Classification of Elements and Periodicity in Properties

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๐Ÿงช Chapter: Classification of Elements and Periodicity in Properties

๐Ÿ”น 1. Need for Classification

  • Elements were discovered over time; their classification was needed for easy study of their properties.

  • Early attempts include:

    • Dobereiner’s Triads

    • Newlands’ Law of Octaves

    • Mendeleev’s Periodic Table

๐Ÿ”น 2. Dobereiner’s Triads

  • Groups of three elements with similar properties.

  • Atomic mass of the middle element is the average of the other two.

  • Example: Li (7), Na (23), K (39); 7 + 39 / 2 = 23

๐Ÿ”น 3. Newlands’ Law of Octaves

  • Every eighth element has properties similar to the first, like musical octaves.

  • Worked only for elements up to calcium.

๐Ÿ”น 4. Mendeleev’s Periodic Table

  • Based on atomic mass.

  • Left gaps for undiscovered elements (like Gallium, Germanium).

  • Limitations:

    • Position of isotopes not explained.

    • Some anomalies (e.g., Co & Ni placement).

      No fixed place for Hydrogen atom. 

๐Ÿ”น 5. Modern Periodic Law

  • “Properties of elements are a periodic function of their atomic numbers.”

  • Proposed by Henry Moseley.

  • Based on atomic number (Z) instead of mass.

๐Ÿ”น 6. Modern Periodic Table

  • Elements arranged in increasing atomic number.

  • 7 periods (horizontal rows).

  • 18 groups (vertical columns).

➤ Periods:

  • Period number = number of shells.

  • 1st period: 2 elements, 2nd & 3rd: 8 elements, etc.

➤ Groups:

  • Same number of valence electrons = same group.

  • Groups 1–2: s-block

  • Groups 13–18: p-block

  • Groups 3–12: d-block (transition elements)

  • Bottom rows: f-block (lanthanides and actinides)

๐Ÿ”น 7. Electronic Configuration and Periodicity

  • Periodic properties depend on valence shell configuration.

๐Ÿ”น 8. Periodic Trends in Properties

๐Ÿ“Œ (i) Atomic Radius

  • ↓ Group: Increases (more shells).

  • → Period: Decreases (increased nuclear charge).

๐Ÿ“Œ (ii) Ionic Radius

  • Cations < Parent atom

  • Anions > Parent atom

๐Ÿ“Œ (iii) Ionization Enthalpy

  • Energy to remove an electron.

  • ↓ Group: Decreases.

  • → Period: Increases.

๐Ÿ“Œ (iv) Electron Gain Enthalpy

  • Energy change when atom gains electron.

  • More negative across a period.

  • Halogens have highest (most negative) values.

๐Ÿ“Œ (v) Electronegativity

  • Tendency to attract shared electrons.

  • ↓ Group: Decreases.

  • → Period: Increases.

๐Ÿ”น 9. Valency

  • Depends on number of valence electrons.

  • In a period: increases from 1 to 4 then decreases to 0.

  • In a group: remains constant.

๐Ÿ”น 10. Anomalous Properties of Second Period Elements

  • Due to:

    • Small size

    • High electronegativity

    • Absence of d-orbitals

๐Ÿ”น 11. Diagonal Relationship

  • Between elements of Period 2 and Group 1/2 and those in Period 3 and Group 2/3.

  • Example: Li ↔ Mg, Be ↔ Al

๐Ÿ“˜ Summary

Property         Trend Across Period   Trend Down Group
Atomic Radius                        Decreases           Increases
Ionization Energy                 Increases           Decreases
Electron Affinity       Becomes more negative    Becomes less negative
Electronegativity                        Increases              Decreases
Metallic Character                 Decreases                           Increases

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