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: Atomic Structure (10 marks)

1. (a) Particles P and Q are isotopes of the same element. [1]
Explanation: Both have the same number of protons (17), so they are atoms of the same element (chlorine). They have different numbers of neutrons (18 and 20), which is the definition of isotopes. [1]

(b) Particle R has 20 protons and 18 electrons.
Charge = +20 − 18 = +2 [1]
Symbol: $^{40}_{20}Ca^{2+}$ (Mass number = 20 + 20 = 40) [1]
Award 1 mark for correct charge, 1 mark for correct symbol with mass number, atomic number, and charge.

2. (a) Group 2, Period 3. [1]
Explanation: Element X has 3 electron shells, so it is in Period 3. It has 2 valence electrons, so it is in Group 2. [1]

(b) Ionic bonding. [1]
Dot-and-cross diagram:

• X (2,8,2) loses its 2 outer electrons to form X²⁺ ion with electronic structure 2,8.
• Y (2,7) gains 1 electron to form Y⁻ ion with electronic structure 2,8.
• Two Y⁻ ions are needed for one X²⁺ ion.
• Formula: XY₂.

Award marks as follows:

• Correct electron transfer shown (X loses 2 electrons, each Y gains 1) [1]
• Correct electronic structures of ions (both achieve stable octet) [1]
• Correct charges and ratio (X²⁺ and 2Y⁻) [1]

Accept dot-and-cross diagram showing outer electrons only with clear key for electrons from each atom.

3. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. [2]
Award 1 mark for "same number of protons" and 1 mark for "different numbers of neutrons".

4. Electronic structure: 2,8,8,1. [1]
Element: Potassium (K). [1]

5. Protons: 13 [1]
Neutrons: 14 (27 − 13) [1]
Electrons: 10 (13 − 3) [1]
Total marks: 2. Award 1 mark for two correct, 2 marks for all three correct.

Section B: Chemical Bonding and Structure (10 marks)

6. (a) Ionic bonding with a giant ionic lattice structure. [1]

(b) Any two properties explained (2 marks each):

• High melting point: There are strong electrostatic forces of attraction between the oppositely charged ions in the giant ionic lattice. A large amount of energy is required to overcome these strong forces. [2]
• Does not conduct when solid: The ions are held in fixed positions in the lattice and are not free to move, so they cannot carry an electric current. [2]
• Conducts when molten: When molten, the ions are free to move and can carry an electric current. [2]
• Soluble in water: Water molecules can attract and separate the ions from the lattice, allowing the substance to dissolve. [2]

Award 2 marks for each property correctly explained with reference to structure and bonding.

7. (a) Diamond has a giant covalent structure. [1] Each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. [1] These strong covalent bonds extend throughout the entire structure, requiring a large amount of energy to break, giving diamond its hardness and very high melting point. [1]

(b) In graphite, each carbon atom is bonded to only three other carbon atoms, leaving one delocalised electron per carbon atom. [1] These delocalised electrons are free to move throughout the structure and can carry an electric current. In diamond, all four valence electrons per carbon atom are used in covalent bonds; there are no free electrons to conduct electricity. [1]

8. (a) Dot-and-cross diagram of NH₃:

• Nitrogen atom (5 valence electrons) in centre.
• Three hydrogen atoms (1 valence electron each) around nitrogen.
• Three shared pairs of electrons (one between N and each H), forming three single covalent bonds.
• One lone pair of electrons on the nitrogen atom.

Award 1 mark for correct electron count and bonding, 1 mark for showing lone pair on nitrogen.

(b) Ammonia has a simple molecular structure. [1] The molecules are held together by weak intermolecular forces of attraction (van der Waals' forces). [1] Only a small amount of energy is required to overcome these weak forces, so ammonia has a low boiling point. [1]
Note: The covalent bonds within the molecules are strong, but these are not broken during boiling.

9. Metals consist of a regular lattice of positive metal ions surrounded by a "sea" of delocalised electrons. [1] The electrostatic attraction between the positive ions and the delocalised electrons is metallic bonding. [1]

10. Ionic compounds are brittle because when a force is applied, layers of ions shift, causing ions of the same charge to align. [1] The repulsion between like charges causes the lattice to shatter. [1]

Section C: Properties, Trends, and Applications (10 marks)

11. (a)
W: Ionic bonding [1]
X: Metallic bonding [1]
Y: Giant covalent bonding [1]
Z: Metallic bonding [1]

(b) Sodium chloride has a giant ionic lattice structure. [1] There are strong electrostatic forces of attraction between the oppositely charged Na⁺ and Cl⁻ ions. A large amount of energy is required to overcome these strong forces, resulting in a high melting point. [1]

12. (a) The atomic radius decreases from sodium to aluminium across Period 3. [1]

(b) Across Period 3, the number of protons in the nucleus increases (nuclear charge increases). [1] Electrons are added to the same electron shell, so the shielding effect remains approximately the same. [1] The increased nuclear charge attracts the electrons more strongly, pulling the electron cloud closer to the nucleus, resulting in a smaller atomic radius. [1]

13. (a) Aluminium has metallic bonding. [1] The structure consists of a regular lattice of positive aluminium ions (Al³⁺) surrounded by a "sea" of delocalised electrons. [1] The electrostatic attraction between the positive ions and the delocalised electrons holds the structure together. [1]

(b) Any two from:

• Recycling aluminium requires much less energy than extracting it from its ore (bauxite) by electrolysis. [1]
• Recycling reduces the need for mining bauxite, which damages the environment and depletes natural resources. [1]
• Recycling reduces the amount of waste sent to landfills. [1]

Award 1 mark for each valid reason, up to 2 marks.

14. Graphite is used as an electrode in electrolysis / in batteries / in electric motor brushes. [1]
Accept any valid use that relies on electrical conductivity.

15. Metals are malleable because the layers of positive ions can slide over each other when a force is applied. [1] The delocalised electrons move with the ions, preventing repulsion and maintaining the metallic bonding. [1]

Section D: Consolidation (10 marks)

16. Sodium chloride has ionic bonding with a giant ionic lattice structure. [1] Carbon dioxide has covalent bonding with a simple molecular structure. [1] In NaCl, strong electrostatic forces between ions require high energy to break; in CO₂, weak intermolecular forces between molecules require little energy to overcome. [1]

17. Solid sodium chloride does not conduct electricity because the ions are held in fixed positions and are not free to move. [1] Sodium metal conducts electricity because it has a "sea" of delocalised electrons that are free to move throughout the structure. [1] The delocalised electrons can carry an electric current. [1]

18. Reactivity increases down Group 1. [1] The atomic radius increases, so the valence electron is further from the nucleus and more easily lost. Increased shielding also reduces the attraction between the nucleus and the valence electron. [1]

19. The statement is generally true but not universally. Most substances with high melting points, like diamond (giant covalent) and sodium chloride (giant ionic), have giant structures. [1] However, some substances with giant structures, like graphite, have high melting points, while all simple molecular substances have low melting points. The statement is valid because strong bonds throughout the structure require high energy to break. [1]

20. Chlorine (Cl₂) is a simple molecular substance with weak intermolecular forces between molecules, so it is a gas at room temperature. [1] Sodium chloride (NaCl) is an ionic compound with a giant ionic lattice structure and strong electrostatic forces between ions, so it is a solid at room temperature. [1]

END OF ANSWER KEY