Secondary 4 Combined Science Chemistry Atomic Structure Bonding Quiz

Secondary 4 Combined Science Chemistry Quiz - Atomic Structure Bonding

ANSWER KEY AND MARKING SCHEME

Total Marks: 40

Section A: Multiple Choice (5 marks)

1. D. O²⁻ [1 mark]

• Ne has 10 electrons. O²⁻ has 8 protons + 2 gained electrons = 10 electrons.
• Na⁺ has 10 electrons (11−1), F⁻ has 10 electrons (9+1), Mg²⁺ has 10 electrons (12−2). All have 10 electrons, but the question asks for "same number" — any correct answer accepted if justified. Accept D as the most straightforward answer.
• Marking note: Award 1 mark for any correct option with valid reasoning if space provided.

2. D. Metallic [1 mark]

• Conducts in both solid and molten states → metallic bonding (delocalised electrons present in all states).
• Malleable → characteristic of metals.
• High melting point → strong metallic bonds.

3. B. Giant covalent, Si and O atoms [1 mark]

• Silicon dioxide has a giant covalent structure with silicon and oxygen atoms covalently bonded in a continuous lattice.
• It does NOT exist as SiO₂ molecules (eliminates C) and is not ionic (eliminates A).

4. C. It is in Group II of the Periodic Table. [1 mark]

• Electronic structure 2,8,2 → 3 shells = Period 3 (not Period 2, eliminates A).
• 2 valence electrons → Group II (C correct).
• Forms 2+ ions, not 2− (eliminates B).
• Neutron number cannot be determined from electronic structure alone (eliminates D).

5. B. Sodium chloride [1 mark]

• Sodium chloride has giant ionic structure with strong electrostatic forces → high melting point (801°C).
• CO₂, H₂O, and CH₄ are simple molecular substances with weak intermolecular forces → low melting points.

Section B: Short Answer (15 marks)

6. (a) Y and Z are atoms. [1 mark for identification]

• Explanation: In atoms, number of protons = number of electrons. Y has 11 protons and 11 electrons; Z has 17 protons and 17 electrons. [1 mark for explanation]

(b) X and Z are isotopes of the same element. [1 mark for identification]

• Explanation: Isotopes have the same number of protons (both have 17 protons) but different numbers of neutrons (X has 18, Z has 20). [1 mark for explanation]

(c) Charge: 1+ [1 mark]

• Explanation: Particle W has 11 protons and 10 electrons. The atom (Y) loses 1 electron to form W⁺. [1 mark for explanation of electron loss]

7. (a) Dot-and-cross diagram for MgCl₂: [3 marks]

• Magnesium atom: 2,8,2 (2 outer electrons shown as •)
• Two chlorine atoms: each 2,8,7 (7 outer electrons each shown as ×)
• Mg loses 2 electrons → Mg²⁺ with no outer electrons (or full outer shell of 8 from second shell)
• Each Cl gains 1 electron → Cl⁻ with 8 outer electrons (full octet)
• Diagram shows: [Mg]²⁺ with two [Cl]⁻ ions, each Cl⁻ surrounded by 8 electrons (1 pair from Mg, 6 of its own)
• Marking:
  • 1 mark: Correct electron transfer (Mg loses 2, each Cl gains 1)
  • 1 mark: Correct charges on ions (Mg²⁺, Cl⁻)
  • 1 mark: Correct electronic structures of ions (Cl⁻ with full octet, Mg²⁺ with empty outer shell or full second shell)

(b) Magnesium chloride has a high melting point because:

• It has a giant ionic lattice structure. [1 mark]
• There are strong electrostatic forces of attraction between the oppositely charged Mg²⁺ and Cl⁻ ions. [1 mark]
• A large amount of energy is required to overcome these strong forces. [Accept as part of 2-mark explanation]

8. (a) Atomic radius decreases across Period 3 (from Na to Ar). [1 mark]

(b) Explanation:

• Across a period, the number of protons (nuclear charge) increases. [1 mark]
• Electrons are added to the same electron shell, so shielding effect remains approximately the same.
• The increased nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus. [1 mark for linking increased nuclear attraction to smaller radius]

(c) Potassium has one more electron shell than sodium (4 shells vs 3 shells). [1 mark]

• The outer electron is further from the nucleus, so atomic radius is larger. [Accept: More electron shells → larger atomic radius]

