Secondary 4 Combined Science Chemistry Periodic Table Quiz

Secondary 4 Combined Science Chemistry Quiz - Periodic Table

ANSWER KEY AND MARKING SCHEME

Total Marks: 40

Section A: Multiple Choice (10 marks)

Marking: 1 mark per correct answer. Total = 10 marks.

Section B: Structured Questions (20 marks)

Question 11 (6 marks)

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

(b) Silicon has a giant covalent / giant molecular structure [1 mark]. It consists of a three-dimensional network of silicon atoms held together by strong covalent bonds [1 mark]. A large amount of energy is required to break these strong covalent bonds, resulting in a high melting point. Phosphorus exists as simple P₄ molecules with weak intermolecular forces of attraction between molecules. Only a small amount of energy is required to overcome these weak forces, resulting in a low melting point [1 mark].

(c) Across Period 3, the elements change from metals (Na, Mg, Al) to metalloid (Si) to non-metals (P, S, Cl) [1 mark]. Metals have delocalised electrons that can move freely and carry charge. Non-metals do not have delocalised electrons / have electrons held in fixed positions in covalent bonds or molecules, so they cannot conduct electricity [1 mark].

Question 12 (6 marks)

(a) 2,8,8,1 [1 mark]

(b) Group: 1 [½ mark]
Period: 4 [½ mark]
Explanation: Potassium has 4 electron shells, so it is in Period 4 [½ mark]. It has 1 valence electron, so it is in Group 1 [½ mark].

(c) Observations: Potassium floats on water / moves rapidly on the water surface [½ mark]; it melts into a silvery ball [½ mark]; it burns with a lilac flame [½ mark]; effervescence / gas bubbles produced [½ mark]; potassium becomes smaller and eventually disappears [½ mark]. (Any 3 observations for 1½ marks)
Equation: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) [1½ marks — ½ for correct formulae, ½ for balancing, ½ for state symbols]

Question 13 (8 marks)

(a) Element W belongs to Group 16 (or Group 6) [1 mark]. It has 6 outer/valence electrons, and the group number for non-metals is determined by the number of valence electrons [1 mark].

(b) Element W is oxygen [1 mark]. Electronic structure: 2,6 [1 mark].

(c) Dot-and-cross diagram of H₂O:

• Oxygen atom with 6 outer electrons (e.g., as dots)
• Two hydrogen atoms each with 1 outer electron (e.g., as crosses)
• Two shared pairs between O and each H (O–H bonds)
• Two lone pairs on oxygen [2 marks — 1 for correct electron arrangement, 1 for correct bonding pairs and lone pairs]

(d) The oxide of oxygen is water (H₂O), which is neutral [1 mark]. Non-metal oxides are generally acidic, but water is an exception / water has a pH of 7 and does not react with acids or bases [1 mark].
Alternative answer if W is interpreted as sulfur: The oxide of sulfur (SO₂, SO₃) is acidic because sulfur is a non-metal, and non-metal oxides react with water to form acidic solutions / react with bases to form salts [2 marks].

Question 14 (10 marks)

(a) Atomic radius decreases across Period 2 from lithium to neon. [1 mark]

(b) Across Period 2, the number of protons / nuclear charge increases [1 mark]. Electrons are added to the same electron shell, so shielding remains approximately constant. The increased nuclear charge exerts a stronger attractive force on the electrons, pulling them closer to the nucleus, resulting in a smaller atomic radius [1 mark].

(c) Neon has 2 electron shells (electronic structure 2,8) while sodium has 3 electron shells (electronic structure 2,8,1) [1 mark]. The addition of a new electron shell in sodium means the outermost electrons are further from the nucleus, resulting in a much larger atomic radius [1 mark].

(d) Argon (proton number 18) has the smallest atomic radius in Period 3 [1 mark]. Across Period 3, atomic radius decreases as nuclear charge increases while electrons are added to the same shell. Argon, being the last element in Period 3, has the highest nuclear charge and therefore the strongest attraction on its electrons, giving it the smallest radius [1 mark].

(e) Potassium has 4 electron shells while sodium has 3 electron shells [1 mark]. The outermost electron in potassium is in the fourth shell, further from the nucleus than the outermost electron in sodium (third shell). Despite potassium having a greater nuclear charge, the increased distance and increased shielding by inner shells result in a larger atomic radius [1 mark].

(f) Chlorine has a greater nuclear charge (17 protons) than sodium (11 protons) [½ mark]. Both have electrons in 3 shells, so shielding is similar. The stronger nuclear attraction in chlorine pulls the electrons closer to the nucleus, resulting in a smaller atomic radius [½ mark].

Question 15 (10 marks)

(a) Across Period 3, the nature of oxides changes from basic (Na₂O, MgO) to amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂). [1 mark]

(b) Sodium is a metal, and metal oxides are basic because they react with acids to form salt and water / contain O²⁻ ions that can accept H⁺ ions [1 mark]. Sulfur is a non-metal, and non-metal oxides are acidic because they react with water to form acidic solutions / react with bases to form salts / contain covalent bonds that react with water to release H⁺ ions [1 mark].

(c) Amphoteric means the oxide can react with both acids and bases / can behave as both an acid and a base [1 mark].
Equation: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l) [2 marks — 1 for correct formulae, 1 for correct balancing and state symbols]

(d) Silicon dioxide has a giant covalent / giant molecular structure [1 mark]. It consists of a three-dimensional network of silicon and oxygen atoms held together by strong covalent bonds throughout the lattice. A large amount of energy is required to break these strong covalent bonds, resulting in a very high melting point [1 mark].

(e) Phosphorus(V) oxide (P₄O₁₀) has a simple molecular structure with weak intermolecular forces of attraction between molecules [1 mark]. Only a small amount of energy is required to overcome these weak forces, resulting in a low melting point. Silicon dioxide has a giant covalent structure with strong covalent bonds requiring much more energy to break [1 mark].

