Chapter 3: Atoms and Molecules Short Questions | science Class 9 CBSE

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Chapter 3: Atoms and Molecules Short Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. Define atom.

The smallest particle of an element.
Cannot be divided in chemical reactions.
Combines to form molecules or ions.

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Question 2

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Q2. What is a molecule of element?

Two or more atoms of the same element bonded together.
Examples: O₂, H₂, N₂.
They retain the properties of the element.

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Question 3

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Q3. Give the chemical formula of water and carbon dioxide.

Water – H₂O.
Carbon dioxide – CO₂.
These are molecular formulas.

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Question 4

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Q4. What is a cation? Give one example.

A positively charged ion formed by loss of electrons.
Example: Na⁺.
Common in ionic compounds.

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Question 5

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Q5. Define valency.

Combining capacity of an element.
Determines how many atoms it can bond with.
Example: Hydrogen has valency 1.

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Question 6

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Q6. Give the formula of magnesium chloride.

Mg²⁺ and Cl⁻ combine in 1:2 ratio.
Formula: MgCl₂.
Obtained using criss-cross method.

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Question 7

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Q7. What is the molecular mass of H₂SO₄?

H = 1, S = 32, O = 16.
Molecular mass = (2×1) + 32 + (4×16) = 98 u.
Used in stoichiometric calculations.

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Question 8

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Q8. Write the symbols for potassium and iron.

Potassium – K.
Iron – Fe.
Based on Latin names.

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Question 9

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Q9. What is a polyatomic ion? Give an example.

An ion made of two or more atoms with net charge.
Example: SO₄²⁻ (sulfate ion).
Acts as a single charged unit in compounds.

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Question 10

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Q10. Name the unit used to express atomic mass.

Atomic mass unit (u).
1 u = 1/12 mass of carbon-12 atom.
Used for atoms and molecules.

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Chapter sections

Chapter 3: Atoms and Molecules | Short Questions

Q1. Define atom.

Q2. What is a molecule of element?

Q3. Give the chemical formula of water and carbon dioxide.

Q4. What is a cation? Give one example.

Q5. Define valency.

Q6. Give the formula of magnesium chloride.

Q7. What is the molecular mass of H₂SO₄?

Q8. Write the symbols for potassium and iron.

Q9. What is a polyatomic ion? Give an example.

Q10. Name the unit used to express atomic mass.