O Level Chemistry Atomic Structure Bonding Quiz

O-Level Chemistry Quiz - Atomic Structure Bonding

ANSWER KEY AND MARKING SCHEME

Total Marks: 40

Section A: Atomic Structure (Questions 1–5)

1. An atom of element X has 19 protons and 20 neutrons.

(a) Answer: ³⁹₁₉X (or ³⁹₁₉K) [1]

• Marking: Accept any format clearly showing mass number 39 and atomic number 19. Symbol X or K both acceptable.

(b) Answer: 19 electrons [1]

• Marking: A neutral atom has equal numbers of protons and electrons.

(c) Answer: 2,8,8,1 [1]

• Marking: Must show four shells with correct electron distribution. Accept 2.8.8.1.

2. Chlorine exists naturally as two isotopes: chlorine-35 and chlorine-37.

(a) Answer: Isotopes are atoms of the same element with the same number of protons (same proton/atomic number) but different numbers of neutrons (different nucleon/mass number). [1]

• Marking: Must mention both: same proton number AND different neutron/nucleon number.

(b) Answer: Both isotopes have the same number of protons (17) and therefore the same number of electrons (17) with the same electronic configuration (2,8,7). Chemical properties are determined by the number and arrangement of electrons, particularly the valence electrons. Since both isotopes have identical electron arrangements, they have identical chemical properties. [2]

• Marking: 1 mark for stating same electron arrangement/configuration; 1 mark for linking electron arrangement to chemical properties.

3. The table below shows information about three particles.

(a) Answer: Particle A is a negative ion. It has 8 protons (+) but 10 electrons (−), giving an overall charge of 2−. [1]

• Marking: Must identify particle A and explain that electrons exceed protons.

(b) Answer: Nucleon number = 11 + 12 = 23 [1]

• Marking: Correct addition of protons and neutrons.

(c) Answer: S²⁻ [1]

• Marking: Particle C has 16 protons (sulfur) and 18 electrons (2− charge). Accept S²⁻ or S⁻².

4. The diagram represents an atom with 5 outer electrons (nitrogen).

(a) Answer: Group V (or 15), Period 2 [1]

• Marking: Group determined by 5 valence electrons; Period determined by 2 electron shells (implied by showing only outer shell with 5 electrons, nitrogen has configuration 2,5).

(b) Answer: Non-metal. The element has 5 valence electrons. Non-metals typically have 4 or more valence electrons and tend to gain or share electrons to achieve a full outer shell. Metals typically have 1–3 valence electrons and lose electrons. [2]

• Marking: 1 mark for correct identification; 1 mark for explanation linking valence electrons to metallic/non-metallic character.

5. An ion Y²⁻ has an electronic configuration of 2,8,8.

(a) Answer: Proton number = 16 [1]

• Marking: The ion has 18 electrons (2+8+8). Since it has a 2− charge, the neutral atom has 16 electrons, so 16 protons.

(b) Answer: Sulfur (S) [1]

• Marking: Element with proton number 16.

Section B: Ionic and Metallic Bonding (Questions 6–10)

6. Sodium reacts with chlorine to form sodium chloride.

(a) Answer: Diagram showing:

• Na atom with 1 outer electron (●) transferring to Cl atom with 7 outer electrons (×)
• Na⁺ ion: [Na]⁺ with no outer electrons shown (or empty outer shell)
• Cl⁻ ion: [Cl]⁻ with 8 outer electrons (1● + 7×) enclosed in brackets with − charge
• Correct use of ● for Na electrons and × for Cl electrons [2]
• Marking: 1 mark for correct electron transfer and ion formation; 1 mark for correct brackets, charges, and dot/cross distinction. Deduct 1 mark if inner shells are shown when instructed "outer electrons only."

(b) Answer: Giant ionic lattice (structure) [1]

• Marking: Accept "giant ionic" or "ionic lattice."

(c) Answer: Sodium chloride has a giant ionic lattice structure. There are strong electrostatic forces of attraction between the oppositely charged Na⁺ and Cl⁻ ions throughout the entire lattice. A large amount of energy is required to overcome these strong forces of attraction, resulting in a high melting point. [2]

• Marking: 1 mark for identifying giant ionic structure and strong electrostatic forces; 1 mark for linking to energy required to overcome forces.

7. Answer: In MgO, the ions are Mg²⁺ and O²⁻, both with double charges (2+ and 2−). In NaCl, the ions are Na⁺ and Cl⁻, both with single charges (1+ and 1−). The electrostatic forces of attraction between ions are stronger when the charges on the ions are greater. Therefore, the ionic bonds in MgO are stronger than those in NaCl, requiring more energy to overcome, resulting in a higher melting point. [2]

• Marking: 1 mark for identifying the difference in ionic charges (Mg²⁺/O²⁻ vs Na⁺/Cl⁻); 1 mark for linking greater charges to stronger electrostatic forces and higher melting point.

