Q29 of 52 Page 43

Can the following groups of elements be classified as Dobereiner's triad ?

(a) Na, Si, Cl


(b) Be, Mg, Ca


Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40


Explain by giving reason.

No. Na, Si, Cl cannot be classified as Dobereiner's triad. Although the atomic mass of silicon (Si) is average of atomic masses of sodium (Na) and chlorine (Cl), all these elements do not have similar properties.


23 (Na) + 35 (Cl)/2 = 29


(b) Yes. Be, Mg, Ca cannot be classified as Dobereiner's triad. They have similar properties and the atomic weights of magnesium (Mg) is roughly the average of the atomic weights of Be and Ca.


9 (Be) + 40 (Ca)/2 = 24.5


Dobereiner’s triad suggested relationship between properties of elements and their atomic weights. The atomic weight of the middle element is approximately equal to the average weights of other two elements.

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27

The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as? Give one example of such a set of elements.

 28

Elements have been arranged in the following sequence on the basis of their increasing atomic masses.

F, Na, Mg, Al, Si, P, S, Cl, Ar, K


(a) Pick two sets of elements which have similar properties.


(b) The given sequence represents which law of classification of elements?

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In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give reason for the same.

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“Hydrogen occupies a unique position in Modern Periodic Table”. Justify the statement.