Johann Wolfgang Döbereiner (1829) proposed one of the earliest attempts to classify elements based on their properties.

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What are Dobereiner’s Triads?

A triad is a group of three elements having:

• Similar chemical properties
• Gradual change in physical properties
• A special relationship in atomic masses

Key Rule (Important for exams):
The atomic mass of the middle element ≈ average of the other two

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Examples of Dobereiner’s Triads

1. Alkali Metal Triad

• Lithium (Li) = 7
• Sodium (Na) = 23
• Potassium (K) = 39

✔ Check:$$\frac{7 + 39}{2} = 23 \approx \text{Na}$$27+39​=23≈Na

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2. Alkaline Earth Metal Triad

• Calcium (Ca) = 40
• Strontium (Sr) = 88
• Barium (Ba) = 137

✔ Check:$$\frac{40 + 137}{2} = 88.5 \approx \text{Sr}$$

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3. Halogen Triad

• Chlorine (Cl) = 35.5
• Bromine (Br) = 80
• Iodine (I) = 127

✔ Check:$$\frac{35.5 + 127}{2} = 81.25 \approx \text{Br}$$

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🔹 Important Characteristics

• Properties change gradually across the triad
• Middle element shows intermediate behavior
• First hint toward periodicity in elements

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🔹 Limitations (VERY IMPORTANT for NEET/JEE)

• Only a few elements could be grouped
• Many known elements did not fit into triads
• Did not explain why periodicity occurs

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