Chapter 4 – Atoms Molecules and Radicals

Selina Concise Chemistry 2023 solutions for class 7 Chemistry. Chapter 4 – Atoms Molecules and Radicals are provided here with simple step-by-step explanations. These solutions for Atoms Molecules and Radicals (4th Chapter) are extremely popular among class 10 students. The Atoms Molecules and Radicals solution is handy for quickly completing your homework and preparing for exams.

Book Name	: Concise Chemistry Middle School
Subject	: Chemistry
Class	: 7
Publisher	: Selina Publisher PVT Ltd
Chapter Name	: Atoms Molecules and Radicals

Exercise

Question 1.

Define the following terms :

1. Atoms
2. Molecules
3. Radicals
4. Valency
5. Periodic table

Answer

1. Atom: The atom is the smallest unit of matter.
2. Molecule: A molecule is the smallest unit of a compound (or an element) which always has an independent existence.
3. Radicals: Radicals are atoms or molecules that have one or more unpaired electrons.
4. Valency: Valency is the combining capacity of an atom or molecule, determined by the number of electrons it can donate, accept or share during a chemical reaction.
5. Periodic table: The periodic table is a tabular arrangement of chemical elements, ordered by their atomic number, electron configurations, and chemical properties.

Question 2.

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

1. Common salt
2. Ammonia
3. Sulphuric acid
4. Glucose
5. Sodium hydroxide
6. Acetic acid

Answer

1. Sodium, chlorine.
2. Nitrogen, hydrogen.
3. Hydrogen, sulphur, oxygen.
4. Carbon, hydrogen, oxygen.
5. Sodium, oxygen, hydrogen.
6. Carbon, hydrogen, oxygen.

Question 3.

What does each of the following represent ?

1. 2C0₂
2. 2H₂S
3. 5H₂S0₄
4. 6NaNO₃

Answer

1. 2C0₂ = 2 molecules of carbon dioxide.
2. 2H₂S = 2 molecules of hydrogen sulphide.
3. 5H₂S0₄ = 5 molecules of sulphuric acid.
4. 6NaNO₃ = 6 molecules of sodium nitrate.

Question 4

Write the symbols and valencies of the following radicals:

1. Magnesium ion
2. Ammonium
3. Carbonate
4. Nitrate
5. Oxide
6. Bisulphate
7. Aluminium ion

Answer

Radical	Symbol	Valency
Magnesium ion	Mg2+	2+
Ammonium	NH4+	1+
Carbonate	CO32-	2-
Nitrate	NO3–	1-
Oxide	O2-	2-
Bisulphate	HSO4–	1-
Aluminium ion	Al3+	3+

Question 5

Name the following radicals :

1. SO₄^2-
2. HCO₃^–
3. OH^–
4. Cr₂O₇^2-

Answer

Radical	Name
SO42-	Sulfate
HCO3–	Bicarbonate or Hydrogen Carbonate
OH–	Hydroxide
Cr2O72-	Dichromate

Question 6

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

Answer:

Valency	Positive Ion	Negative Ion
+1	Sodium ion	Hydroxide ion (-1)
+2	Magnesium ion	Carbonate ion (-2)
+3	Aluminium ion	Nitride ion (-3)
-1	Fluoride ion	Chloride ion
-2	Oxide ion	Sulfate ion
-3	Nitride ion	Phosphate ion

Question 7

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

1. CaO
2. Ca(OH)₂
3. CaC0₃
4. CaCl₂

Answer:

Radical	Valency with Ca
O	2
OH	1
CO3	2
Cl	1

Question 8

Write the names of the following compounds :

1. (NH₄)₂SO₄
2. Ca(NO₃)₂
3. FeS
4. Na₃PO₄
5. NH₄OH
6. CuCO₃
7. HgO
8. ZnCl₂
9. ZnS
10. H₂S

Answer

Formula	Name
(NH4)2SO4	Ammonium sulfate
Ca(NO3)2	Calcium nitrate
FeS	Iron(II) sulfide or ferrous sulfide
Na3PO4	Sodium phosphate
NH4OH	Ammonium hydroxide or ammonia solution
CuCO3	Copper(II) carbonate or cupric carbonate
HgO	Mercury(II) oxide or mercuric oxide
ZnCl2	Zinc chloride
ZnS	Zinc sulfide
H2S	Hydrogen sulfide or hydrosulfuric acid

