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Chapter 4

Atomic Structure

Class 7 - Concise Chemistry Selina


Question 1

Define the following terms:

(a) Atoms

(b) Molecules

(c) Radicals

(d) Valency

(e) Periodic table


(a) An atom is the smallest particle of an element that exhibits all the properties of that element. It may or may not exist independently but takes part in every chemical reactions.

(b) A molecule is the smallest particle of an element or compound, made up of two or more atoms of an element or compound which has independent existence exhibiting all of their properties.

(c) A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.

(d) Valency is the combining capacity of an element or of a radical.

(e) Periodic table is a tabular arrangement of elements in vertical columns and horizontal rows indicating the regular trends in the properties of elements.

Question 2

Write the names of the elements present in the following compounds.

(a) Common salt

(b) Ammonia

(c) Sulphuric acid

(d) Glucose

(e) Sodium hydroxide

(f) Acetic acid


Common saltsodium, chlorine
AmmoniaNitrogen, hydrogen
Sulphuric acidHydrogen, sulphur, oxygen
GlucoseCarbon, hydrogen, oxygen
Sodium hydroxideSodium, oxygen, hydrogen
Acetic acidcarbon, hydrogen, oxygen

Question 3

What does each of the following represent?

(a) 2CO2

(b) 2H2S

(c) 5H2SO4

(d) 6NaNO3


(a) 2 molecules of carbon dioxide

(b) 2 molecules of hydrogen sulphide

(c) 5 molecules of sulphuric acid

(d) 6 molecules of sodium nitrate

Question 4

Write the symbols and valencies of the following radicals:

(a) Magnesium ion

(b) Ammonium

(c) Carbonate

(d) Nitrate

(e) Oxide

(f) Bisulphate

(g) Aluminium ion


Magnesium ionMg2+2

Question 5

Name the following radicals:

(a) SO42-

(b) HCO3-

(c) OH-

(d) Cr2O72-


(a) Sulphate ion

(b) Bicarbonate ion

(c) Hydroxide ion

(d) Dichromate ion

Question 6a

Name one ion for each of the valencies +1, +2 and +3.


Sodium ion (Na+) has valency +1.

Magnesium ion (Mg2+) has valency +2.

Aluminium ion (Al3+) has valency +3.

Question 6b

Name one ion for each of the valencies -1, -2 and -3.


Fluoride ion (F-) has valency -1.

Sulphide ion (S2-) has valency -2.

Nitride ion (N3-) has valency -3.

Question 7

The valency of calcium is 2. Write the valencies of other radicals in the following compounds:

(a) CaO

(b) Ca(OH)2

(c) CaCO3

(d) CaCl2


(a) Oxide (O2-) valency is 2

(b) Hydroxide (OH-) valency is 1

(c) Carbonate (CO32-) valency is 2

(d) Chloride (Cl-) valency is 1

Question 8

Write the names of the following compounds.

(a) (NH4)2SO4

(b) Ca(NO3)2

(c) FeS

(d) Na3PO4

(e) NH4OH

(f) CuCO3

(g) HgO

(h) ZnCl2

(i) ZnS

(j) H2S


(a) Ammonium sulphate

(b) Calcium nitrate

(c) Iron sulphide

(d) Sodium phosphate

(e) Ammonium hydroxide

(f) Copper carbonate

(g) Mercuric oxide

(h) Zinc chloride

(i) Zinc sulphide

(j) Hydrogen sulphide

Question 9

Write the molecular formula of:

(a) Sodium sulphide

(b) Magnesium oxide

(c) Calcium hydroxide

(d) Hydrogen chloride

(e) Sulphuric acid

(f) Iron (II) sulphide

(g) Iron (III) sulphate

(h) Nitric acid

(i) Calcium phosphate

(j) Aluminium sulphate

(k) Magnesium nitride


(a) Na2S

(b) MgO

(c) Ca(OH)2

(d) HCl

(e) H2SO4

(f) FeS

(g) Fe2(SO4)3

(h) HNO3

(i) Ca3(PO4)2

(j) Al2(SO4)3

(k) Mg3N2

Question 10

The valency of sodium is one. Write the molecular formula for the following compounds of sodium.

(a) sodium oxide

(b) sodium sulphate

(c) sodium carbonate

(d) sodium hydroxide

(e) sodium nitrate


(a) Na2O

(b) Na2SO4

(c) Na2CO3

(d) NaOH

(e) NaNO3

Question 11

What is variable valency? Give two examples of elements showing variable valency.


Certain elements exhibit more than one valency that means these elements show variable valency.

Examples: Iron and copper.

Iron (Fe)Ferrous, Ferric2, 3
Copper (Cu)Cuprous, Cupric1, 2

Question 12

Give the group number of the following elements present in periodic table.

(a) Magnesium

(b) Carbon

(c) Sulphur

(d) Neon


(a) IIA

(b) IVA

(c) VIA

(d) Zero

Question 13

An element belongs to group VA. What would be its valency? Name two such elements.


Its valency will be 3. Nitrogen(N) and Phosphorus(P) belongs to group VA.

Question 14

An element belongs to group II. What would be its valency? Write the formulae of molecules of compounds it will form with elements in VA, VIA and VII A groups.


The element will have valency 2.

Let the element be A which belongs to group II,

B belongs to group VA which have valency 3,

C belongs to group VIA which have valency 2,

D belongs to group VII A which have valency 1.

Formula of compounds are A3B2, AC, AD2.

Objective Type Questions

Question 1

Fill in the blanks:

(a) Atoms are ............... .

(b) An ion with positive charge is called ............... .

(c) An ion with negative charge is called ............... .

(d) 2H2 means two ............... of hydrogen.

(e) ............... is a triatomic molecule.

(f) Metals have ............... valency.

(g) Chemical name of caustic soda is ............... .


(a) indivisible

(b) Cation

(c) Anion

(d) molecules

(e) Ozone

(f) electropositive

(g) Sodium hydroxide(NaOH)

Question 2a

The valency of iron in Fe2O3 is:

  1. 1
  2. 2
  3. 3
  4. 6



Reason — The compound is ferric oxide where Fe has valency 3.

Question 2b

Which of the following has valency 4?

  1. Aluminium
  2. Oxygen
  3. Carbon
  4. Phosphorus



Reason — Carbon belongs to group IVA and so its valency is 4.

Question 2c

The sulphate radical is written as SO42-. What is the formula of calcium sulphate?

  1. Ca(SO4)2
  2. Ca2(SO4)
  3. Ca(SO4)3
  4. CaSO4



Reason — Calcium radical is Ca+2. So formula of calcium sulphate is CaSO4.

Question 2d

Which of the following exhibits variable valency?

  1. Calcium
  2. Copper
  3. Carbon
  4. Chlorine



Reason — Copper exhibits variable valency i.e. 1 and 2.

Question 3

State the term for the following:

(a) The number of atoms present in a molecule of an element.

(b) The symbolic representation of a molecule.

(c) A group of atoms that react as a single unit.

(d) The combining capacity of an element.

(e) The tabular arrangement of elements in horizontal rows and vertical columns.


(a) Atomicity

(b) Molecular formula

(c) Radicals

(d) Valency

(e) Periodic table