(i) About more than one hundred years ago Lavisher discovered that some elements resembled in their properties. He classified the thirty then known elements into metal and non-metals to study their acid and basic properties.
But his classification could not account for those elements which had the properties of both metals and non-metals.
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(ii) Determiner’s triads:
Determiner in 1829 observed that there were certain groups of three similar elements called triads. The elements of a triad had similar properties and when arranged in order of increasing atomic masses, the atomic mass and properties of the middle element was roughly the arithmetic mean of the other two.
For example, lithium (atomic mass = 7), sodium (atomic mass = 23) and potassium (atomic mass = 39) formed such a triad. The atomic mass of sodium is equal to the average of the atomic mass of lithium and Potassium.
In the triad, calcium (atomic mass = 40), strontium (atomic mass = 88) and barium (atomic mass = 137), the atomic weight of the middle element strontium is very close to the average of the atomic weights of barium and calcium
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The elements chlorine (35.5), bromine (80) and iodine (127) formed another triad. However, only a limited number of elements could be arranged in this fashion.
(iii) Newlands’ law of octaves:
Newland in 1864 found that if the elements were arranged in the order of increasing masses, every eighth element starting from a given one would be a repetition of the first with regard to its properties. Therefore, Newlands’ law came to be known as law of octaves.
However, this law was applicable to a few elements only. It could not be applied to elements of higher atomic masses. With the discovery of noble gases, it became the ninth element and not the eighth element that will be similar to the first one.
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(iv) Lother Meyer’s atomic volume curve:
Luther Meyer plotted a graph of the atomic volume of different elements against their respective atomic mass
He found that elements with similar properties occupy corresponding positions on the peaks and troughs. For example, elements lithium, sodium, potassium, rubidium, caesium and francium occupy only peak.
Chemically similar elements, fluorine (F), chlorine (CI) and bromine (Br) also occupy the similar positions as on the troughs in the curve. However, only a limited number of such groups could be made with certainty.