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a) Identify the following compounds metallic, ionic or covalent.

• (i) Sulphuric acid (H₂SO₄) Sulphuric acid is composed of hydrogen, sulfur, and oxygen, all of which are non-metals. Bonds formed between non-metal atoms through electron sharing are covalent.

  • Covalent

• (ii) Potassium ion (K⁺) Potassium is an alkali metal. The K⁺ ion is formed when a potassium atom loses an electron. Ions are the constituent particles of ionic compounds, which are formed by electrostatic attraction between cations and anions.

  • Ionic

• (iii) Calcium carbonate (Ca(SO₄)) The chemical formula given, Ca(SO₄), corresponds to Calcium Sulfate, not Calcium Carbonate. Assuming the compound is Calcium Sulfate (CaSO₄), Calcium (Ca) is a metal, and the Sulfate ion (SO₄²⁻) is a polyatomic anion. The bond between a metal cation and a polyatomic anion is ionic.

  • Ionic

b) Draw the dot and cross Lewis structure for the following compounds.

• (i) Ca(SO₄) Calcium sulfate is an ionic compound. It consists of a calcium cation (Ca²⁺) and a sulfate anion (SO₄²⁻). Calcium loses two valence electrons to become Ca²⁺. The sulfate ion contains covalent bonds between sulfur and oxygen atoms, with an overall 2- charge.

  $$\text{Ca}^{2+} \quad \begin{bmatrix} \quad \quad \cdot \cdot \\ \cdot \cdot \text{O} \underset{\times}{\overset{\times}{\text{S}}} \text{O} \cdot \cdot \\ \quad \quad \cdot \cdot \\ \quad \quad \cdot \cdot \\ \quad \quad \text{O} \\ \quad \quad \cdot \cdot \end{bmatrix}^{2-}$$

  (Note: In the sulfate ion, sulfur's valence electrons are represented by crosses, and oxygen's valence electrons and the two extra electrons for the 2- charge are represented by dots. Each oxygen atom has three lone pairs, and sulfur forms four single bonds with the oxygen atoms.)

• (ii) NH₄(Cl) Ammonium chloride is an ionic compound. It consists of an ammonium cation (NH₄⁺) and a chloride anion (Cl⁻). The ammonium ion has covalent bonds between nitrogen and hydrogen atoms, with an overall +1 charge. The chloride ion is a chlorine atom that has gained an electron, resulting in a -1 charge.

  $$\begin{bmatrix} \quad \text{H} \\ \text{H} \underset{\cdot}{\overset{\cdot}{\text{N}}} \text{H} \\ \quad \text{H} \end{bmatrix}^{+} \quad \begin{bmatrix} \cdot \cdot \\ \times \text{Cl} \times \\ \cdot \cdot \end{bmatrix}^{-}$$

  (Note: In the ammonium ion, nitrogen's valence electrons are represented by dots, and hydrogen's valence electrons are represented by crosses. One electron is removed for the +1 charge. In the chloride ion, chlorine's original valence electrons are dots, and the gained electron is a cross, completing its octet.)

c) Ammonia reacts with boron-trifluoride to form ammonia-boron-trifluoride. Use dot and cross to draw the molecular structure of the atoms to show the bonding of the following.

• (i) Ammonia (NH₃) Ammonia is a covalent molecule. Nitrogen shares electrons with three hydrogen atoms and has one lone pair of electrons.

  $$\begin{array}{c} \text{H} \\ \times \\ \text{H} \underset{\cdot}{\overset{\cdot}{\text{N}}} \cdot \text{H} \\ \end{array}$$

  (Note: Nitrogen's valence electrons are shown as dots, and hydrogen's valence electrons are shown as crosses.)

• (ii) Boron-trifluoride (BF₃) Boron trifluoride is a covalent molecule. Boron shares electrons with three fluorine atoms. Boron typically has an incomplete octet in BF₃.

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