If the first and third members of a Doberiner triad are Phosphorus and Antimony, the second member of this triad is:

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RRB ALP Previous Paper 8 (Held On: 13 Aug 2018 Shift 3)
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  1. Iodine
  2. Sulphur
  3. Calcium
  4. Arsenic

Answer (Detailed Solution Below)

Option 4 : Arsenic
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Detailed Solution

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  • Doberiner made groups of three elements each having similar chemical properties and called them triads.
  • Doberiner arranged the 3 elements in a triad in an increasing order of atomic mass and showed that the atomic mass of the middle element was equal to the mean of the atomic masses of the two elements.
  •  

    Triad

    Element – 1

    Element – 2

    Element  3

    Actual atomic mass(a)

    Mean = (a + c)/2

    Actual atomic mass (c)

    Li, Na, K

    Lithium (Li)

    Sodium (Na)

    Potassium (K)

    6.9

    (6.9 + 39.1)/2

    = 23.0

    39.1

    P, As, Sb

    Phosphorus(P)

    Arsenic (As)

    Antimony (Sb)

    30.9

    (30.9+121.7)/2

    = 76.9

    121.7

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