CBSE Class 10 Science 2020 · Paper 31/4/1 · Question 3

Periodic Classification of Elements 4 marks Medium

Answer question numbers 3(a) – 3(d) on the basis of your understanding of the following paragraph and the related studied concepts : Around the year 1800, only 30 elements were known. Dobereiner in 1817 and Newlands in 1866 tried to arrange the then known elements and framed laws which were rejected by the scientists. Even after the rejection of the proposed laws, many scientists continued to search for a pattern that correlated the properties of elements with their atomic masses. The main credit for classifying elements goes to Mendeleev for his most important contribution to the early development of a Periodic table of elements wherein he arranged the elements on the basis of their fundamental property, the atomic mass and also on the similarity of chemical properties. The formulae of their hydrides and oxides were treated as basic criteria for the classification of the elements. However, Mendeleev’s classification also had some limitations as it could not assign the position to isotopes. He also left some gaps in the periodic table.

(a)State Mendeleev’s Periodic Law.[1]
(b)Why did Mendeleev leave some gaps in the Periodic table ?[1]
(c)If the letter ‘R’ was used to represent any of the elements in the group, then the hydride and oxide of carbon would respectively be represented as (i) $RH_{4}$ , $RO$ (ii) $RH_{4}$ , $RO_{2}$ (iii) $RH_{2}$ , $RO_{2}$ (iv) $RH_{2}$ , $RO$ [1]
(d)Isotopes are (i) Atoms of an element with similar chemical properties but different atomic masses. (ii) Atoms of different elements with similar chemical properties but different atomic masses. (iii) Atoms of an element with different chemical properties but same atomic masses. (iv) Atoms of different elements with different chemical properties but same atomic masses.[1]

Answer

Properties repeat periodically with atomic mass; gaps were left for undiscovered elements; carbon's hydride and oxide are option (ii); isotopes are option (i).

(a) The properties of elements are the periodic function of their atomic masses. (b) Mendeleev left the gaps for elements that had not been discovered then. He predicted their existence and properties (eka-boron, eka-aluminium and eka-silicon), and scandium, gallium and germanium, discovered later, filled them. (c) Option (ii):

RH_{4}

RO_{2}

(carbon forms

CH_{4}

and

CO_{2}

). (d) Option (i): Atoms of an element with similar chemical properties but different atomic masses.

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CBSE Class 10 Science 2020 · Paper 31/4/1 · Question 3

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