8. Aluminium is a metal widely used in aircraft construction.

(a) Answer: Diagram showing:

• Regular arrangement of Al³⁺ cations in a lattice pattern
• Delocalised electrons shown as small dots or a "sea" between the cations
• Labels: "Al³⁺ cations" (or "metal cations/positive ions") and "delocalised electrons" (or "sea of electrons") [2]
• Marking: 1 mark for correct lattice of positive ions; 1 mark for delocalised electrons and correct labels.

(b) Answer: Aluminium has a giant metallic lattice structure with a sea of delocalised electrons. These delocalised electrons are free to move throughout the metal lattice and carry electrical charge when a potential difference is applied. [1]

• Marking: Must mention delocalised/free-moving electrons.

9. Answer: Metals have a giant metallic lattice structure consisting of positive metal ions arranged in regular layers, surrounded by a sea of delocalised electrons. When a force is applied, the layers of metal ions can slide over each other without breaking the metallic bonds because the delocalised electrons can move and continue to hold the ions together in the new positions. This allows metals to be hammered into different shapes without breaking. [2]

• Marking: 1 mark for describing the layer structure of metal ions; 1 mark for explaining that layers can slide while delocalised electrons maintain bonding.

10. Answer: In pure copper, all atoms are the same size and are arranged in regular layers. When a force is applied, these layers can slide over each other easily. In brass, zinc atoms are of a different size from copper atoms. The presence of differently-sized zinc atoms in the copper lattice disrupts the regular arrangement of layers. This makes it more difficult for the layers to slide over each other when a force is applied, making brass harder and stronger than pure copper. [2]

• Marking: 1 mark for describing regular layers in pure copper and easy sliding; 1 mark for explaining how different-sized atoms in the alloy disrupt layer sliding.

Section C: Covalent Bonding and Giant Covalent Structures (Questions 11–15)

11. Draw dot-and-cross diagrams.

(a) Answer: Diagram of H₂O showing:

• O atom with 6 outer electrons (e.g., ●), sharing one pair with each H atom
• Each H atom with 1 outer electron (×), sharing with O
• Two bonding pairs and two lone pairs on O
• Correct dot/cross distinction [2]
• Marking: 1 mark for correct electron arrangement and bonding pairs; 1 mark for correct lone pairs and dot/cross distinction.

(b) Answer: Diagram of CO₂ showing:

• C atom with 4 outer electrons (e.g., ●), sharing all four with O atoms
• Each O atom with 6 outer electrons (×), sharing two with C (double bonds)
• Two double bonds (C=O), each O with two lone pairs
• Correct dot/cross distinction [2]
• Marking: 1 mark for correct double bonds; 1 mark for correct lone pairs and dot/cross distinction.

12. Methane, CH₄.

(a) Answer: Simple molecular structure [1]

• Marking: Accept "simple molecular" or "simple covalent molecular."

(b) Answer: Methane has a simple molecular structure consisting of discrete CH₄ molecules. The atoms within each molecule are held together by strong covalent bonds, but there are only weak intermolecular forces of attraction between the molecules. Only a small amount of energy is required to overcome these weak intermolecular forces during boiling, resulting in a low boiling point. [2]

• Marking: 1 mark for identifying weak intermolecular forces between molecules; 1 mark for linking weak forces to low energy requirement and low boiling point. Do NOT accept "breaking covalent bonds" — this is a common error.

13. Diamond and graphite.

(a) Answer: Giant covalent structure (or giant molecular structure) [1]

• Marking: Accept "giant covalent" or "giant molecular." Do not accept "giant ionic" or "simple molecular."

(b) Answer: In diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a rigid three-dimensional giant covalent network structure. All bonds are strong covalent bonds throughout the entire structure, making diamond extremely hard. In graphite, each carbon atom is covalently bonded to three other carbon atoms, forming layers of hexagonal rings. The layers are held together by weak intermolecular forces (van der Waals forces). These weak forces between layers allow the layers to slide over each other easily, making graphite soft and slippery. [3]

• Marking: 1 mark for describing diamond's tetrahedral 3D network of strong covalent bonds; 1 mark for describing graphite's layered structure with covalent bonds within layers; 1 mark for identifying weak forces between graphite layers and linking to slipperiness.

14. Answer: Silicon dioxide has a giant covalent (giant molecular) structure. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms, forming a continuous three-dimensional network of strong covalent bonds throughout the entire structure. A large amount of energy is required to break these strong covalent bonds, resulting in a very high melting point. [2]

• Marking: 1 mark for identifying giant covalent structure with strong covalent bonds throughout; 1 mark for linking to the large amount of energy needed to break bonds.

