Classification of Elements - Part II

2. Newlands’ Law of octaves
After Dobereiner’s ideas were ruled out, an English chemist John Alexander Reina Newlands in 1864 noted that every eighth element showed similar physical and chemical properties, when the elements are placed in the increasing order of their atomic masses. This was called as the Newlands’ law of octaves. The law states that when elements are placed in the increasing order of atomic masses, the properties of the eight element are repeated.

Newlands arranged the elements then known in the following manner.

Li         Be            B            C            N            O            F

Na       Mg            Al            Si           P            S            Cl        

K         Ca

Row of elements had seven elements and the eighth fell under the first element. In those days, the number of elements known were very limited and no elements from the noble or inert gas elements such as helium (He), neon (Ne), argon (Ar), etc. were known. 

First let us see the elements in the first column. Li is the first element. The eighth element after Li is Na. Similarly, the eighth element after Na is K. So from the Newlands' law of octaves, we should expect the elements Li, Na and K to have similar chemical properties. This they do have. All the elements are metallic, highly reactive and show a valence of +1. They are known as alkali elements.

Next, if we take beryllium (Be) as the first element, the eighth element from Be is magnesium (Mg). If we continue in the similar fashion, the eighth element after Mg is calcium Ca. According to Newlands’ law, the elements Be, Mg and Ca should display similar chemical and physical properties. They do. The elements Be, Mg, Ca fall under the group of alkali-earth metals. All these elements are metallic in nature, their oxides are alkaline in nature and they have a valence of +2.

Now look at another vertical column that has carbon (C) as the first element. The eighth element from C is silicon (Si). It is seen that C and Si are similar in properties. Both of them show tetra-valency. They are non-metals and form oxides easily. Thus Newlands' law of octaves hold good.

Similarly lets see the last group of halogens starting with flourine (F). The eighth element after F is chlorine (Cl). As we have already seen that F and Cl display similar properties. Both of them are highly reactive, when dissolved in water, form acids, and have valence of  -1.  Thus the Newlands' law of octaves was obeyed.

Newlands' Law of octaves failed for the following reasons :

1. It was not valid for elements that had atomic masses higher than Ca.
2. When more elements were discovered, such as elements from the noble gases such as He, Ne, Ar, they could not be accommodated in his table.

But the most important contribution in the process of classification of elements was the periodicity he saw in every eighth element. The modern periodic table, that we shall study in later sections, drew heavily from the concept of periods of eight. Also it must be noted that Dobereiner’s triads occurred in the octaves of Newlands.

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