CBSE Class 10 Science 2018 · Paper 31/1 · Question 17

(a) Döbereiner’s triads. Advantage: elements with similar properties were grouped in threes, and the atomic mass of the middle element was roughly the average of the other two, so it could be predicted (lithium 6.9, sodium 23.0, potassium 39.1). Limitation: he could identify only three triads from the elements known at that time. Newlands’ law of octaves. Advantage: with the elements in increasing atomic mass, every eighth element had properties similar to the first, which related the properties of elements to their atomic masses. Limitation: it was applicable only up to calcium (and he assumed that only 56 elements existed, with no new elements to be discovered). Mendeleev’s periodic table. Advantage: elements with similar properties were grouped together, and gaps were left for elements not yet discovered, whose existence and properties he predicted. Limitation: no fixed position for hydrogen; no place for isotopes; atomic masses do not increase in a regular manner. Any one correct advantage and one correct limitation of each attempt earns the marks. (b) Henry Moseley. (c) Modern periodic law: properties of elements are a periodic function of their atomic number.