CBSE Class 10 Science 2018 · Paper 31/1/C · Question 17(b)

(a) Mendeléev arranged the elements in increasing order of their atomic masses and grouped together elements with similar chemical properties; among chemical properties he used chiefly the formulae of the oxides and hydrides formed by the elements. His periodic law states that the properties of elements are a periodic function of their atomic masses. (b) Achievements: 1. He left gaps in his table for elements not yet discovered and predicted their properties (eka-boron, eka-aluminium, eka-silicon); when scandium, gallium and germanium were discovered, their properties matched the predictions closely. For example, eka-aluminium was predicted to have atomic mass $68$ and oxide ${\text{E}}_2{\text{O}}_3$; gallium has atomic mass $69.7$ and oxide ${\text{Ga}}_2{\text{O}}_3$. 2. When the noble gases (helium, neon, argon) were discovered later, they could be placed in a new group without disturbing the existing order. (c) Challenges: 1. Isotopes: isotopes of an element, such as chlorine-35 and chlorine-37, have different atomic masses but the same chemical properties, so a classification based on atomic mass cannot place them properly. 2. Position of hydrogen: hydrogen resembles both the alkali metals and the halogens, so it could not be given a fixed position. Other correct observations, such as the irregular increase of atomic mass from one element to the next, or cobalt (58.9) having to be placed before nickel (58.7), also earn credit.