Class 10 Science

Periodic Classification of Elements

NCERT Exemplar Problem

Short Answer

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.

Answer: Such an arrangement of elements is called triad, as in Dobereiner’s Triads. Example: Lithium (Li) 7.0, Sodium (Na) 23.0, Potassium (K) 39.0 make a triad in which atomic mass of sodium is equal to average of atomic masses of lithium and potassium.

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.

Answer: F and Cl have similar properties, another set is Na and Ar

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

Answer: Newlands' Law of Octaves

Question 29: Can the following groups of elements be classified as Dobereiner’s triad?

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

Explain by giving reasons.

(a) Na, Si, Cl

Answer: Atomic mass of silicon is average of atomic masses of sodium and chlorine. However, they don’t have similar properties. Hence, these elements cannot be classified as Dobereiner’s triad.

(b) Be, Mg, Ca

Answer: Atomic mass of magnesium is average of atomic masses of beryllium and calcium. These elements have similar properties. Hence, they can be classified as Dobereiner’s triad.

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 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give reasons for the same.

Answer: Mendeleev put certain elements in wrong order of their atomic masses. He did it in order to put elements with similar properties under similar groups. So, cobalt was placed before nickel in spite of having higher atomic mass.

Question 31: “Hydrogen occupies a unique position in Modern Periodic Table”. Justify the statement.

Answer: Hydrogen resembled alkali metals as well as halogens. Hydrogen makes compounds in the same way as alkali metals do. Hydrogen makes diatomic molecule and also makes covalent compounds, the way halogens do. So, we can say that hydrogen occupies a unique position in Modern Periodic Table.

Question 32: Write the formulae of chlorides of Eka-silicon and Eka-aluminum, the elements predicted by Mendeleev.

Answer: GeCl4, GaCl3

Question 33: Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.


AGroup 133
BGroup 144
CGroup 22

Question 34: If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?

Answer: Since the element is tetravalent, hence formula for is chloride is XCl4 and it will be covalent bond.

Question 35: Compare the radii of two species X and Y. Give reasons for your answer.

  1. X has 12 protons and 12 electrons.
  2. Y has 12 protons and 10 electrons.

Answer: Here, Y is cation of X, i.e. is formed after X donated 2 electrons. So, atomic radius of Y is less than that of X.

Question 36: Arrange the following elements in increasing order of their atomic radii.

(a) Li, Be, F, N

Answer: F < N < Be < Li

(b) Cl, At, Br, I

Answer: Cl < Br < I < I

Question 37: Identify and name the metals out of the following elements whose electronic configurations are given below.

  1. 2, 8, 2
  2. 2, 8, 1
  3. 2, 8, 7
  4. 2, 1

Answer: (a) Magnesium, (b) Sodium, (d) Lithium while (c) is chlorine (a non-metal)

Question 38: Write the formula of the product formed when the element A (atomic number 19) combines with the element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed?

Answer: AB

electron dot structure

Question 39: Arrange the following elements in the increasing order of their metallic character: Mg, Ca, K, Ge, Ga

Answer: Ge < Ga < Mg < ca < K

Question 40: Identify the elements with the following property and arrange them in increasing order of their reactivity.

  1. An element which is a soft and reactive metal
  2. The metal which is an important constituent of limestone
  3. The metal which exists in liquid state at room temperature

Answer: (a) Sodium or Potassium (b) Calcium (c) Mercury

Hg < Ca < Na < K

Question 41: Properties of the elements are given below. Where would you locate the following elements in the periodic table?

  1. A soft metal stored under kerosene
  2. An element with variable (more than one) valency stored under water
  3. An element which is tetravalent and forms the basis of organic chemistry
  4. An element which is n inert gas with atomic number 2
  5. An element whose thin oxide layer is sued to make other elements corrosion resistant by the process of ‘anodising’.


S. No.ElementGroupPeriod