Periodic Table (The Genesis of Modern Chemistry) - 1

Dobereiner’s Triads

- Grouped three elements with similar properties called triads.

- The Atomic weight of the middle element has the mean atomic weight of the extreme elements of the triad.

- E.g. – 1. (Calcium, Strontium and Barium)

  • Valency - +2

  • Metallic

  • Hydroxide was Basic

Atomic Wt. of Strontium = 88
Mean of Atomic Weight of Ca and Ba = (40 +137)/2 = 88.5
which is almost equal to Weight of Strontium.

2. (Chlorine, Bromine and Iodine)

  • Valency - -1

  • Non- Metallic

  • Halide was Acidic


Atomic Wt. of Bromine = 80
Mean of Atomic Weight of Cl and I = (35.5 + 127)/2 = 81.25
which is almost equal to Weight of Bromine.

Failures of the Dobereiner’s Triads

· All known elements could not be segregated into triads.

· Even in the same group, the theory failed.(Like in Fluorine, Chlorine, Bromine)

 

Newland’s Law of Octave

· If we arrange the elements in increasing order of their atomic weight, then every 8th element will resemble the properties of the 1st element starting from anywhere.

Merits of Law of Octaves

· First time the elements were arranged in an order with relation to the Atomic wt. (Properties of Elements were related to Atomic weight).

· Lighter elements were grouped together like Lithium, Sodium, and Potassium.

· Periodicity - It refers to trends or recurring variations in element properties with increasing atomic number or other factor. Periodicity is caused by regular and predictable variations in element atomic structure.

Demerits of Law of Octaves

· Discovery of Noble Gases – No space to accommodate Noble Gases

· Discovery of heavier Metals






 

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