 

Question 9

Write the molecular formulae of:

1. Sodium sulphide
2. Magnesium oxide
3. Calcium hydroxide
4. Hydrogen Chloride
5. Sulphuric acid
6. Iron (II) sulphide
7. Iron (III) sulphate
8. Nitric acid
9. Calcium Phosphate
10. Aluminium Sulphate
11. Magnesium nitride

Answer

Compound	Molecular Formula
Sodium sulphide	Na2S
Magnesium oxide	MgO
Calcium hydroxide	Ca(OH)2
Hydrogen Chloride	HCl
Sulphuric acid	H2SO4
Iron (II) sulphide	FeS
Iron (III) sulphate	Fe2(SO4)3
Nitric acid	HNO3
Calcium Phosphate	Ca3(PO4)2
Aluminium Sulphate	Al2(SO4)3
Magnesium nitride	Mg3N2

 

Question 10

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

1. sodium oxide
2. sodium sulphate
3. sodium carbonate
4. sodium hydroxide
5. sodium nitrate

Answer

Compound	Molecular Formula
Sodium oxide	Na2O
Sodium sulphate	Na2SO4
Sodium carbonate	Na2CO3
Sodium hydroxide	NaOH
Sodium nitrate	NaNO3

Question 11

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

Answer

Variable valency refers to the ability of an element to exhibit more than one valency or combining capacity in different chemical compounds.

Example – Iron (Fe) and Copper (Cu)

Question 12

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

1. Magnesium
2. Carbon
3. Sulphur
4. Neon

Answer

Element	Group Number
Magnesium	2
Carbon	14
Sulphur	16
Neon	18

Question 13

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

Answer

An element belonging to group VA of the periodic table would have a valency of 3 or 5.

Two elements: Nitrogen and phosphorus.

Question 14

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

Answer

An element belonging to group II of the periodic table would have a valency of 2.

The formulas of the molecules of compounds it will form with elements in VA, VIA, and VIIA groups are:

• With group VA: The formula of the compound will be M₃X₂, where M represents the group II element and X represents the group VA element. For example, the compound formed between calcium (Ca) and nitrogen (N) will be Ca₃N₂.
• With group VIA: The formula of the compound will be MX₂, where M represents the group II element and X represents the group VIA element. For example, the compound formed between beryllium (Be) and oxygen (O) will be BeO.
• With group VIIA: The formula of the compound will be MX₂, where M represents the group II element and X represents the group VIIA element. For example, the compound formed between magnesium (Mg) and chlorine (Cl) will be MgCl₂.

OBJECTIVE TYPE QUESTIONS

Question 1

Fill in the blanks:

1. Atoms are ____.
2. An ion with a positive charge is called ____.
3.  An ion with a negative charge is called ____.
4. 2H₂ means two ____ of hydrogen.
5. ____ is a triatomic molecule.
6. Metals have ____ valency.
7. The chemical name of caustic soda is sodium hydroxide ____.

Answer

1. neutral
2. cation
3. anion
4. molecules
5. Ozone
6. variable
7. NaOH

Question 2

Tick (√) the correct answer.

(a) The valency of iron in Fe₂0₃ is

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

Answer

3

(b) Which of the following has valency 4 ?

1. aluminium
2. oxygen
3. carbon
4. phosphorus

Answer

carbon

(c) The sulphate radical is written as S0₄^2-. What is the formula of calcium sulphate?

1. Ca(S04)₂
2. Ca₂(S0₄)
3. Ca(S0₄)₃
4. CaS0₄

Answer

CaS0₄

(d) Which of the following exhibit variable valency ?

1. calcium
2. copper
3. carbon
4. chlorine

Answer

copper

Question 3

State the term for the following:

1. The number of atoms present in a molecule of an element.
2. The symbolic representation of a molecule.
3. A group of atoms that react as a single unit.
4. The combining capacity of an element.
5. The tabular arrangement of elements in horizontal rows and vertical columns.

Answer

1. the atomicity
2. molecular formula.
3. molecule
4. valency
5. periodic table