CIE iGCSE Co-Ordinated Science C2.4 Ions and ionic bonds Exam Style Questions Paper 1
▶️ Answer/Explanation
Magnesium hydroxide is an ionic compound consisting of magnesium ions (\( \text{Mg}^{2+} \)) and hydroxide ions (\( \text{OH}^- \)).
To balance the charges:
- Magnesium has a charge of \( 2+ \) (it loses two electrons to achieve a noble gas configuration).
- The hydroxide ion has a charge of \( 1- \) (it consists of one oxygen atom and one hydrogen atom with a single negative charge).
To form a neutral compound, the total positive charge must equal the total negative charge. Therefore, two hydroxide ions (\( 2 \times 1- = 2- \)) are needed to balance one magnesium ion (\( 2+ \)).
This gives the formula \( \text{Mg(OH)}_2 \). The brackets are used to show that there are two hydroxide groups. This formula also correctly shows that there are twice as many oxygen and hydrogen atoms than magnesium atoms (1 Mg : 2 O : 2 H).
Option A (MgOH) would have equal numbers of atoms, not twice as many. Option B (MgO\(_2\)H\(_2\)) shows the correct ratio but is incorrectly written — the correct notation is \( \text{Mg(OH)}_2 \). Option D (Mg\(_2\)(OH)) has twice as many magnesium atoms.
✅ Answer: (C)
▶️ Answer/Explanation
Sodium chloride is an ionic compound formed between a metal (sodium, Na) and a non-metal (chlorine, Cl). Sodium is in Group I and has one electron in its outer shell. Chlorine is in Group VII and has seven electrons in its outer shell.
During bond formation, sodium transfers its one outer-shell electron to chlorine. This gives sodium a full outer shell (becoming a Na\(^+\) ion with a \(2,8\) configuration) and gives chlorine a full outer shell (becoming a Cl\(^-\) ion with a \(2,8,8\) configuration). The electrostatic attraction between the oppositely charged ions forms the ionic bond.
In a dot-and-cross diagram:
- Sodium (Na) loses its outer-shell electron, so its outer shell is shown as empty (or with only the inner shells).
- Chlorine (Cl) gains the electron, so its outer shell is shown with a full complement of eight electrons.
- The transferred electron from sodium is typically shown as a cross (×) to distinguish it from the electrons originally belonging to chlorine (dots).
Diagram B correctly shows sodium with an empty outer shell after losing its electron, and chlorine with a full outer shell after gaining the electron. The other diagrams incorrectly show electron sharing (covalent bonding, not ionic) or incorrect electron arrangements.
✅ Answer: (B)
▶️ Answer/Explanation
✅ Answer: (D)
▶️ Answer/Explanation
For a compound to conduct electricity when molten, it must contain ions that are free to move. Ionic compounds conduct electricity when molten because the ions are free to move and carry charge. Covalent compounds do not conduct electricity because they have no free ions or electrons.
Looking at each option:
- A. carbon and hydrogen — These are both non-metals, so they form covalent compounds (e.g., methane, \( \text{CH}_4 \)). Covalent compounds do not conduct electricity when molten because they have no free ions.
- B. lithium and bromine — Lithium is a metal (Group I) and bromine is a non-metal (Group VII). They form an ionic compound (lithium bromide, LiBr). Ionic compounds conduct when molten.
- C. magnesium and chlorine — Magnesium is a metal (Group II) and chlorine is a non-metal (Group VII). They form an ionic compound (magnesium chloride, \( \text{MgCl}_2 \)). Ionic compounds conduct when molten.
- D. potassium and iodine — Potassium is a metal (Group I) and iodine is a non-metal (Group VII). They form an ionic compound (potassium iodide, KI). Ionic compounds conduct when molten.
Therefore, only the compound formed by carbon and hydrogen (covalent) does not conduct electricity when molten.
✅ Answer: (A)
▶️ Answer/Explanation
✅ Answer: (C)