CIE iGCSE Co-Ordinated Science C2.4 Ions and ionic bonds Exam Style Questions Paper 4
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C2.2 — Atomic structure (Part (a) & (b))
• Topic C2.3 — Isotopes (Part (c))
• Topic C2.5 — Covalent bonds / Dot-and-cross diagrams (Part (d))
• Topic C2.4 — Ionic bonds / C2.5 — Covalent bonds / Intermolecular forces (Part (e))
▶️ Answer/Explanation
(a) Table 6.1 completed:
Electrons have a relative charge of -1 and negligible mass; neutrons are neutral; protons have a charge of +1 and a relative mass of 1.
(b) 10 electrons and 12 neutrons.
Sodium has 11 protons (atomic number 11). The ion Na⁺ has lost one electron, so it has 10 electrons. The mass number 23 means 23 − 11 = 12 neutrons.
(c) \( ^{35}_{18}\text{Cl} \) is not an isotope of chlorine because it has 18 protons instead of 17.
Isotopes of the same element have the same proton number but different neutron numbers. The symbol with 18 protons is actually an isotope of argon, not chlorine.
(d) Dot-and-cross diagram for Cl₂: Two chlorine atoms each contribute one unpaired electron to form a shared pair (covalent bond). Each chlorine atom has three lone pairs of electrons around it.
Cl : Cl (with each Cl having three additional pairs of dots/crosses)
Each chlorine atom has 7 outer electrons; they share one pair to achieve a full outer shell of 8 electrons (stable octet).
(e) Chlorine is covalent with weak intermolecular forces between molecules, so little energy is needed to separate them, making it a gas.
Sodium chloride is ionic with strong electrostatic forces between oppositely charged ions throughout a giant lattice, requiring much more energy to overcome, making it a solid.
In covalent chlorine, the molecules are held together by weak van der Waals forces. In ionic sodium chloride, strong ionic bonds hold the lattice together, giving it a high melting point.
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C2.2 — Atomic structure and the Periodic Table (Part (a)(i), (a)(ii), (a)(iii), (c)(i), (c)(ii))
• Topic C2.4 — Ions and ionic bonds (Part (b))
• Topic C2.6 — Giant covalent structures (Part (c)(iii))
• Topic C4.1 — Electrolysis (Part (c)(iv))
▶️ Answer/Explanation
(a)(i) Ar (argon).
Argon (Ar) is a noble gas with the electronic configuration 2,8,8. It has 8 electrons in its outer shell (a full outer shell), making it chemically stable and unreactive.
(a)(ii) Na, K, or Al (sodium, potassium, or aluminium).
Sodium (Na), potassium (K), and aluminium (Al) are metals. Metals tend to lose electrons from their outer shells to form positive ions (cations). Sodium loses 1 electron to form Na+, potassium loses 1 to form K+, and aluminium loses 3 to form Al3+.
(a)(iii) Al (aluminium).
The electronic configuration 2,8,3 means there are 13 electrons in total. The element with 13 electrons is aluminium (Al), which has the atomic number 13.
(b)
Sodium chloride is an ionic compound. Sodium (Na) has 1 electron in its outer shell and chlorine (Cl) has 7 electrons in its outer shell. Sodium transfers its single outer electron to chlorine. Sodium becomes a positive ion (Na+) with a full outer shell (2,8), and chlorine becomes a negative ion (Cl−) with a full outer shell (2,8,8). The dot-and-cross diagram shows the electron transfer with crosses representing sodium’s electron and dots representing chlorine’s electrons.
(c)(i) Carbon has 2 occupied electron shells.
The period number of an element in the Periodic Table corresponds to the number of occupied electron shells. Fig. 6.2 shows a carbon atom with 2 electron shells (the inner shell and the outer shell). Therefore, carbon is in Period 2.
(c)(ii) Both isotopes have the same electronic configuration.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Carbon-12 has 6 protons and 6 neutrons, while carbon-13 has 6 protons and 7 neutrons. Chemical properties are determined by the number and arrangement of electrons in the atom. Since both isotopes have the same number of electrons (6) and the same electronic configuration (2,4), they have identical chemical properties.
(c)(iii) Graphite has electrons that can move.
Graphite has a giant covalent structure where each carbon atom is bonded to three other carbon atoms in a hexagonal ring structure. This leaves one delocalised electron per carbon atom that is not used in bonding. These delocalised electrons are free to move throughout the structure, allowing graphite to conduct electricity.
(c)(iv) Anode: oxygen.
Cathode: copper.
During the electrolysis of aqueous copper(II) sulfate using graphite (inert) electrodes:
• At the anode (positive electrode), hydroxide ions (OH−) from the water are discharged. They lose electrons and form oxygen gas: 4OH− → O2 + 2H2O + 4e−.
• At the cathode (negative electrode), copper(II) ions (Cu2+) are discharged. They gain electrons and form copper metal: Cu2+ + 2e− → Cu.