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Molecules, Atomicity and Ions — Long Answer Questions (Class 9, CBSE)

Medium Level (Application & Explanation)

Q1. What is a molecule? Explain how a molecule is different from an atom, giving examples of both elements and compounds.

Answer:

  • A molecule is a group of two or more atoms chemically bonded together. It is the smallest part of an element or compound that can exist independently and still show the properties of that substance.
  • An atom is the smallest particle of an element that retains the chemical identity of that element. Atoms become molecules when they bond with other atoms.
  • Examples: A single helium atom is He (a monoatomic element). An oxygen molecule is O₂ (two O atoms bonded). A water molecule is H₂O (two H atoms and one O atom bonded).
  • Important differences:
    • A molecule may contain the same type of atoms (elemental molecules like O₂) or different types (compound molecules like H₂O).
    • An atom cannot be split by chemical methods; a molecule can undergo chemical change to form different substances.
  • In short, an atom is a single particle, while a molecule is a bonded collection of atoms that shows definite chemical properties.

Q2. Define atomicity. Describe and classify the atomicity of the following elements: helium, oxygen, phosphorus and sulphur, with reasons for their atomicity.

Answer:

  • Atomicity is the number of atoms present in one molecule of an element. It tells us whether a substance is monoatomic, diatomic, triatomic, tetra-atomic or polyatomic.
  • Helium (He): Atomicity = 1; it is monoatomic because noble gases exist as single atoms due to a stable electron configuration.
  • Oxygen (O₂): Atomicity = 2; it is diatomic in its common form because two oxygen atoms share electrons to achieve stability.
  • Phosphorus (P₄): Atomicity = 4; white phosphorus usually exists as P₄ molecules (tetra-atomic) where four P atoms bond in a tetrahedral shape to achieve stability.
  • Sulphur (S₈): Atomicity = 8; sulphur commonly exists as S₈ rings (poly-atomic) that give it more stable bonding arrangements.
  • Reasoning: Atomicity depends on how atoms achieve stable electron arrangements by sharing or arranging bonds; elements differ because of their valence electrons and preferred bonding patterns.

Q3. From mass ratios to formulae: Explain step-by-step how to find the simplest whole-number ratio of atoms from given mass ratios. Apply this method to carbon dioxide (CO₂) where carbon and oxygen combine in the mass ratio 3:8.

Answer:

  • Step 1: Write the mass ratio of the elements given. For CO₂, C:O = 3 : 8 by mass.
  • Step 2: Divide each mass by the atomic mass (relative atomic mass) of that element to get number of moles (proportional to number of atoms). Atomic masses: C = 12, O = 16.
    • C: 3 / 12 = 0.25
    • O: 8 / 16 = 0.5
  • Step 3: Find the simplest whole-number ratio by dividing by the smallest of these numbers (0.25).
    • C: 0.25 / 0.25 = 1
    • O: 0.5 / 0.25 = 2
  • Step 4: Interpret the ratio as the formula: C : O = 1 : 2, so the molecular formula (simplest whole-number ratio) is CO₂.
  • Conclusion: This method converts mass ratios into relative numbers of atoms using atomic masses, then reduces to the simplest whole-number ratio to obtain the formula.

Q4. Describe how ionic compounds are formed using magnesium and sulphur as an example. Explain the charges involved and why the compound formed is electrically neutral.

Answer:

  • Ionic compounds form when a metal loses electrons to become a cation, and a non-metal gains electrons to become an anion. The electrostatic attraction between oppositely charged ions forms the ionic bond.
  • Example: Magnesium (Mg) has two valence electrons. It tends to lose two electrons to achieve a stable configuration, forming Mg²⁺. Sulphur (S) has six valence electrons and tends to gain two electrons to complete its octet, forming S²⁻.
  • These ions combine in a 1:1 ratio because one Mg²⁺ balances one S²⁻, producing the neutral ionic compound MgS.
  • The compound is electrically neutral because total positive charge (+2 from Mg²⁺) equals total negative charge (−2 from S²⁻).
  • Ionic compounds like MgS have fixed ratios determined by ion charges, strong electrostatic forces, and typically form crystalline solids with high melting points.

Q5. What are polyatomic ions? Give three common examples, describe their charges, and show how to write correct formulas for compounds that contain them (use Na₂SO₄ and Ca(NO₃)₂ as examples).

Answer:

  • Polyatomic ions are groups of atoms bonded together that carry an overall charge. They behave as single charged units in ionic compounds.
  • Common examples:
    • Nitrate (NO₃⁻): one nitrogen and three oxygens with a −1 charge.
    • Sulfate (SO₄²⁻): one sulfur and four oxygens with a −2 charge.
    • Ammonium (NH₄⁺): one nitrogen and four hydrogens with a +1 charge.
  • Writing formulas: Combine the polyatomic ion with appropriate cations so the total positive and negative charges balance.
    • Sodium sulfate: sodium is Na⁺ (each Na gives +1). Sulfate is SO₄²⁻ (−2). Two Na⁺ are needed to balance one SO₄²⁻. Formula: Na₂SO₄.
    • Calcium nitrate: calcium is Ca²⁺ (each Ca gives +2). Nitrate is NO₃⁻ (−1). Two NO₃⁻ are needed to balance one Ca²⁺. Formula: Ca(NO₃)₂ (brackets used to show two nitrate groups).
  • Important: When writing formulas with polyatomic ions, include brackets if more than one polyatomic ion appears and make sure overall charge is zero.

