• Law: Properties of elements are a periodic function of their atomic masses
• Elements arranged in increasing atomic mass
• Similar properties appear at regular intervals → periodicity

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🧪 Key Features

1. Groups & Periods

• Vertical columns → Groups (I to VIII)
• Horizontal rows → Periods
• Elements in same group → similar valency & chemical properties

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2. Subgroups (A & B)

• Each group divided into:
  • A (main group) → normal elements
  • B (transition elements) → less regular properties

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3. Gaps for Undiscovered Elements ⭐ (VERY IMPORTANT)

• Mendeleev left blank spaces for unknown elements
• Predicted their properties accurately
  Example:
  • Eka-aluminium → Gallium
  • Eka-silicon → Germanium

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4. Correction of Atomic Mass

• If element didn’t fit → atomic mass was corrected
  • Example: Tellurium–Iodine anomaly handled by placement based on properties

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5. Periodicity Trend

• Properties repeat due to similar valence shell configuration (not known then)

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⚠️ Limitations (High-weightage for exams)

1. Hydrogen Position ❓

• Resembles Group 1 and Group 17 → no fixed position

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2. Isotopes Problem

• Isotopes have same properties but different masses
• Could not be placed properly

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3. Increasing Atomic Mass Order Violations

• Example:
  • Argon (39.9) before Potassium (39.1)
• Order violated to maintain chemical similarity

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4. No Explanation of Periodicity

• Reason (electronic configuration) was unknown

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5. Lanthanides & Actinides

• No proper place in table

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