Selina Class 7 ICSE Solutions Chemistry : Chapter 4- Atoms, Molecules and Radicals

Class 7: Chemistry Chapter 4 solutions. Complete Class 7 Chemistry Chapter 4 Notes.

Selina Class 7 ICSE Solutions Chemistry : Chapter 4- Atoms, Molecules and Radicals

Selina 7th Chemistry Chapter 4, Class 7 Chemistry Chapter 4 solutions

EXERCISE

Question 1.
Define the following terms :
Answer:

1. Atom : An atom is the smallest indivisible unit of an
   OR
   Atom is the smallest unit of matter.
2. Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existance.
3. Radicals : 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.
4. Valency : It is the number of electrons donated or accepted by the valence shell of an atom during chemical combination.
5. Periodic table represents the tabular arrangment of elements in horizontal rows called periods and vertical columns called groups in order to classify the elements and their systematic study.

Question 2.
Write the names of the elements present in the following compounds.
Answer:

1. Common salt : Sodium, chlorine.
2. Ammonia : Nitrogen, hydrogen.
3. Sulphuric acid : Hydrogen, sulphur, oxygen.
4. Glucose : Carbon, hydrogen, oxygen.
5. Sodium hydroxide : Sodium, oxygen, hydrogen.
6. Acetic acid : Carbon, hydrogen, oxygen.

Question 3.
What does each of the following represent ?
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:
Answer:


Question 5.
Name the following radicals :
Answer:

1. SO₄^2- = Sulphate
2. HC0₃^– = Bicarbonate
3. OH- = Hydroxide
4. Cr₂0₇^2-  = 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:

1. +1 = Sodium Na+
   +2 = Calcium Ca+2
   +3 = Aluminium Al+3
2. -1 = Chlorine Cl-
   -2 = Oxygen 0-2
   -3 = Nitrogen N3_

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:

1. O= 2
2. OH = 1
3. CO₃ = 2
4. Cl = l

Question 8.
Write the names of the following compounds :
Answer:

Question 9.
Write the molecular formulae of:
Answer:

Question 10.
The valency of sodium is one, write the molecular formula for the following compounds of sodium.

1. sodium oxide : Na₂0
2. sodium sulphate : Na₂S0₄
3. sodium carbonate : Na₂CO₃
4. sodium hydroxide : NaOH
5. sodium nitrate : NaN0₃

Question 11.
What is variable valency ? Give two examples of elements showing variable valency.
Answer:
There are some elements with more than one valency. They are said to have variable valency, e.g. Iron, copper.

Question 12.
Give the group number of following elements present in periodic table

1. Magnesium : IIA
2. Carbon : IVA
3. Sulphur : VIA
4. Neon : Zero

Question 13.
An element belongs to group VA. What would be its valency? Name two such elements.
Answer:
Elements of group VA has valency 3.
Two elements : Nitrogen and phosphorus.

Question 14.
An element belongs to group II. What would be its valency? Write the formula of molcules of compounds it will form with elements in VA, VIA and VIIA groups.
Answer:

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
Answer:

1. Atoms are neutral.
2. An ion with positive charge is called cation.
3. An ion with negative charge is called anion.
4. 2H₂ means two atoms of hydrogen.
5. Ozone is a triatomic molecule.
6. Metals have variable valency.
7. Chemical name of caustic soda is sodium hydroxide NaOH.

2. Tick (√) the correct answer.

(a) The valency of iron in Fe203 is

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

(b) Which of the following has valency 4 ?

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

(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

(d) Which of the following exhibit variable valency ?

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

3. State the term for the following:

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

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Selina Class 7 ICSE Solutions Chemistry : Chapter 4- Atoms, Molecules and Radicals

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Chapterwise Selina Publishers ICSE Solutions for Class 7 Chemistry :

• Chapter 1- Matter and Its Composition
• Chapter 2- Physical and Chemical Changes
• Chapter 3- Elements, Compounds and Mixtures
• Chapter 4- Atoms, Molecules and Radicals
• Chapter 5- Language of Chemistry
• Chapter 6- Metals and Non-metals
• Chapter 7- Air and Atmosphere

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