Periodic Classification of Elements

Need and Early attempts

Need of classification of elements became necessary with increase in discovery of elements.

Early attempts At the Classfication of Elements

Doereiner's Triads

Johann Wolfgand Doereiner, a German Chemists identified some groups having three elements each with similar properties.

He arranged three elements in a group in the increasing order of their atomic mass and found that the atomic mass of the middle element was roughly the average of the atomic masses of other two elements.

Drawbacks of Dobereiner Triads

Dobereiner could identify only three triads among the elements known at that time. Because of this Dobereiner's Classification of element was not found to be useful.

Newland's Law of Octaves

John Newlands, an English scientist, arranged elements known at that time in the increasing order of their atomic mass. He found that every eighth element had properties similar to that of the first. He compared this to the octaves found in music therefore he called it the 'Law of Octaves'. It is known as Newlands' Law of Octaves.

In Newlands' Octave, sodium is the eighth element after lithium and hence has similar properties. Similarly, Beryllium and magnesium resembles each other.

Drawbacks of Newlands' Octaves

Newlands' Octaves was applicable only upto calcium (Ca), as after calcium (Ca) every eighth element did not possess the properties similar to that of the first.

At the time of Newlands only 56 elements were known. After discovery of several new elements, their properties did not fit into the Law of Octaves.

Newlands placed cobalt (Co) and nickel (Ni) in the same group as they have similar properties, but kept iron (Fe) very far from these elements while iron, cobalt and nickel have similar properties.

Cobalt (Co), and nickel (Ni) are placed in same column as chlorine (Cl), fluorine (F) and Bromine, while cobalt and nickel has completely different properties than those of chlorine, fluorine and bromine.

Hence, Newlands' Law Octaves work well with lighter elements only.

Mendeleev's Periodic Table

Dmitri Ivanovich Medeleev, as Russian chemist successfully classify elements in a table, called Mendeleev's Periodic Table.

Mendeleev arranged elements in the increasing order of their atomic mass starting from hydrogen, which has the lowest atomic mass similar to that of the Newlands.

Among chemical properties Mendeleev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected oxygen and hydrogen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by elements were treated as one of the basic properties of an element for its classification.

Mendeleev's Law of Periodic Table: 'the properties of elements are the periodic function of their atomic masses'.

Mendeleev's Periodic Table: At a glance

Mendeleev's Periodic table contains vertical column and horizontal rows as similar to a table.

He called vertical columns 'Groups' and horizontal rows 'Periods'.

There are eight groups and six periods in the Medeleev's Periodic table.

From 1^st to 7^th each groups are divided into two subgroups called 'A' and 'B'.

From 4^th to 6^th periods each are divided into two series called 'First series' and 'Second series'.

The 8^th group is called transition series.

Achievements of Medeleev's Periodic Table

(a) Mendeleev placed some element having slightly greater atomic mass before the element having slightly lower atomic mass so that elements with similar properties could be grouped together. For example: cobalt is placed before nickel while atomic mass of cobalt is 58.9 and atomic mass of nickel is 58.7

(b) Mendeleev left some gaps in his Periodic Table for those elements that had not been discovered at that time. And he predicted boldly about the existence of those elements to be discovered in future.

(c) Mendeleev named them by prefixing 'Eka(one)' to the name of preceding element in the same group. For example: scandium, gallium and germanium, discovered later, have properties similar to Eka-boron, Eka-aluminium and Eka-silicon, respectively.

(d) Inert gases were not discovered at the time of Mendeleev. After discovery of inert gases, they could be placed as a separate group without disturbing the existing order of elements in the Mendeleev's Periodic Table.

Limitation of Mendeleev's Classification of Elements

(a) Hydrogen was placed with alkali metals as hydrogen combines with halogen, oxygen and sulphur similar to alkali metals. On the other hand, just like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compound.

Thus, Mendeleev could not assign and explain the position of Hydrogen in his Periodic Table.

(b) Long after Mendeleev had proposed his Periodic Classification of Elements, Isotopes were discovered.

Isotopes of an element have similar properties but different atomic masses.

Thus, isotopes of all elements posed a challenge to Medeleev?s Periodic Law. If isotopes were incorporated in the Mendeleev's Periodic Table, it would disturb the order of elements because elements were arranged in the increasing order of their atomic masses.

(c) Some elements do not placed in Periodic table in correct order of their increasing atomic mass. For example Cobalt (Atomic mass: 58.93) was placed before Nickel (Atomic mass: 58.71).

Instead of above limitation Mendeleev's Periodic Table was the best classification of elements and proved a milestone in the field of Chemistry. Mendeleev's Periodic Table also led to discovery of new element and further development of Classification of Elements in better way.