Periodic table:
As many as 118 elements,natural and synthetic,have so far been discovered.The tabular form of this arrangement in which elements are arranged in horizontal rows and vertical columns is called periodic table.Some of the earlier important attempts to classify the elements are briefly summed up as follows.
1.Dobereiner triads:
In 1829,dobereiner, a german chemist,was the first to suggest that the properties of the elements might be related to their atomic masses.He showed that there were several groups of three elements which had very similar properties and the atomic mass of the middle element was nearly equal to the arithmetic mean of the atomic masses of the other two elements.Some examples of these triads are:
lithium=7
sodium =23
potassium=39
7+39=46/2=23= atomic mass of sodium
The major drawback of dobereiner's classification was that all the known elements could not be arranged in triads.
2.Newlands law of octave:
In 1865,an english chemist,newland proposed a co-relation between the magnitude of the mass and properties of elements.He showed that when elements (leaving hydrogen) were arranged in order of increasing atomic masses,every succeeding eighth element had properties similar to those of the first.On analogy with the musical scale,he called this relationship as the law of octaves.
Thus,the properties of sodium and potassium are similar to those of lithium, whereas the properties of magnesium and calcium resemble to those of beryllium and so on.The chief drawback of the newlands, system was that the regularity broke down after calcium.
3.Lother meyer's rearragement:
We see that the most electropositive alkali metals (li,na,k,rb and cs) occupy the peaks on the curves,the less electropositive alkaline earth metals (be,mg,ca,sr and ba) occupy the ascending positions on the curve.All these observations led lother meyer to conclude that the atomic volumes of elements are a periodic function of their atomic masses.He arranged the known elements in the tabular form in order of their increasing atomic masses.All these attempts provided the same conclusion that there are certain regularities among the elements.