High Complexity (Analytical & Scenario-Based)

Q6. Given the mass ratio for sodium chloride is 23 : 35.5 (Na : Cl), show how to determine the molecular formula of sodium chloride and explain why the formula is NaCl rather than Na₂Cl or NaCl₂.

Answer:

  • Step 1: Write the mass ratio: Na : Cl = 23 : 35.5.
  • Step 2: Divide each by the atomic masses to get proportional numbers of atoms: atomic mass Na = 23, Cl = 35.5.
    • Na: 23 / 23 = 1
    • Cl: 35.5 / 35.5 = 1
  • Step 3: The simplest whole-number ratio of atoms is 1 : 1, giving the formula NaCl.
  • Why not Na₂Cl or NaCl₂? Those would imply unequal charge balance or different mass ratios. Sodium tends to form Na⁺ and chlorine forms Cl⁻. A 1:1 ionic combination of Na⁺ and Cl⁻ gives electrical neutrality. Na₂Cl would mean excess positive charge (+2 from two Na⁺ vs −1 from one Cl⁻) — not neutral. NaCl₂ would mean excess negative charge.
  • Therefore the mass ratio and the ion charges both support the correct, neutral formula NaCl.

Q7. Scenario: You are given a pure sample of an element that forms molecules containing four identical atoms. The sample burns in air to give an oxide with formula P₂O₅ when mass analysis shows P:O = 2:5. Identify the element, explain its atomicity, and describe why its molecules are composed of four atoms (P₄).

Answer:

  • Identification: The element is phosphorus. The oxide formula P₂O₅ and mass ratio P:O = 2:5 match known phosphorus oxide compositions.
  • Atomicity: White phosphorus commonly exists as P₄, so the atomicity is 4 (tetra-atomic). This means each stable molecule contains four phosphorus atoms bonded together in a tetrahedral shape.
  • Why P₄? Phosphorus atoms have five valence electrons and prefer to form three single bonds to complete octet-like arrangements. In the P₄ tetrahedral molecule, each phosphorus atom bonds to three others, satisfying bonding tendencies and creating a relatively stable molecular structure for elemental phosphorus.
  • When phosphorus burns in oxygen, it forms oxides such as P₂O₅ (the empirical formula), which reflect how phosphorus atoms combine with oxygen. The molecular nature (P₄) explains observed chemical behaviour, volatility, and reactivity of elemental phosphorus.

Q8. Analytical question: Magnesium sulphide is reported to have a mass ratio Mg : S = 3 : 4. Using atomic masses Mg = 24 and S = 32, determine the chemical formula of magnesium sulphide and explain every step.

Answer:

  • Step 1: Write the mass ratio given: Mg : S = 3 : 4 (by mass).
  • Step 2: Convert to relative number of atoms by dividing by atomic masses:
    • Mg: 3 / 24 = 0.125
    • S: 4 / 32 = 0.125
  • Step 3: Divide both values by the smallest (0.125) to get simplest whole-number ratio:
    • Mg: 0.125 / 0.125 = 1
    • S: 0.125 / 0.125 = 1
  • Step 4: The simplest ratio is 1 : 1, so the chemical formula is MgS.
  • Explanation: Although the mass numbers 3 and 4 look different, after accounting for atomic masses of Mg and S, they correspond to equal numbers of atoms. This matches the ionic expectation: magnesium tends to form Mg²⁺, sulphur forms S²⁻, and one Mg²⁺ balances one S²⁻ to make the neutral compound MgS.

Q9. Scenario-based reaction analysis: Write and balance the chemical equation for the reaction of sodium metal with chlorine gas to form sodium chloride. Explain in detail how electrons are transferred and why the balanced equation reflects charge neutrality.

Answer:

  • Chemical equation (unbalanced skeleton): Na + Cl₂ → NaCl.
  • Sodium (Na) is a metal with one valence electron; it tends to lose one electron to form Na⁺. Chlorine (Cl₂) is a diatomic non-metal; each chlorine atom needs one electron to become Cl⁻.
  • For each Cl₂ molecule (2 chlorine atoms), two electrons are needed (one for each Cl). Two sodium atoms each lose one electron; together they supply the two electrons. Therefore the balanced equation is: 2 Na + Cl₂ → 2 NaCl.
  • Electron transfer details:
    • 2 Na → 2 Na⁺ + 2 e⁻ (two sodium atoms lose two electrons)
    • Cl₂ + 2 e⁻ → 2 Cl⁻ (chlorine molecule gains two electrons, one per atom)
  • The final ions Na⁺ and Cl⁻ combine in a 1:1 ratio to form electrically neutral NaCl units. ...

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