9. (a) (i) Substance A: Ionic bonding [1 mark]

• Reason: High melting point, conducts when molten but not when solid, soluble in water — all characteristic of ionic compounds. [1 mark]

(ii) Substance B: Covalent (simple molecular) [1 mark]

• Reason: Low melting point (−7°C), does not conduct in any state, insoluble in water — characteristic of simple molecular substances with weak intermolecular forces. [1 mark]

(iii) Substance C: Metallic bonding [1 mark]

• Reason: Conducts electricity in both solid and molten states, high melting point — characteristic of metals with delocalised electrons. [1 mark]

(iv) Substance D: Covalent (giant) [1 mark]

• Reason: Very high melting point (1610°C), does not conduct in any state, insoluble — characteristic of giant covalent structures like silicon dioxide. [1 mark]

(b) Diagram of solid sodium chloride: [2 marks]

• Regular arrangement of alternating Na⁺ and Cl⁻ ions in a lattice.
• Ions labelled clearly.
• Marking:
  • 1 mark: Correct alternating arrangement of positive and negative ions
  • 1 mark: Ions correctly labelled (Na⁺ and Cl⁻)

Section C: Structured Questions (20 marks)

10. (a) Group VI, Period 3 [1 mark for both correct]

• Electronic structure 2,8,6 → 3 shells = Period 3; 6 valence electrons = Group VI.

(b) (i) Formula: Na₂Q (or Na₂S if Q identified as sulfur) [1 mark]

• Sodium is Group I (1+ ion), Q is Group VI (2− ion) → Na₂Q.

(ii) Ionic bonding [1 mark]

(iii) Dot-and-cross diagram: [3 marks]

• Two sodium atoms each lose 1 electron → two Na⁺ ions
• One Q atom gains 2 electrons → Q²⁻ ion with 8 outer electrons (full octet)
• Diagram shows electron transfer from Na (•) to Q (×)
• Marking:
  • 1 mark: Correct electron transfer (2 Na each lose 1, Q gains 2)
  • 1 mark: Correct charges (Na⁺, Q²⁻)
  • 1 mark: Correct electronic structures (Na⁺ with empty outer shell, Q²⁻ with full octet)

(c) (i) Solid: Does not conduct electricity. [1 mark]

• Ions are held in fixed positions in the lattice and cannot move.

(ii) Dissolved in water: Conducts electricity. [1 mark]

• Ions are free to move in aqueous solution and can carry electric current.

11. (a) Diamond vs graphite hardness:

• Diamond: Each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a rigid three-dimensional giant covalent network. [1 mark]
• All bonds are strong covalent bonds throughout the structure, making it extremely hard. [1 mark]
• Graphite: Carbon atoms are arranged in layers. Each carbon is bonded to three others within the layer (strong covalent bonds), but layers are held together by weak intermolecular forces. [1 mark]
• Layers can slide over each other easily, making graphite soft and slippery. [Accept as part of 3-mark explanation]

(b) Electrical conductivity:

• Graphite: Each carbon atom uses 3 of its 4 valence electrons for covalent bonding. The fourth electron is delocalised and can move freely between layers. [1 mark]
• These delocalised electrons can carry electric current.
• Diamond: All 4 valence electrons per carbon atom are used in covalent bonding. There are no delocalised electrons. [1 mark]
• Therefore, diamond cannot conduct electricity.

12. (a) Sodium [1 mark]

• Electronic structure 2,8,1 → atomic number 11 → sodium.

(b) 1+ [1 mark]

• One valence electron, loses 1 electron to achieve stable octet.

(c) Na₂O [1 mark]

• Sodium is Group I (1+), oxygen is Group VI (2−) → Na₂O.

13. (a) Solid sodium chloride does not conduct electricity because the ions (Na⁺ and Cl⁻) are held in fixed positions in the giant ionic lattice and cannot move. [1 mark]

• In molten sodium chloride, the ions are free to move and can carry electric current. [1 mark]

(b) Diamond has a very high melting point because it has a giant covalent structure. [1 mark]

• A large amount of energy is required to break the many strong covalent bonds between carbon atoms throughout the structure. [1 mark]

14. (a) Substance W is most likely to be a gas at room temperature. [1 mark]

• Explanation: Its melting point (−182°C) is well below room temperature (25°C), so it exists as a gas at 25°C. [1 mark]

(b) Ionic bonding [1 mark]

• Conducts when molten (ions free to move) but not when solid (ions fixed) → characteristic of ionic compounds.

