Modern Periodic Table
Arrangement of Elements & Periodicity | Classification of Elements
1. Modern Periodic Law
In 1913, Henry Moseley showed that the Atomic Number ($Z$) is a more fundamental property than Atomic Mass. He observed a regular pattern in X-ray spectra: $\sqrt{\nu} \propto Z$.
Law: The physical and chemical properties of elements are periodic functions of their Atomic Numbers.
2. Structure of the Periodic Table
The long form of the periodic table is based on electronic configuration.
A. Periods (Horizontal Rows)
- There are 7 Periods.
- The period number corresponds to the highest Principal Quantum Number ($n$) of the elements.
- Magic Numbers: 2, 8, 8, 18, 18, 32 (Number of elements in periods 1 to 6).
B. Groups (Vertical Columns)
- There are 18 Groups (numbered 1 to 18 by IUPAC).
- Elements in the same group have the same number of valence electrons and similar chemical properties.
3. Classification into Blocks
Based on the subshell in which the last electron enters, elements are classified into four blocks:
1. s-Block Elements
- Groups: 1 (Alkali Metals) & 2 (Alkaline Earth Metals).
- Config: $ns^{1-2}$
- Reactive metals with low ionization enthalpy.
2. p-Block Elements
- Groups: 13 to 18.
- Config: $ns^2 np^{1-6}$
- Contains Metals, Non-metals, and Metalloids.
- Representative Elements: s-block + p-block (except Group 18).
3. d-Block Elements (Transition Elements)
- Groups: 3 to 12.
- Config: $(n-1)d^{1-10} ns^{0-2}$
- Metals that form colored ions, variable oxidation states, and act as catalysts.
- Note: Zn, Cd, Hg are d-block but strictly not transition elements (full d-orbital).
4. f-Block Elements (Inner Transition Elements)
- Position: Placed separately at the bottom (belong to Group 3).
- Config: $(n-2)f^{1-14} (n-1)d^{0-1} ns^2$
- Lanthanoids ($4f$): Rare earth elements.
- Actinoids ($5f$): Radioactive elements.
4. Summary of Periodic Trends
| Property | Left to Right (Period) | Top to Bottom (Group) |
|---|---|---|
| Atomic Radius | Decreases | Increases |
| Ionization Energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic Character | Decreases | Increases |
Practice Quiz
Test your knowledge on the Periodic Table.
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