Early attempts at the classification of elements

• Only 30 elements were known in 1800. As more and more elements were discovered, remembering the elements and their properties were burdensome for scientists.
• So, they started gathering information about the elements and categorizing it.
• Many elements present in the Earth’s crust were isolated by scientists till the 18th century. At first these elements were classified as metals and non-metals, broadly.
• However, the number of elements increased as the years passed and this broad classification was not an efficient way to group all the elements.
• So, various tests were carried out by scientists to group the elements based on certain observations.
• Some of the popular methods were Dobereiner’s triads and Newland’s law of octaves.
• The categorization of elements in a tabular form according to their properties was a popular method of categorization and this is known as the periodic table.
• There are 118 elements present in the periodic table, out of which 94 elements are natural elements and 24 elements are synthetic elements.

DOBEREINER’S TRIADS

• Johann Wolfgang Dobereiner was a German chemist who tried grouping similar elements based on properties.
• He conducted many experiments and observed the trends.
• He finally categorized elements into groups of three in 1817, based on the atomic masses of the elements in each triad.
• These groups were called triads, because of three elements in each group.
• According to Dobereiner’s law of triads: When elements are arranged in the order of increasing atomic masses, groups of three elements known as triads having similar chemical properties are obtained.
• So, according to the law, the atomic mass of the middle element of the triad is equal to the arithmetic mean of the atomic masses of the other two elements.

(i) Alkali metal group (Dobereiner’s Triad)

(ii) Alkaline earth metal (Dobereiner’s Triad)

(iii) Halogen Group (Dobereiner’s Triad)

NEWLANDS’ LAW OF OCTAVES

• John Newlands was an English scientist who arranged the 56 elements known at that time, in the order of increasing atomic masses in 1866.
• The first element was hydrogen which has the lowest atomic mass and the last element was thorium which was the element known to have highest atomic mass at that time.
• He called this law as the ‘Law of Octaves’ better known as ‘Newland’s Law of Octaves’ because every eighth element exhibited similar properties as compared to the first element.
• According to the Newlands’ law of octaves: When elements are arranged in the order of increasing atomic masses, the properties of the eight elements (starting from a given element) are a repetition of the properties of the first element.
• Since the properties of lithium and sodium are same, sodium is placed at the eighth position with respect to lithium.
• Newlands’ classification law of octaves concluded the presence of a systematic relationship between the order of atomic masses and repetition of properties of elements.