Class 11 Chemistry: Classification of Elements and Their Periodic Properties

Topic Overview

• Chapter: Classification of Elements and their Periodic Properties
• Previous Topics Covered:
  • Classification Systems: Dobereiner's triads, Newlands' Law of Octaves, Mendeleev's and Modern Periodic Table
  • Modern Periodic Table: s block, p block, d block, and f block classification based on electronic configuration and chemical properties
  • Important Trick: Identified period number and group number of an element from the periodic table for competitive exams
• Current Topic: Periodic Properties

Periodic Properties

Definition

• Properties of elements that repeat in a group after certain atomic numbers.
• Examples: Lithium (atomic no. 3) and Sodium (atomic no. 11) have similar properties.
• Called periodic because they repeat periodically as you go down a group.

Important Periodic Properties

1. Atomic Size (Atomic Radii)
2. Ionization Enthalpy
3. Electronegativity
4. Electron Gain Enthalpy
5. Electropositivity (Metallic Character)
6. Valency/Valence Electrons

Variation in Periodic Properties

• Left to Right: Properties may increase or decrease across a period.
• Top to Bottom: Properties may increase or decrease down a group.

Atomic Size (Atomic Radii)

Definition

• Distance between nucleus and the outermost shell (valence shell) of an atom.
• Difficult to measure directly due to impracticality of extracting single atoms.

Types of Atomic Radii

1. Covalent Radii
   • Definition: Half the distance between nuclei of two covalently bonded atoms.
   • Example: H2 molecule, internuclear distance is 72 pm, covalent radii is 36 pm.
2. Metallic Radii
   • Definition: Half the distance between nuclei of two atoms in a metallic bond.
3. Van der Waals Radii
   • Definition: Half the distance between nuclei of atoms in separate molecules held by van der Waals forces.
   • Used For: Noble gases

Comparative Sizes

• Van der Waals Radii > Metallic Radii > Covalent Radii
• Noble gases are considered larger due to using van der Waals radii for size measurement.

Trend Observations in Periodic Table

1. Left to Right (Across a Period): Atomic size decreases
   • Reason: Nuclear charge increases, pulling shell closer to nucleus.
2. Top to Bottom (Down a Group): Atomic size increases
   • Reason: Increase in number of shells.

Ionic Radii

Definitions

• Cation: Ion with positive charge; formed by loss of electrons.
  • Example: Na → Na⁺ by losing 1 electron.
• Anion: Ion with negative charge; formed by gain of electrons.
  • Example: F → F⁻ by gaining 1 electron.

Comparative Sizes

• Cations are smaller than the parent atom.
• Anions are larger than the parent atom.

Example Problems

• Comparative Size: I⁻ > I > I⁺

Isoelectronic Species

Definition

• Species with the same number of electrons (can be ions or neutral atoms).

Trend

• Nuclear Charge Increases: Size decreases.
• Higher atomic number among isoelectronic species indicates smaller size due to increased nuclear charge.

Example

• Species: F⁻, O²⁻, N³⁻, Mg²⁺, Al³⁺ (All have 10 electrons)
• Order of Size: N³⁻ > O²⁻ > F⁻ > Ne > Mg²⁺ > Al³⁺

Conclusion

• Next topic: Ionization Enthalpy.