Actinides)
3. Features of Periods and Groups
| Feature | Periods | Groups |
|---|---|---|
| Number | 7 | 18 |
| Represents | Principal quantum number (n) | Valence shell configuration |
| Example | 2nd period → Li to Ne | Group 17 → Halogens (F, Cl, Br, I, At) |
4. Important Trends in the Periodic Table
(a) Atomic Radius
- ↓ Group → increases (new shells added)
- → Period → decreases (nuclear charge ↑)
(b) Ionization Enthalpy (IE)
- ↓ Group → decreases (outer electrons farther)
- → Period → increases (nuclear charge ↑)
(c) Electron Gain Enthalpy (EGE)
- → Period → generally becomes more negative
- ↓ Group → becomes less negative
- Exception: Noble gases have positive EGE.
(d) Electronegativity
- → Period → increases
- ↓ Group → decreases
- Pauling scale: F = 4.0 (highest)
(e) Metallic and Nonmetallic Character
- Metallic character ↓ across a period, ↑ down a group.
- Nonmetallic character shows reverse trend.
(f) Valency
- Depends on group number:
- Group 1 → valency 1
- Group 14 → valency 4
- Group 17 → valency 1
- Group 18 → valency 0
5. Anomalies and Exceptions
- Diagonal relationship: Li–Mg, Be–Al (similar properties)
- d-Block contraction: due to poor shielding of d-electrons.
- Lanthanide contraction: causes Zr–Hf similarity.
6. Applications
- Predicting properties of elements.
- Classifying unknown elements.
- Understanding chemical reactivity.
- Basis for electronic configuration and chemical bonding.
7. Modern Periodic Table Snapshot
| Block | Range | Example Elements | Characteristic |
|---|---|---|---|
| s-block | 1–2 | Na, Mg | Highly reactive metals |
| p-block | 13–18 | B, C, N, O, F | Includes nonmetals, metalloids |
| d-block | 3–12 | Fe, Cu, Zn | Transition metals |
| f-block | Lanthanoids, Actinoids | Ce, U | Inner transition metals |
8. Mathematical Expression
where ( Z ) = atomic number, ( p ) = number of protons, ( n ) = neutrons.
9. Quick Revision Points
- Elements arranged by atomic number.
- Periodicity due to repetition of similar electronic configuration.
- Noble gases show complete outer shells → inert nature.
- Across a period: metallic → nonmetallic transition.
- Down a group: atomic size ↑, ionization energy ↓.
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