  Question

The atomic masses of three elements X, Y and Z having similar chemical properties are 7, 23 and 39 respectively. (a) Calculate the average atomic mass of elements X and Z. (b) How does the average atomic mass of elements X and Z compare with the atomic mass of element Y? (c) Which law of classification of elements is illustrated by this example? (d) What could the elements X, Y and Z be? (e) Give another example of a set of elements which can be classified according to this law.

Solution

(a) The atomic masses of three elements X, Y and Z are 7, 23 and 39 respectively. The average atomic mass of X and Z is nothing but the arithmetic mean of the atomic masses of X and Z. Therefore, the average atomic mass of  X and Z = $\frac{7+39}{2}=23$. (b) From the above calculation, we observe that the average atomic mass of the elements X and Z is equal to the atomic mass of the element Y. (c) Dobereiner's law of triads is illustrated by the above example. According to Dobereiner's law, when elements are arranged in an increasing order of atomic masses, a three element group (triads), having the same chemical properties is formed. The atomic mass of the middle element is equal to the average of the atomic masses of the other two elements. (d) The elements X,Y and Z , having atomic masses 7, 23, 39 are lithium (Li), sodium (Na) and potassium (K). They belong to the alkali metal group (group 1), with the same valency of 1 and exhibit similar chemical properties. (e) The elements calcium (Ca), strontium (Sr) and barium (Ba) are a set of alkali earth metals (group 2 elements), with atomic masses 40,88 and 137 respectively, which are classified according to Dobereiner's law of triads. ScienceLakhmir Chemistry 2013Standard X

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