CBSE – 10th Standard Science

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Class 10 Chapter 5 – Periodic Classification of Elements

 

Periodic Classification of Elements

In the year 1817, Johann Wolfgang Döbereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified some groups having three elements each. So he called these groups ‘triads’.

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MAKING ORDER OUT OF CHAOS – MENDELÉEV’S PERIODIC TABLE

When Mendeléev started his work, 63 elements were known. He examined the relationship between the atomic masses of the elements and their physical and chemical properties. Among chemical properties, Mendeléev
concentrated on the compounds formed by elements with oxygen and hydrogen.

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The maximum number of electrons that can be accommodated in a shell depends on the formula 2n2 where ‘n’ is the number of the given shell from the nucleus

For example,

K Shell – 2 × (1)2 = 2, hence the first period has 2 elements.

L Shell – 2 × (2)2 = 8, hence the second period has 8 elements.

M Shell – 2 × (3)2 = 18, but the outermost shell can have only
8 electrons, so the third period also has only 8 elements.

The position of an element in the Periodic Table tells us about its chemical reactivity.

The valence electrons determine the kind and number of bonds formed by an element.

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