Secondary 4 Combined Science Chemistry Atomic Structure Bonding Quiz

Secondary 4 Combined Science Chemistry Quiz - Atomic Structure Bonding

Answer Key

Section A: Multiple Choice Questions (Questions 1–5)

1. C [1]
Electrons occupy most of the volume of the atom (electron cloud), while the nucleus is extremely small and dense. Option A is incorrect because the nucleus contains protons and neutrons, not electrons. Option B is incorrect because neutrons are neutral. Option D is incorrect because mass number = protons + neutrons.

2. A [1]
12 protons means 12 electrons in a neutral atom. Electronic configuration: 2, 8, 2 (filling shells in order: K=2, L=8, M=2).

3. B [1]
Sodium (metal) and chlorine (non-metal) form an ionic bond through electron transfer. The other pairs consist of non-metals only and form covalent bonds.

4. B [1]
A covalent bond is the sharing of electrons between two non-metal atoms. Option A describes ionic bonding. Option C describes ionic bonding. Option D describes metallic bonding.

5. C [1]
Silicon dioxide (SiO₂) has a giant covalent (macromolecular) structure. NaCl is ionic. CO₂ and H₂O are simple molecular.

Section B: Short Answer Questions (Questions 6–10)

6. Isotopes are atoms of the same element (same number of protons/atomic number) that have different numbers of neutrons (and therefore different mass numbers). [2]
Marking: 1 mark for "same element/same number of protons"; 1 mark for "different number of neutrons/different mass number".

7.
(a) 17 protons [1]
Atomic number = number of protons.

(b) 18 neutrons [1]
Mass number = protons + neutrons → 35 − 17 = 18 neutrons.

(c) 17 electrons [1]
In a neutral atom, number of electrons = number of protons.

8. Any two of the following: [2]

• Ionic compounds have high melting/boiling points; covalent (simple molecular) compounds have low melting/boiling points. [1]
• Ionic compounds conduct electricity when molten or in aqueous solution; covalent compounds do not conduct electricity in any state. [1]
• Ionic compounds are usually soluble in water; many covalent compounds are insoluble in water. [1]
• Ionic compounds form crystals; covalent compounds may be gases, liquids, or solids at room temperature. [1]
  Marking: 1 mark per valid difference, max 2 marks.

9. Sodium chloride has a giant ionic lattice structure. There are strong electrostatic forces of attraction (ionic bonds) between the oppositely charged Na⁺ and Cl⁻ ions. A large amount of energy is required to overcome these strong forces, hence the high melting point. [2]
Marking: 1 mark for identifying strong electrostatic forces/ionic bonds between ions; 1 mark for stating that a large amount of energy is needed to overcome these forces.

10.

      ..
   H : O : H
      ..

[2]
Oxygen has 6 outer shell electrons (3 lone pairs shown as 6 dots, but in dot-and-cross: 2 lone pairs shown as 4 dots, and 2 shared electrons). Each hydrogen shares 1 electron with oxygen. Oxygen achieves a stable octet (8 electrons in outer shell); each hydrogen achieves a stable duplet (2 electrons).
Marking: 1 mark for correct number of bonds (2 O–H bonds); 1 mark for showing all outer shell electrons correctly (lone pairs on O).

Section C: Structured Response Questions (Questions 11–15)

11.
(a) Particle A is a positive ion. [1] It has 11 protons (positive charges) but only 10 electrons (negative charges), giving it a net positive charge of +1. [1]
Marking: 1 mark for identifying A; 1 mark for explanation based on unequal protons and electrons.

(b) Particles B and C are isotopes. [1] They have the same number of protons (11) but different numbers of neutrons (12 and 13 respectively). [1]
Marking: 1 mark for identifying B and C; 1 mark for explanation (same protons, different neutrons).

(c) Sodium; chemical symbol: Na [1]
Atomic number 11 corresponds to sodium.

12.
(a) Ionic bonding [1]

(b) Magnesium (Mg) has 2 outer shell electrons. It loses these 2 electrons to form a Mg²⁺ ion. [1] Oxygen (O) gains these 2 electrons to form an O²⁻ ion. The oppositely charged ions are held together by strong electrostatic forces of attraction. [1]
Marking: 1 mark for describing electron transfer (Mg loses 2e⁻, O gains 2e⁻); 1 mark for stating electrostatic attraction between ions.

(c) Any two of: [2]

• High melting point [1]
• Conducts electricity when molten or in aqueous solution [1]
• Usually soluble in water [1]
• Forms crystalline solids [1]
  Marking: 1 mark per valid property, max 2 marks.

13.
(a) Giant covalent (macromolecular) structure [1]

(b) Each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. [1] The covalent bonds are very strong and a large amount of energy is needed to break them, making diamond very hard. [1]
Marking: 1 mark for describing the 3D network of strong covalent bonds; 1 mark for linking strong bonds to hardness.