Section C: Data-Based Questions (10 marks)

Question 16 (10 marks)

(a) Atomic radius decreases across Period 2 from lithium to neon. [1 mark]

(b) Across Period 2, the number of protons / nuclear charge increases [1 mark]. Electrons are added to the same electron shell, so shielding remains approximately constant. The increased nuclear charge exerts a stronger attractive force on the electrons, pulling them closer to the nucleus, resulting in a smaller atomic radius [1 mark].

(c) Neon has 2 electron shells (electronic structure 2,8) while sodium has 3 electron shells (electronic structure 2,8,1) [1 mark]. The addition of a new electron shell in sodium means the outermost electrons are further from the nucleus, resulting in a much larger atomic radius [1 mark].

(d) Argon (proton number 18) has the smallest atomic radius in Period 3 [1 mark]. Across Period 3, atomic radius decreases as nuclear charge increases while electrons are added to the same shell. Argon, being the last element in Period 3, has the highest nuclear charge and therefore the strongest attraction on its electrons, giving it the smallest radius [1 mark].

(e) Potassium has 4 electron shells while sodium has 3 electron shells [1 mark]. The outermost electron in potassium is in the fourth shell, further from the nucleus than the outermost electron in sodium (third shell). Despite potassium having a greater nuclear charge, the increased distance and increased shielding by inner shells result in a larger atomic radius [1 mark].

(f) Chlorine has a greater nuclear charge (17 protons) than sodium (11 protons) [½ mark]. Both have electrons in 3 shells, so shielding is similar. The stronger nuclear attraction in chlorine pulls the electrons closer to the nucleus, resulting in a smaller atomic radius [½ mark].

Question 17 (10 marks)

(a) Across Period 3, the nature of oxides changes from basic (Na₂O, MgO) to amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂). [1 mark]

(b) Sodium is a metal, and metal oxides are basic because they react with acids to form salt and water / contain O²⁻ ions that can accept H⁺ ions [1 mark]. Sulfur is a non-metal, and non-metal oxides are acidic because they react with water to form acidic solutions / react with bases to form salts / contain covalent bonds that react with water to release H⁺ ions [1 mark].

(c) Amphoteric means the oxide can react with both acids and bases / can behave as both an acid and a base [1 mark].
Equation: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l) [2 marks — 1 for correct formulae, 1 for correct balancing and state symbols]

(d) Silicon dioxide has a giant covalent / giant molecular structure [1 mark]. It consists of a three-dimensional network of silicon and oxygen atoms held together by strong covalent bonds throughout the lattice. A large amount of energy is required to break these strong covalent bonds, resulting in a very high melting point [1 mark].

(e) Phosphorus(V) oxide (P₄O₁₀) has a simple molecular structure with weak intermolecular forces of attraction between molecules [1 mark]. Only a small amount of energy is required to overcome these weak forces, resulting in a low melting point. Silicon dioxide has a giant covalent structure with strong covalent bonds requiring much more energy to break [1 mark].

Question 18 (6 marks)

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

(b) Silicon has a giant covalent / giant molecular structure [1 mark]. It consists of a three-dimensional network of silicon atoms held together by strong covalent bonds [1 mark]. A large amount of energy is required to break these strong covalent bonds, resulting in a high melting point. Phosphorus exists as simple P₄ molecules with weak intermolecular forces of attraction between molecules. Only a small amount of energy is required to overcome these weak forces, resulting in a low melting point [1 mark].

(c) Across Period 3, the elements change from metals (Na, Mg, Al) to metalloid (Si) to non-metals (P, S, Cl) [1 mark]. Metals have delocalised electrons that can move freely and carry charge. Non-metals do not have delocalised electrons / have electrons held in fixed positions in covalent bonds or molecules, so they cannot conduct electricity [1 mark].

Question 19 (6 marks)

(a) 2,8,8,1 [1 mark]

(b) Group: 1 [½ mark]
Period: 4 [½ mark]
Explanation: Potassium has 4 electron shells, so it is in Period 4 [½ mark]. It has 1 valence electron, so it is in Group 1 [½ mark].

(c) Observations: Potassium floats on water / moves rapidly on the water surface [½ mark]; it melts into a silvery ball [½ mark]; it burns with a lilac flame [½ mark]; effervescence / gas bubbles produced [½ mark]; potassium becomes smaller and eventually disappears [½ mark]. (Any 3 observations for 1½ marks)
Equation: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) [1½ marks — ½ for correct formulae, ½ for balancing, ½ for state symbols]

Question 20 (8 marks)

(a) Element W belongs to Group 16 (or Group 6) [1 mark]. It has 6 outer/valence electrons, and the group number for non-metals is determined by the number of valence electrons [1 mark].

(b) Element W is oxygen [1 mark]. Electronic structure: 2,6 [1 mark].

(c) Dot-and-cross diagram of H₂O:

• Oxygen atom with 6 outer electrons (e.g., as dots)
• Two hydrogen atoms each with 1 outer electron (e.g., as crosses)
• Two shared pairs between O and each H (O–H bonds)
• Two lone pairs on oxygen [2 marks — 1 for correct electron arrangement, 1 for correct bonding pairs and lone pairs]

(d) The oxide of oxygen is water (H₂O), which is neutral [1 mark]. Non-metal oxides are generally acidic, but water is an exception / water has a pH of 7 and does not react with acids or bases [1 mark].
Alternative answer if W is interpreted as sulfur: The oxide of sulfur (SO₂, SO₃) is acidic because sulfur is a non-metal, and non-metal oxides react with water to form acidic solutions / react with bases to form salts [2 marks].

END OF ANSWER KEY