NCERT Class X Science Class: Chapter –5. Periodic Classification of Elements – Part-6

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Question 26:

Which of the following set of elements is written in order of their increasing metallic character?

Answer: A

Image increasing metalic character

Image Increasing Metalic Character

Image increasing metalic character

Short Answer Questions

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.


The arrangement of these elements is known as Dobereiner triad. Example, Lithium, Sodium and Potassium

Question 28:

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

  1. Pick two sets of elements which have similar properties.

  2. The given sequence represents which law of classification of elements?


  1. F and Cl

  2. Na and K.

  3. Newland’s law of octaves

Question 29:

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

  1. Na, Si, Cl

  2. Be, Mg, Ca

Atomic mass of

Explain by giving reason.


  1. No, because all these elements do not have similar properties although the atomic mass of silicon is average of atomic masses of

  2. Yes, because they have similar properties and the mass of is roughly the average of the atomic mass of.

Question 30:

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


There are Some pairs of elements, like cobalt and nickel, in which element with higher atomic Mass was placed before the lower atomic mass element. It was done in order to maintain the similarity in chemical properties of elements belonging to same group

Question 31:

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


Hydrogen occupies a unique position in the modern periodic table due to the following reasons (i) Both hydrogen and alkali metals have similar outer electronic configuration as both have one electron in the valence shell. Therefore, some of the properties of hydrogen are similar to those of alkali metals and hence, it can be placed in group along with alkali metals. (ii) Both hydrogen and halogens have similar outer electronic configuration (both have one electron less than the nearest inert gas configuration). Therefore, some of the properties of hydrogen are similar to those of halogens and hence, it can be placed in group 17 along with halogens. (iii) In some properties, it differs from both hydrogen and halogens, e.g., the oxide of hydrogen, i.e., H2O is neutral but the oxides of alkali metals (i.e., etc.) are basic while those of halogens (i.e., etc.) are acidic.

Question 32:

Write the formulae of chlorides of , the elements predicted by Mendeleev.


Eka-silicon is Germanium. The formula for chloride is

Eka-aluminium is Gallium and formula for chloride is

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