(c) All four outer shell electrons of each carbon atom are involved in covalent bonding. There are no free/delocalised electrons available to carry charge. [1]
Marking: 1 mark for stating no free/delocalised electrons.

14.
(a) Copper (Cu) conducts electricity when solid. [1] Copper has a giant metallic structure with delocalised electrons that are free to move and carry charge through the solid. [1]
Marking: 1 mark for identifying copper; 1 mark for delocalised electrons explanation.

(b) Sodium chloride (NaCl) conducts electricity when dissolved in water. [1] When dissolved, the Na⁺ and Cl⁻ ions are free to move and carry charge through the solution. [1]
Marking: 1 mark for identifying NaCl; 1 mark for mobile ions explanation.

(c) Carbon dioxide (CO₂) has the lowest melting point. [1] CO₂ is a simple molecular substance with weak intermolecular forces of attraction between molecules. Only a small amount of energy is needed to overcome these weak forces. [1]
Marking: 1 mark for identifying CO₂; 1 mark for weak intermolecular forces explanation.

15.
(a) 2, 8, 8, 1 [1]
19 electrons: K=2, L=8, M=8, N=1.

(b) Group 1 [1]; Period 4 [1]
1 outer shell electron → Group 1. 4 electron shells → Period 4.

(c) Formula: KCl [1]; Type of bonding: Ionic [1]
Potassium (K) loses 1 electron to form K⁺; chlorine (Cl) gains 1 electron to form Cl⁻. The ions are held together by ionic bonds.

Section D: Application and Data-Based Questions (Questions 16–20)

16.
(a) Substance P: Giant ionic structure [1]
Substance Q: Giant covalent (macromolecular) structure [1]
Substance R: Simple molecular structure [1]
P: high MP/BP, conducts when liquid → ionic. Q: very high MP/BP, does not conduct → giant covalent. R: very low MP/BP, does not conduct → simple molecular.

(b) In the solid state, the ions are held in fixed positions in the lattice and cannot move to carry charge. [1] When liquid (molten), the ions are free to move and can carry electrical charge. [1]
Marking: 1 mark for fixed ions in solid; 1 mark for mobile ions in liquid.

17.
(a) Dot-and-cross diagram for NH₃:

      H
      |
  H : N :
      |
      H

Nitrogen has 5 outer shell electrons (3 shared with H atoms, 1 lone pair). Each hydrogen shares 1 electron with nitrogen. Nitrogen achieves an octet; each hydrogen achieves a duplet.
Marking: 1 mark for correct bonding (3 N–H bonds); 1 mark for showing lone pair on N and all outer shell electrons.

(b) Ammonia has a simple molecular structure. [1] The intermolecular forces of attraction (van der Waals' forces) between NH₃ molecules are weak. Only a small amount of energy is needed to overcome these weak forces, hence the low melting point. [1]
Marking: 1 mark for identifying simple molecular structure/weak intermolecular forces; 1 mark for linking to low energy requirement.

18.
(a) Covalent bonding [1]

(b) Each carbon atom in graphite is bonded to three other carbon atoms. One electron from each carbon atom is delocalised. [1] These delocalised electrons are free to move between the layers and can carry electrical charge. [1]
Marking: 1 mark for delocalised electrons; 1 mark for free movement carrying charge.

(c) The layers of carbon atoms are held together by weak intermolecular forces. [1] These layers can slide over each other easily, making graphite soft and slippery. [1]
Marking: 1 mark for weak forces between layers; 1 mark for layers sliding over each other.

19. Test 1: Test solubility in water.

• NaCl dissolves in water; glucose dissolves in water; SiO₂ does not dissolve. [1]
  Test 2: Test electrical conductivity of the aqueous solution.
• NaCl solution conducts electricity (ions are free to move); glucose solution does not conduct electricity (no ions formed). [1]
  Test 3: Test melting point / heat the solids.
• SiO₂ has a very high melting point (giant covalent); glucose has a low melting point (simple molecular). [1]
  Alternative: Test conductivity of solid — none conduct, but copper would (not applicable here).
  Marking: 1 mark for each valid test with correct observation, max 4 marks. Accept any logical series of tests that correctly identifies all three solids.

20.
(a) W (atomic number 6): 2, 4 [1]
X (atomic number 8): 2, 6 [1]
Y (atomic number 12): 2, 8, 2 [1]
Z (atomic number 17): 2, 8, 7 [1]

(b) Elements: Y (magnesium) and Z (chlorine) [1]
Formula: MgCl₂ [1]
Y is a metal (Group 2), Z is a non-metal (Group 7). They form an ionic compound. Mg loses 2 electrons, each Cl gains 1 electron → MgCl₂.

(c) Elements: W (carbon) and X (oxygen) [1]
Formula: CO₂ [1]
W and X are both non-metals. They form a covalent compound. C shares 4 electrons, each O shares 2 electrons → CO₂.