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.
(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 the arithmetic mean of the atomic masses of X and Z.
Therefore, the average atomic mass of X and Z = (7+39)/2=23 (which is the atomic mass of Y).
(b) From the above calculation, we can understand 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 in the above example.
(Extra information: Dobereiner's law of triads states that when elements are arranged in 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 can be lithium (Li), sodium (Na), and potassium (K).
(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, that can also be classified according to Dobereiner's law of triads.
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