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

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

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Question 1

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Q1. State the laws of chemical combination and explain each with examples.

• The two laws of chemical combination are: Law of Conservation of Mass and Law of Constant Proportions.
• Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.
• For example, 5.3 g of sodium carbonate and 6 g of acetic acid produce 11.3 g of products, showing total mass remains unchanged.
• Law of Constant Proportions states that in a pure compound, elements are always present in a fixed proportion by mass.
• For example, in water, the ratio of hydrogen to oxygen is always 1:8 by mass.
• These laws were validated through numerous experiments by Lavoisier and Proust.
• Dalton's atomic theory later explained these laws at the atomic level.

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Question 2

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Q2. What are the postulates of Dalton’s atomic theory?

• All matter is made of very tiny particles called atoms.
• Atoms are indivisible and cannot be created or destroyed in chemical reactions.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in small whole number ratios to form compounds.
• The relative number and kinds of atoms are constant in a given compound.
• Dalton’s theory explained the laws of conservation of mass and definite proportions.

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Question 3

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Q3. Define an atom. Why is it difficult to see atoms with the naked eye?

• An atom is the smallest particle of an element that cannot normally exist independently and retains all its chemical properties.
• Atoms are extremely small, typically of the order of 10⁻¹⁰ meters.
• Millions of atoms stacked together form a layer as thin as a sheet of paper.
• Their size is beyond the resolving power of the human eye.
• Special instruments like scanning tunneling microscopes are needed to observe atoms.
• Atomic radius is measured in nanometres (1 nm = 10⁻⁹ m).
• For example, the radius of a hydrogen atom is approximately 10⁻¹⁰ m.

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Question 4

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Q4. What are modern symbols of elements and how are they derived?

• Modern symbols of elements are derived from English or Latin names.
• Each element has a one- or two-letter symbol; the first letter is always capitalized.
• Examples: Hydrogen (H), Oxygen (O), Calcium (Ca), Chlorine (Cl).
• Latin-based symbols: Iron (Fe - Ferrum), Sodium (Na - Natrium), Potassium (K - Kalium).
• Symbols are approved by IUPAC.
• These symbols represent one atom of the element.
• They are used universally in writing chemical equations and formulae.

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Question 5

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Q5. What is atomic mass? How is the standard atomic mass unit defined?

• Atomic mass is the average mass of an atom of an element compared to 1/12th the mass of one carbon-12 atom.
• The standard unit is called unified mass or 'u'.
• 1 u = 1/12th of the mass of one atom of carbon-12.
• Atomic mass of hydrogen = 1 u, oxygen = 16 u, carbon = 12 u.
• Initially, oxygen-16 was used as the standard but later replaced by carbon-12 for universal consistency.
• Atomic masses are used to calculate molecular and formula masses.
• They help in balancing chemical equations and stoichiometric calculations.

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Question 6

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Q6. Define molecule. Differentiate between molecules of elements and molecules of compounds.

• A molecule is the smallest particle of an element or a compound capable of independent existence and shows all properties of the substance.
• Molecules of elements contain atoms of the same kind.
• Example: O₂, H₂, N₂, Cl₂ (diatomic), P₄ (tetra-atomic), S₈ (polyatomic).
• Molecules of compounds contain atoms of different elements in fixed ratios.
• Example: Water (H₂O), Carbon dioxide (CO₂), Ammonia (NH₃).
• Atomicity refers to the number of atoms in a molecule.
• Molecules of elements can be monoatomic (He), diatomic (O₂), or polyatomic (P₄).

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Question 7

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Q7. What are ions? Explain with examples and distinguish between cations and anions.

• Ions are charged species formed by loss or gain of electrons.
• Cations are positively charged ions (e.g., Na⁺, Ca²⁺).
• Anions are negatively charged ions (e.g., Cl⁻, SO₄²⁻).
• Ions can be monoatomic (Na⁺, Cl⁻) or polyatomic (NH₄⁺, SO₄²⁻).
• Example: In NaCl, Na⁺ is the cation and Cl⁻ is the anion.
• Ionic compounds consist of a regular arrangement of cations and anions.
• Ions are represented with their respective charges as superscripts.

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Question 8

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Q8. What is valency? How is it useful in writing chemical formulae?

• Valency is the combining capacity of an element.
• It determines how many atoms of an element will combine with atoms of another.
• It is used to derive chemical formulae of compounds.
• For example, valency of H is 1, O is 2, so formula of water is H₂O.
• Formulae are written by criss-crossing the valencies of elements.
• Example: Al³⁺ and O²⁻ give Al₂O₃ after cross multiplication.
• In case of polyatomic ions, brackets are used (e.g., Ca(OH)₂).

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Question 9

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Q9. Write the rules for writing chemical formulae of ionic compounds with examples.

• The cation (metal) is written first, followed by the anion (non-metal).
• The valencies are criss-crossed to balance charges.
• Simplify the ratio to smallest whole numbers.
• Do not mention charges in the final formula.
• Use brackets for polyatomic ions if more than one is needed.
• Examples: Na⁺ and Cl⁻ → NaCl, Mg²⁺ and Cl⁻ → MgCl₂, Ca²⁺ and OH⁻ → Ca(OH)₂.
• Example with polyatomic ions: (NH₄)₂SO₄ from NH₄⁺ and SO₄²⁻.

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Question 10

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Q10. Define molecular mass. How is it calculated? Give two examples.

• Molecular mass is the sum of atomic masses of all atoms in a molecule.
• It is expressed in unified mass units (u).
• Example 1: H₂O = 2×1 + 1×16 = 18 u.
• Example 2: HNO₃ = 1×1 + 1×14 + 3×16 = 63 u.
• Helps in determining moles and stoichiometry in reactions.
• Molecular mass is applicable to covalent compounds.
• It represents relative mass of one molecule of a substance.

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Chapter 3: Atoms and Molecules | Long Questions

Q1. State the laws of chemical combination and explain each with examples.

Q2. What are the postulates of Dalton’s atomic theory?

Q3. Define an atom. Why is it difficult to see atoms with the naked eye?

Q4. What are modern symbols of elements and how are they derived?

Q5. What is atomic mass? How is the standard atomic mass unit defined?

Q6. Define molecule. Differentiate between molecules of elements and molecules of compounds.

Q7. What are ions? Explain with examples and distinguish between cations and anions.

Q8. What is valency? How is it useful in writing chemical formulae?

Q9. Write the rules for writing chemical formulae of ionic compounds with examples.

Q10. Define molecular mass. How is it calculated? Give two examples.

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