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Standard X

Chemistry

Dobereiner's Triads

The three ele...

Question

Question 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.

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Solution

The arrangement of these elements is known as Dobereiner triad. Example: Lithium, sodium, and potassium.
In 1829, a German chemist Johann Wolfgang Dobereiner noted that there were triads of elements that showed similar chemical properties. He also noted that the atomic weightof the central element of the triad was the arithmetic mean of the atomic weights of the other two members.

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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? Give one example of such a set of elements. [2 MARKS]

Q. 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.

Q. The atomic masses of three elements

A

B

and

C

having similar chemical properties are

7

23

and

39

respectively.
(a) Calculate the average atomic mass of elements

A

and

C

.
(b) Compare the average atomic mass with atomic mass of

B

.
(c) What could the elements

A

B

and

C

be?

Question 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?

Q. (a) Explain why:
(i) All the elements of a group have similar chemical properties.
(ii) All the elements of a period have different chemical properties.

(b) The atomic radii of three elements X, Y and Z of a period of the periodic table are 186 pm; 104 pm and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.

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Dobereiner's Triads

Standard X Chemistry

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Question 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.

Question 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.