Chapter 1 – Classification of Elements and Periodicity
1. Need for Classification of Elements
As the number of discovered elements increased, it became difficult to study them individually.
Classification helps in:
- Systematic study of elements
- Understanding similarities and trends
- Predicting properties of unknown elements
2. Early Attempts at Classification
Döbereiner’s Triads
Elements were grouped in sets of three with similar properties.
The atomic mass of the middle element was approximately the average of the other two.
$\text{Atomic mass of middle} \approx \dfrac{m_1 + m_3}{2}$
Limitation: Only a few elements could be arranged in triads.
Newlands’ Law of Octaves
When elements were arranged in increasing atomic mass, every 8th element had similar properties.
Worked only up to calcium.
3. Mendeleev’s Periodic Law
The physical and chemical properties of elements are periodic functions of their atomic masses.
Achievements:
- Predicted undiscovered elements
- Corrected atomic masses
- Left gaps for future elements
Limitation: Could not explain isotopes.
4. Modern Periodic Law
The physical and chemical properties of elements are periodic functions of their atomic numbers.
5. Modern Periodic Table
- 7 periods
- 18 groups
- Elements arranged by increasing atomic number
6. Electronic Configuration and Periodicity
Electronic configuration determines position of element in periodic table.
Valence shell configuration governs chemical properties.
7. Periodic Trends
Atomic Radius
Atomic radius is half the distance between nuclei of two bonded atoms.
Trend:
- Decreases across a period
- Increases down a group
Ionic Radius
Cations are smaller than atoms; anions are larger.
8. Ionization Enthalpy
$X(g) \rightarrow X^+(g) + e^-$
Trend:
- Increases across a period
- Decreases down a group
9. Electron Gain Enthalpy
$X(g) + e^- \rightarrow X^-(g)$
Halogens have maximum electron gain enthalpy.
10. Electronegativity
Electronegativity is the tendency of an atom to attract electrons towards itself.
Fluorine is the most electronegative element.
11. Valency
Valency is the combining capacity of an element.
12. Metallic and Non-metallic Character
Metallic character increases down a group and decreases across a period.
13. Reactivity of Elements
- Alkali metals → highly reactive
- Halogens → highly reactive non-metals
14. Periodic Trends Summary Table
| Property | Across Period | Down Group |
|---|---|---|
| Atomic Radius | Decreases | Increases |
| Ionization Energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
15. Common JEE Traps
- Confusing atomic size trend in d-block
- Forgetting half-filled stability
- Wrong comparison of electron affinity
16. Final Exam Checklist
You are fully prepared if you can:
- Explain all periodic trends with reasons
- Predict properties from position
- Compare elements accurately
- Handle exceptions confidently