(c) Substance Y has a high melting point because it has a giant covalent structure with strong covalent bonds between atoms throughout the lattice. [1 mark]

• A large amount of energy is needed to break these strong covalent bonds. [1 mark]

15. (a) Metallic bonding [1 mark]

• Diagram shows positive ions in a sea of delocalised electrons.

(b) Any two of:

• Conducts electricity (in solid and molten states)
• Malleable / ductile
• High melting point
• Shiny / lustrous [1 mark each, maximum 2 marks]

(c) In metallic bonding, the layers of positive ions can slide over each other when a force is applied. [1 mark]

• The delocalised electrons can move and adjust to the new arrangement, preventing the structure from breaking apart. [1 mark]

Section D: Data-Based Questions (10 marks)

16. (a) Substance Q is most likely to be a metal. [1 mark]

• Explanation: It conducts electricity in the solid state (delocalised electrons present) and when molten, and is insoluble in water — all characteristic of metals. [1 mark]

(b) Substance P is most likely to be an ionic compound. [1 mark]

• Explanation: It does not conduct when solid (ions fixed) but conducts when molten and when dissolved in water (ions free to move) — characteristic of ionic compounds. [1 mark]

(c) Covalent (simple molecular) bonding that dissociates into ions in water, OR a polar covalent compound that ionises in water. [1 mark]

• Accept: Acid / hydrogen chloride type bonding where molecules react with water to form ions.

17. (a) Element L (2,8,2) and element K (2,8,7) would react to form an ionic compound. [1 mark]

• Explanation: L is a metal (Group II, loses 2 electrons to form 2+ ion), K is a non-metal (Group VII, gains 1 electron to form 1− ion). Transfer of electrons forms ionic bond. [1 mark]

(b) LK₂ (or MgCl₂ if elements identified) [1 mark]

• L²⁺ and K⁻ → LK₂.

(c) Dot-and-cross diagram for J₂M (or Na₂O): [2 marks]

• Two J atoms each lose 1 electron → two J⁺ ions
• One M atom gains 2 electrons → M²⁻ ion with 8 outer electrons
• Marking:
  • 1 mark: Correct electron transfer (2 J each lose 1, M gains 2)
  • 1 mark: Correct charges and electronic structures

18. (a) Melting point increases from sodium to aluminium. [1 mark]

• Na (98°C) → Mg (650°C) → Al (660°C).

(b) Silicon has a giant covalent structure. [1 mark]

• A large amount of energy is required to break the many strong covalent bonds between silicon atoms throughout the structure, resulting in a very high melting point. [1 mark]

(c) Argon is a noble gas with a monatomic structure. [1 mark]

• It exists as individual atoms with very weak intermolecular forces (van der Waals forces) between atoms. Very little energy is required to overcome these weak forces, resulting in a very low melting point. [1 mark]

19. (a) The statement is incorrect because some substances with covalent bonds have giant covalent structures and very high melting points. [1 mark]

• Examples: Diamond and silicon dioxide have giant covalent structures with strong covalent bonds throughout, requiring large amounts of energy to break, resulting in high melting points. [1 mark]

(b) Simple molecular covalent: Consists of small discrete molecules with strong covalent bonds within molecules but weak intermolecular forces between molecules. [1 mark]

• Giant covalent: Consists of a continuous network of atoms held together by strong covalent bonds throughout the entire structure. [1 mark]

(c) Diamond / Graphite / Silicon dioxide / Silicon [1 mark for any correct example]

20. (a) Particle T is a positive ion. [1 mark]

• Explanation: T has 12 protons but only 10 electrons, giving it an overall charge of 2+ (more protons than electrons). [1 mark]

(b) U and T are isotopes of the same element. [1 mark]

• Explanation: Both have 12 protons (same element) but different numbers of neutrons (U has 13, T has 12). [1 mark]

(c) Charge: 1− [1 mark]

• Explanation: Particle V has 17 protons and 18 electrons. The atom (W) gains 1 electron to form V⁻. [Accept: 1− charge, formed by gaining one electron]

END OF ANSWER KEY