Periodic Classification of Elements
PERIODIC CLASSIFICATION OF ELEMENTS
Introduction
There are 115 elements that are known at present. Some elements have similar properties whereas some others have completely contrasting properties
Scientists began to look for some pattern in the properties of these elements
Dobereiner’s triads
In 1817, Johann Wolfgang Dobereiner, a German chemist, classified elements into groups based on their properties. He kept all elements having similar properties in one group. Most of his groups had three elements each. Thus, he called these groups as triads
He also gave a law known as the Law of Triads.
It states that when three elements in a triad are listed in the increasing order of their atomic masses, the atomic mass of the middle element will roughly be the average of the atomic masses of the other two elements
Other metals Non-metals
Element Atomic mass Element Atomic mass
Calcium 40 Chlorine 35.5
Strontium 88 Bromine 80
Barium 137 Iodine 127
Average mass of calcium and barium =
= 88.5
Hence, Dobereiner was able to identify only three triads from the elements known at that time as shown
Li Ca Cl
Na Sr Br
K Ba I
Table 1: Dobereiner’s triads
Limitations of Dobereiner’s classification of elements:
• All known elements could not be classified into groups of triads on the basis of their properties.
• Not all groups obeyed the Law of Triads. For example, nitrogen family does not obey the Law of Triads
Although Dobereiner tried to classify elements into groups based on their properties, his classification was not very useful.
In 1866, John Newlands, an English scientist, arranged the known elements in an increasing order of their atomic masses.
He began with hydrogen, which has the lowest atomic mass. He observed that if the elements are arranged in the increasing order of their atomic masses, then every eighth element (starting from a given element) had properties similar to those of the first element. Therefore, he arranged the elements in
Introduction
There are 115 elements that are known at present. Some elements have similar properties whereas some others have completely contrasting properties
Scientists began to look for some pattern in the properties of these elements
Dobereiner’s triads
In 1817, Johann Wolfgang Dobereiner, a German chemist, classified elements into groups based on their properties. He kept all elements having similar properties in one group. Most of his groups had three elements each. Thus, he called these groups as triads
He also gave a law known as the Law of Triads.
It states that when three elements in a triad are listed in the increasing order of their atomic masses, the atomic mass of the middle element will roughly be the average of the atomic masses of the other two elements
Other metals Non-metals
Element Atomic mass Element Atomic mass
Calcium 40 Chlorine 35.5
Strontium 88 Bromine 80
Barium 137 Iodine 127
Average mass of calcium and barium =
= 88.5
Hence, Dobereiner was able to identify only three triads from the elements known at that time as shown
Li Ca Cl
Na Sr Br
K Ba I
Table 1: Dobereiner’s triads
Limitations of Dobereiner’s classification of elements:
• All known elements could not be classified into groups of triads on the basis of their properties.
• Not all groups obeyed the Law of Triads. For example, nitrogen family does not obey the Law of Triads
Although Dobereiner tried to classify elements into groups based on their properties, his classification was not very useful.
In 1866, John Newlands, an English scientist, arranged the known elements in an increasing order of their atomic masses.
He began with hydrogen, which has the lowest atomic mass. He observed that if the elements are arranged in the increasing order of their atomic masses, then every eighth element (starting from a given element) had properties similar to those of the first element. Therefore, he arranged the elements in