Periodic Classification of Elements -Chapter-5-Notes

Döbereiner’s triads

These groups of three elements having similar properties were called Döbereiner’s triads after his name. He further observed that when the elements of any particular triad were arranged in order of their increasing atomic masses, the atomic mass of the middle element was roughly the mean or average of the atomic masses of the other two elements.

Limitations: Döbereiner could identify only four triads from the elements known at that time. In other words, all the elements known at that time could not be arranged as Döbereiner’s triads. For example, the three elements, nitrogen (N), phosphorus (P) and arsenic (As) have similar properties. Therefore they can be regarded to form a triad. However, the actual atomic mass of phosphorus (31.0 u) is much lower than the average (44.45 u) of the atomic masses of nitrogen and arsenic. Thus, these three elements do not constitute a Döbereiner’s triad in spite of their similar chemical properties.

This, this system of classification of elements into triads could not be applied to all the elements known at that time and hence this was rejected.

Newlands’s law of octaves

When elements were arranged in order of their increasing relative atomic masses, the properties of every eighth element were similar to the first one like the eighth note of a musical scale.

Limitations: 

1. Newlands’s law of octaves was applicable only to lighter elements having atomic masses upto 40 u, i.e., upto calcium.

2. It was assumed by Newlands that only 56 elements existed in nature and no new elements would be discovered in future. But, later on, several new elements were discovered whose properties did not fit into the law of octaves.

3. In order to fit elements into his table 5.1, Newlands not only placed two elements in the same slot but also replaced some unlike elements in the same column.

4. Noble gases were not known when Newlands gave his law of octaves. However, when noble gases were discovered around the year 1900, the properties of the eighth element were no similar to the first one. Actually, now it was the first and the ninth element which has similar properties.

Mendeleev’s periodic law

A periodic table may be defined as an arrangement or a chart which classifies all the known elements on the basis of their properties in such a way that the elements with similar properties are placed in the same vertical column and dissimilar elements are separated from one another.

Mendeleev’s periodic table

At the time when Mendeleev gave his periodic table, 63 elements were known. He arranged these elements in increasing order of their atomic masses in horizontal rows (now can periods) in such a way that elements with similar properties fell under the same vertical columns called groups.

Characteristic features of Mendeleev’s periodic table:

(1) Eight vertical columns called groups:  These are designated as I, II, III, IV, V, VI, VII and VIII. Except group VIII, each group is divided into two sub-groups designated as A and B. The elements which lie on the left hand side of each group constitute sub-group A. These are called normal or representative elements. The elements which lie on right hand side of each group constitute group B. These are called transition elements. This sub-division is made on the basis of difference in their properties.

Group VIII contains nine transition elements in three sets each containing three elements. These three sets lie in the 4th, 5th and 6th period.

Please note that noble gases (or inert gases or the rare gases of the atmosphere) were not known at the time of Mendeleev. Therefore, there was no group of noble gases in Mendeleev’s original periodic table. However, when these gases were discovered around the year 1900, a new group called the zero group was added to the Mendeleev’s periodic table.

(2) Seven horizontal columns called periods:These are numbered from 1 to 7.

Achievements of Mendeleev’s periodic table

When developing the periodic table. Mendeleev kept two things in mind:

(i) Increasing atomic masses

(ii) Grouping of similar elements together.

Limitations of Mendeleev’s classification of elements (or periodic table)

1. Anomalous position of hydrogen.

2. Position of isotopes.

3. Wrong order of atomic masses of some elements could not be explained.

4. Uncertainty in prediction of new elements.

Modern periodic law- atomic number as the basis of classification of elements

Atomic number is equal to the number of protons in the nucleus.

Atomic number is the number of electrons in the extranuclear part of the atom.

The atomic number of an element is a more fundamental property than its atomic mass.

The atomic number of every element is fixed and no two elements can have the same atomic number.

Modern periodic law

The properties of elements are a periodic function of their atomic numbers.

Position of elements in the Modern periodic table

The modern periodic table consists of 18 vertical columns called groups and 7 horizontal rows called periods. The groups are numbered from 1-18 while periods are numbered from 1 to 7.

Q1. Limitations of Döbereiner’s triads?

Q2. What is Modern Periodic Law?

Q3. Limitations of Mendeleev’s classification of elements?