Classification of Elements - Part I

In the early part  of the 19^th century, many chemists noted that chemical properties of elements showed some similarities. The elements could be formed into groups. In 1817, Dobereiner showed that elements came in groups of three, now known as Dobereiner’s triads. In 1863, a 44 year old French geologist, A. E. Béguyer de Chancourtois created a list of the elements, arranged by increasing atomic weight. The list was wrapped around a cylinder so that several sets of similar elements lined up, creating the first geometric representation of the periodic law.  In England, a 32 year old analytical chemist John A. R. Newlands was also wrapping the elements, noting that chemical groups repeated every eight elements. He named this the octave rule, and compared it to a musical scale. Chemists Dmitrii I. Mendeleev, a Russian, and German Lothar Meyer, were working independently in 1868 and 1869 on the arrangement of elements into seven columns, corresponding to various chemical and physical properties. Their tables were similar - Meyer's table was an accurate accounting of the known facts about each element, such as melting point and atomic volume. The table clearly showed the existence of periodic chemical families.

What we will study in this chapter
1. Dobereiner’s triads
2. Newlands’ Law of octaves
3. Lothar Mayer’s atomic volume curves
4. Mendeleev’s periodic table
5. Modern periodic table  

1. Dobereiner’s triads
Dobereiner in 1817 observed that certain elements, which had similar chemical properties, could be grouped together. When these elements were arranged in increasing order of their atomic masses, they generally occurred in groups of three. These groups were called triads. He noticed that the atomic mass of the middle element of the triad was the arithmetic mean of the other two elements of the triad.  This was known as the Dobereiner’s law of triads. The law states that : when elements are placed in order of the ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle element of the triad is equal to the mean of the atomic masses of the other two elements of the triad.

Drawbacks of Deberneir’s law of triad, was that it was valid only for a few groups of elements known during that time. Also with more accurate measurements of atomic masses showed that the mid element of the triad did not really have the mean value of the sum of the other two elements of the triad.

Examples of Dobereiner Triads : 

In the alkali metal group, consider elements lithium (Li), sodium (Na)  and potassium (K). All these elements are metals, they are highly reactive and they show valency of +1. The Dobereiner’s triad for alkali metal group can be shown as:

From the Dobereiner’s law of triads, the atomic mass of the middle element, in this case Na, should be the arithmetic mean of Li and K.

Thus
                                                  
arithmetic mean of Li and K =         7 + 39     =   23
                                                      2

It can be seen that

Arithmetic mean of atomic masses of Li and K = atomic mass of Na.

Now consider elements in the halogen group : chlorine (Cl), bromine (Br) and iodine (I). All these elements are non-metallic, they are very reactive and form acids with water, they have a valency of –1. Due to their similar chemical properties, these three elements formed another of Dobereiner’s  triad. So see if the Cl, Br, I obey the Dobereiner’s law of triad, consider the following table.

For Dobereiner’s law to be valid

                                                            
A (Br)  =         A (Cl) + A (I)         =          35.5 + 127        =      81.2
                             2                                   2

The actual atomic mass of Br is 80.

Thus the atomic mass of the middle element of the triad,  is nearly equal to the arithmetic mean of the atomic masses of the other two elements of the triad.  Hence the Dobereiner’s law holds true for halogen triads.

Consider another group of elements : sulphur (S), selenium (Se) and tellurium (Te). All these elements are non-metals, tending to show metallic behavior. When you arrange them in the ascending order of their atomic masses, they obey Dobereiner’s law. See the table given below.  

We can  verify that

                                            
A (Se)  =       A (S) + A (Te)                                     
                             2  

Dobereiner’s law of triads failed for the following reasons :  

• all the then known elements could not be arranged in the form of triads.

• for very low mass or for very high mass elements, the law was not holding good. Take the example of F, Cl, Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.

• as the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly valid.

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