15. Answer: Carbon dioxide has a simple molecular structure consisting of discrete CO₂ molecules held together by weak intermolecular forces. Only a small amount of energy is needed to overcome these weak forces, resulting in a low melting point. Silicon dioxide has a giant covalent structure with strong covalent bonds between Si and O atoms throughout a continuous three-dimensional network. A large amount of energy is required to break these strong covalent bonds, resulting in a very high melting point. [2]

• Marking: 1 mark for correctly identifying the different structure types (simple molecular vs. giant covalent); 1 mark for linking structure type to the strength of forces/bonds that must be overcome.

Section D: Structure-Property Relationships and Applications (Questions 16–20)

16. Table analysis.

(a) Answer: Substance W has a giant ionic lattice structure with ionic bonding. Evidence: It has a high melting point (801°C), indicating strong forces between particles. It does not conduct electricity in the solid state (ions are held in fixed positions and cannot move), but conducts when molten or aqueous (ions become free to move and carry charge). These properties are characteristic of ionic compounds. [2]

• Marking: 1 mark for identifying giant ionic structure; 1 mark for using evidence from the table (high m.p., conductivity only when molten/aqueous) to justify.

(b) Answer: In graphite, each carbon atom is covalently bonded to three other carbon atoms, forming layers. Each carbon atom has one delocalised electron that is free to move within the layer (between the layers of carbon atoms). These delocalised electrons can move and carry electrical charge parallel to the layers, but cannot move easily perpendicular to the layers (between layers). Therefore, graphite conducts electricity along the layers but not across them. [2]

• Marking: 1 mark for identifying delocalised electrons within layers; 1 mark for explaining that conduction occurs along layers only.

17. Poly(ethene).

(a) Answer: Both poly(ethene) and ethene have simple molecular structures with weak intermolecular forces between molecules. However, poly(ethene) molecules are very long polymer chains with much larger molecular sizes compared to small ethene molecules. The intermolecular forces between long poly(ethene) chains are much stronger (more extensive) than those between small ethene molecules. More energy is required to overcome these stronger intermolecular forces, so poly(ethene) is a solid at room temperature while ethene is a gas. [2]

• Marking: 1 mark for recognising both have intermolecular forces (simple molecular); 1 mark for explaining that longer chains have stronger/more extensive intermolecular forces.

(b) Answer: Poly(ethene) has a simple molecular structure with no free-moving ions or delocalised electrons. All electrons are held in covalent bonds or as lone pairs within the molecules, so there are no mobile charge carriers to conduct electricity. [1]

• Marking: Must mention absence of mobile charge carriers (ions or delocalised electrons).

18. Answer: Iodine has a simple molecular structure consisting of discrete I₂ molecules. There are no free-moving ions or delocalised electrons in solid iodine because all electrons are held within the covalent bonds or as lone pairs on the iodine atoms. When dissolved in water, iodine molecules remain as neutral I₂ molecules; they do not dissociate into ions. Since there are no mobile charge carriers (ions or delocalised electrons) in either the solid or aqueous state, iodine does not conduct electricity. [2]

• Marking: 1 mark for identifying simple molecular structure with no mobile charge carriers; 1 mark for explaining that iodine does not form ions in water.

19. Answer: Sodium chloride is an ionic compound. In the solid state, the Na⁺ and Cl⁻ ions are held in fixed positions in the giant ionic lattice and cannot move, so it does not conduct. When dissolved in water, the ions dissociate and become free to move, allowing the solution to conduct electricity. Hydrogen chloride is a simple covalent molecule. As a gas, it consists of neutral HCl molecules with no mobile charge carriers, so it does not conduct. However, when dissolved in water, HCl molecules ionise/dissociate to form H⁺ and Cl⁻ ions, which are free to move and carry charge, so the solution conducts electricity. [2]

• Marking: 1 mark for explaining NaCl behaviour (ionic, ions fixed in solid, mobile in solution); 1 mark for explaining HCl behaviour (covalent molecules, ionises in water to form mobile ions).

20. Material selection.

(a) Answer: Copper (or aluminium). Copper has a giant metallic lattice structure with a sea of delocalised electrons. These delocalised electrons are free to move throughout the metal lattice and can carry electrical charge efficiently when a potential difference is applied, making copper an excellent electrical conductor suitable for wiring. [1]

• Marking: Must name a suitable metal and link to delocalised electrons and electrical conductivity. Accept copper or aluminium.

(b) Answer: Graphite. Graphite has a layered giant covalent structure. The layers of carbon atoms are held together by weak intermolecular forces, allowing the layers to slide over each other easily. This makes graphite soft and slippery, ideal for use as a lubricant. [1]

• Marking: Must name graphite and link to layered structure and weak forces between layers allowing sliding.

END OF ANSWER KEY