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Chapter 4 – Atoms Molecules and Radicals

Atoms Molecules and Radicals
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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

RadicalSymbolValency
Magnesium ionMg2+2+
AmmoniumNH4+1+
CarbonateCO32-2-
NitrateNO3–1-
OxideO2-2-
BisulphateHSO4–1-
Aluminium ionAl3+3+

Question 5

Name the following radicals :

  1. SO42-
  2. HCO3–
  3. OH–
  4. Cr2O72-

Answer

RadicalName
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:

ValencyPositive IonNegative Ion
+1Sodium ionHydroxide ion (-1)
+2Magnesium ionCarbonate ion (-2)
+3Aluminium ionNitride ion (-3)
-1Fluoride ionChloride ion
-2Oxide ionSulfate ion
-3Nitride ionPhosphate 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:

RadicalValency with Ca
O2
OH1
CO32
Cl1

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

FormulaName
(NH4)2SO4Ammonium sulfate
Ca(NO3)2Calcium nitrate
FeSIron(II) sulfide or ferrous sulfide
Na3PO4Sodium phosphate
NH4OHAmmonium hydroxide or ammonia solution
CuCO3Copper(II) carbonate or cupric carbonate
HgOMercury(II) oxide or mercuric oxide
ZnCl2Zinc chloride
ZnSZinc sulfide
H2SHydrogen 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

CompoundMolecular Formula
Sodium sulphideNa2S
Magnesium oxideMgO
Calcium hydroxideCa(OH)2
Hydrogen ChlorideHCl
Sulphuric acidH2SO4
Iron (II) sulphideFeS
Iron (III) sulphateFe2(SO4)3
Nitric acidHNO3
Calcium PhosphateCa3(PO4)2
Aluminium SulphateAl2(SO4)3
Magnesium nitrideMg3N2

 

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

CompoundMolecular Formula
Sodium oxideNa2O
Sodium sulphateNa2SO4
Sodium carbonateNa2CO3
Sodium hydroxideNaOH
Sodium nitrateNaNO3

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

ElementGroup Number
Magnesium2
Carbon14
Sulphur16
Neon18

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 S042-. 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

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