Topic 2 Practice Paper: Atoms, elements and compounds — mark scheme (print)

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QQ/IGCSE-CHEM/T2

IGCSE · Cambridge IGCSE Chemistry (Extended (Core + Supplement))

Mark scheme

Atoms, elements and compounds

Topic 2 Practice Paper · Maximum mark: 30

Notes for markers

Award one mark for each point marked [1], up to the marks shown for the part.
Accept answers worded differently that show the same understanding.
Units are required where the answer line shows one. ecf: allow an error carried forward.

Section A: Multiple-choice questions[10 marks]

Question	Answer
1	BBrass is a mixture of copper and zinc, so its parts can be separated by physical means.	1
2	AAn electron carries a single negative charge, while a neutron is neutral and a proton is positive.	1
3	CThe neutron sits in the nucleus alongside the proton but carries no charge.	1
4	BThere are seven electrons in the outer shell, so the element belongs in Group VII of the Periodic Table.	1
5	DIsotopes share the same proton number but differ in neutrons, so the atom with seventeen protons and nineteen neutrons is the isotope.	1
6	AIn the solid the ions are locked in the lattice, but melting frees them to move and carry the current.	1
7	CEach oxygen atom needs two electrons, so the two atoms share two pairs, forming a double bond.	1
8	BLittle energy is needed to overcome the attractions between separate molecules, so the substance boils at a low temperature.	1
9	DGraphite forms layers of hexagonal rings, and one electron per carbon is delocalised and free to move along the layers.	1
10	ABecause the rows of ions can move without breaking the metallic bond, the metal is malleable and ductile.	1

Section B: Structured questions[20 marks]

Question	Answer and marking points	Marks
11(a)	2,4 [1]	1
11(b)	Both particles have 6 protons, so they are the same element, but they have different numbers of neutrons (6 and 8). same number of protons [1] different number of neutrons [1]	2
11(c)	A charge of 1+. 1+ / single positive charge [1]	1
	Total for Question 11	4
12(a)	A sodium atom loses one electron from its outer shell to form a 1+ ion, and a chlorine atom gains one electron to complete its outer shell and form a 1− ion. sodium loses one electron / forms a 1+ ion [1] chlorine gains one electron / forms a 1− ion [1]	2
12(b)	It has a giant ionic lattice containing many strong electrostatic attractions between oppositely charged ions, and all of these must be overcome to melt it. giant lattice of ions [1] many strong electrostatic attractions must be overcome [1]	2
12(c)	Its ions are fixed in the lattice and cannot move to carry charge. ions cannot move in the solid [1]	1
	Total for Question 12	5
13(a)	The two atoms share a pair of electrons, which gives both of them a full outer shell. shared pair of electrons [1] each atom gains a full / noble-gas outer shell [1]	2
13(b)	The forces between separate molecules are weak, so little energy is needed to separate the molecules from one another. weak forces between molecules [1] little energy needed to overcome them [1]	2
13(c)	No, because it contains molecules rather than free-moving ions or electrons. no, and a reason about molecules / no free ions [1]	1
	Total for Question 13	5
14(a)	Similarity: both are giant covalent networks of carbon atoms joined by strong covalent bonds. Difference: in diamond each carbon atom is bonded to four others in a three-dimensional network, while in graphite each carbon atom is bonded to only three others, forming layers. both are giant covalent networks with strong covalent bonds between carbon atoms [1] diamond has a three-dimensional network of four bonds per atom while graphite has layers [1]	2
14(b)	Each carbon atom has one delocalised electron that is free to move along the layers, so graphite conducts electricity. delocalised electrons [1] free to move so charge can flow [1]	2
14(c)	Positive copper ions in the giant lattice are attracted to a sea of delocalised electrons, and the layers of ions can slide over one another without breaking that attraction. positive ions in a sea of delocalised electrons [1] layers of ions slide over one another [1]	2
	Total for Question 14	6

— End of mark scheme —