PERIODIC CLASSIFICATION OF ELEMENTS.pdf

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PERIODIC CLASSIFICATION OF ELEMENTS Metals

non-metals Elements Metalloides

Inert gases Matter Homogenous Mixture Heterogenou s Compounds

Dobereiner’s Dobereiner ’s Triads Triads 

In 1817, Johann Wolfgang Dobereiner, a German chemist.

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Identified some groups having three elements each called ‘triads’.

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In triads when elements are arranged in order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

Limitation: Only three triads were identified at that time

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Newlands’’ Law of Octaves Newlands 

In 1866, John Newland an English scientist, arranged the then known elements in the order of increasing atomic masses.

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He started with element Hydrogen (A=1) to Thorium (A=56).

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He found that every eighth element had properties similar to that of the first.

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He compared this to the octaves found in music. Therefore, he called it the ‘ Law of Octaves’.

Limitations: 

Law of Octave was applicable only up to Calcium.

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It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future. 2


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Adjusted two elements in the same slot, but also put some unlike elements under the same note.

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Law of Octaves worked well with lighter elements only.

Mendeleev’s Periodic Table 

Dmitri Ivanovich Mendeleev, a Russian chemist.

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Arranged 63 elements

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Elements were arranged on the the basis of – 

Atomic masses

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Their physical and chemical properties

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The formulae of Hydrides and Oxides formed by an element were treated as the basic properties for classification. Group I Oxides R2O Hydride RH

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II RO RH2

III R2O3 RH3

IV RO2 RH4

V R2O5 RH3

VI RO3 RH2

VII R2O7 RH

He observed there there occurs a periodic periodic recurrence elements with with similar physical and chemical properties.

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‘The properties of elements are the periodic function of their atomic masses’.

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Periodic table contains 8 vertical columns called ‘groups’ and 6

horizontal rows called ‘periods’. 3

VIII RO4


Achievements of Mendeleev’s Periodic Table 

Placed slightly greater atomic mass before an element with slightly lower atomic mass. Example; cobalt (A= 58.9) before nickel ( A=58.7).

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Left some gaps in his Periodic Table for Eka – Eka –boron boron

for scandium

Eka- aluminium

for gallium

Eka- silicon

for germanium

Inert gases could be placed in a new group without disturbing the existing order.

Limitations 

Could not assign a correct position to Hydrogen (H form compound like Alkali metals and exist in Diatomic form). Compounds of H HCl H2O H2S

Compounds of Na NaCl Na2O Na2S

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Isotopes of all elements posed a challenge to Mendeleev’s

Periodic Law. 

Atomic masses do not increase in a regular manner going

from one element to the next.

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The Modern Periodic Table 

In1913, Henry Moseley showed that

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‘ Properties of elements are a periodic function of their atomic number.’ number.’

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Trends in Modern Periodic Table

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PERIODIC CLASSIFICATION OF ELEMENTS.pdf

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