Introduction to Periodic Classification of Elements

Introduction

The elements are the basic units of all types of matter. In 1800, only 31 elements were discovered. Fifty years later by 1850, scientists had discovered 63 chemical elements and the numbers kept increasing. With the discovery and study of more and more elements and their compounds, the various data about them also increased. It became progressively difficult to organise all the known elements but different scientists made attempts to look for some trends and patterns in their properties.

Initially, the elements were simply classified as metals and non-metals. However, some elements possessed properties which could neither be classified as metals nor nonmetals which were later named as metalloids. As this kind of classification was insufficient for systematic studies of the elements and their compounds, several chemists attempted to make a rational and systematic classification of the physical and chemical properties of elements and tabulate the results in the form of a table. Such tables helped even to predict the new elements, opening new insights into understanding the properties of the matter. Thus, periodic classification of elements evolved.

History of Periodic Classification of Elements

Classification of elements into groups and development of periodic law and periodic table are the consequences of systematizing the knowledge gained by a number of scientists through their observation and experiments. Few important attempts for the classification of elements are described below :

• Dobereiner's Triads:

Johann Wolfgang Dobereiner was the scientist to make the earliest attempt for the classification of the elements discovered till then. In 1829, he noticed that some elements formed groups of three with related properties. He termed those groups the 'Triads'. Some triads classified by Dobereiner are:

a) Chlorine, Bromine, and Iodine

b) Calcium, Strontium, and Barium

c) Sulfur, Selenium and Tellurium

d) Lithium, Sodium, and Potassium

He noticed that the element of each of the triads had an atomic mass approximately equal to the arithmetic mean of other two elements. The Dobereiner's relationship referred to the Law of Triads, seems to work only for a few elements out of those known at that time. Some of Dobereiner's triads are shown in the table below. It is interesting to note that the Law of Triads is valid even on using the atomic number as the basis for classification.

• de Chancourtois Telluric Helix

The first person to notice the periodicity of the properties of the elements was a French geologist named Alexandre-Emile Beguyer de Chancourtois. He pointed out in 1862 that,similar elements seem to occur at regular intervals when they are ordered by their atomic masses.He devised a table called Telluric Helix. With the elements arranged in a spiral on a cylinder by order of increasing atomic mass, de Chancourtois saw those elements with similar properties lined up vertically. He did not include the chart he developed in his publication and thus, his discovery was neglected till Mendeleev's Table was published.

• Newlands' Law of Octaves

In 1866, the English chemist, John Alexander Newland arranged the elements in increasing order of their atomic masses and noted that every eighth element has properties similar to the first element. It was called Newland Law of Octaves as the relationship was just like every eighth, not that resembles the first in octaves of music. Newlands' Law of Octaves seemed to be true only for elements up to calcium. A few octaves based on Newlands' law are shown in the table below. The numbers in parentheses are atomic masses.

According to the Law of Octaves, the elements in the groups indicated by I, II, III (such as properties of the H, F, Cl; Li, Na, K; and Be, Mg, Ca etc.) should be similar which is actually the case.

Table showing octaves of some elements

• Lothar Meyer's Atomic Volume Curve

In 1864, a German chemist Lothar Meyer published a chart in which he plotted various physical properties (such as atomic volume, melting point and boiling point) as a function of their atomic masses. He noted that the elements with the similar position on the curve as shown in the curve below. The atomic volumes were calculated by dividing atomic masses by the solid state density of the elements.

The following conclusions can be drawn from Lothar Meyer's curve:

a) The alkali metals (Li, Na, K, Rb, and Cs) occupy the peaks on the curve.

b) The alkaline earth metals (Be, Mg, Ca, Sr, and Ba) and the elements forming basic oxides occupy the descending position on the curve.

c) The halogens (F, Cl, Br, I) and the elements forming acidic oxides occupy the ascending position on the curve.

Thus, Lothar Meyer's classification for the first time confirmed a definite pattern and periodicity in the properties of the elements. He is regarded as co-creator of the Periodic System of Elements.

Reference

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).