Class 10 Science

Periodic Classification of Elements

NCERT Exemplar Problem

Long Answer

Question 42: An element is placed in 2nd Group and 3rd Period of the Periodic Table, burns in presence of oxygen to form a basic oxide.

(a) Identify the element

Answer: Magnesium

(b) Write the electronic configuration

Answer: 2, 8, 2

(c) Write the balanced chemical equation when it burns in the presence of air

Answer: 2Mg(s) + O2(g) → 2MgO (s)

(d) Write a balanced equation when this oxide is dissolved in water

Answer: MgO(s) + H2O(l) → Mg(OH)2(aq)

(e) Draw the electron dot structure for the formation of this oxide


Electron Dot Structure Magnesium Oxide

Question 43: An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide.

(a) Where in the periodic table are elements X and Y placed?

Answer: X belongs to Group 17 and Period 3, while Y belongs to Group 2 and Period 4

(b) Classify X and Y as metal (s), non-metal (s) or metalloid (s).

Answer: X is non-metal, while Y is metal

(c) What will be the nature of oxide of element Y? Identify the nature of the bonding in the compound formed.

Answer: As Y is a metal, so it makes basic oxide. The halide compound has ionic bond.

(d) Draw the electron dot structure of the divalent halide.


electron dot structure

Question 44: Atomic numbers of a few elements are given here: 10, 20, 7, 14

  1. Identify the elements
  2. Identify the Group number of these elements in the Periodic Table.
  3. Identify the Periods of these elements in the Periodic Table.
  4. What would be the electronic configuration for each of these elements?
  5. Determine the valency of these elements.


Atomic NumberElementGroupPeriodElectronic configValency
10Neon1822, 80
20Calcium242, 8, 8, 22
7Nitrogen1522, 53
14Silicon1432, 8, 44

Question 45: Complete the following cross word puzzle:


(1) An element with atomic number 12

(3) Metal used in making cans and member of Group 14

(4) A lustrous non-metal which has 7 electrons in its outermost shell


(2) Highly reactive and soft metal which imparts yellow colour when subjected to flame and kept in kerosene.

(5) The first element of second Period

(6) An element which is used in making fluorescent bulbs and is second member of Group 18 in the Modern Periodic Table

(7) A radioactive element which is the last member of halogen family

(8) Metal which is an important constituent of steel and forms rust when exposed to moist air

(9) The first metalloid in Modern Periodic Table whose fibres are used in making bullet-proof vests


3 T8 I ND9 B 5 L

Question 46: Li H B N F Ne Be C O Mg Na Cl Si P Ar Al Ca K S He

(a) In this series, symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table.

Answer: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca

(b) Arrange them in the order of their group also.


Group 1H, Li, Na, K
Group 2Be, Mg, Ca
Group 13B, Al
Group 14C, Si
Group 15N, P
Group 16O, S
Group 17F, Cl
Group 1He, Ne, Ar

Question 47: Mendeleev predicted the existence of certain elements not know at that time and named two of them as Eka-silicon and Eka-aluminium.

  1. Name the elements which have taken the place of these elements.
  2. Mention the group and the period of these elements in the Modern Periodic Table.
  3. Classify these elements as metals, non-metals or metalloids.
  4. How many valence electrons are present in each one of them?
ElementGroupPeriodTypeValence electrons
Germanium (Ge)144Metalloid4
Gallium (Ga)134Metal3

Question 48: Electropositive nature of the element (s) increases down the group and decreases across the period. Electronegativity of the elements decreases down the group and increases across the period. Atomic size increases down the group and decreases across a period (left to right). Metallic character increases down the group and decreases across a period.

On the basis of above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9.

  1. Name the most electropositive element among them.
  2. Name the most electronegative element.
  3. Name the element with the smallest atomic size.
  4. Name the element which is a metalloid.
  5. Name the element which shows maximum valency.
Answer: (a) Lithium, (b) Fluorine, (c) Fluorine, (d) Boron, (e) Carbon

Question 49: An element X, which is a yellow solid at room temperature, shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants.

(a) Identify the element X

Answer: Sulphur (16)

(b) Write the electronic configuration of X

Answer: 2, 8, 6

(c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals.

Answer: 2FeSO4 (s) + heat → FeO3(s) + SO2(g) + SO3(g)

(d) What would be the nature (acidic/basic) of oxides formed?

Answer: Acidic

(e) Locate the position of the element in the Modern Periodic Table.

Answer: Group 16, Period 3

Question 50: An element X of group 15 exists as diatomic molecule and combines with hydrogen at 773 K in the presence of the catalyst to form a compound, ammonia which has a characteristic pungent smell.

(a) Identify the element X. How many valence electrons does it have?

Answer: Nitrogen (7)

(b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?

Answer: Covalent Bond

electron dot structure of nitrogen molecule

(c) Draw the electron dot structure for ammonia and what type of bond is formed in it?

Answer: Covalent Bond

electron dot structure of ammonia

Question 51: Which group of elements could be placed in Mendeleev’s Periodic Table without disturbing the original order? Give reason.

Answer: Noble gases were discovered much later after Mendeleev. After the discovery of noble gas, they were placed in a separate group called Zero Group, after VIII group, without making any disturbance to the arrangement of any elements in the Mendeleev’s Periodic Table. This could be possible because Mendeleev postulated that there is a periodic recurrence of elements with similar physical and chemical properties.

Question 52: Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at “Periodic Law”?

Answer: 63 elements were known at that time. Mendeleev arranged those elements in order of their relative atomic masses in a table. Mendeleev studied formation of hydrides and oxides of these elements. Then he arranged elements of similar property in a group. He observed that elements were automatically arranged in order of their increasing atomic masses. Thus, Mendeleev arrived at Periodic Law.