Chemical Bonding, Formulas & Chemical Equations
Ionic (electrovalent) bonding, covalent bonding, metallic bonding, writing chemical formulas using valency, balancing chemical equations, and physical properties of bonded substances.
In Ghana, the extraction of gold at Obuasi and Tarkwa relies on sodium cyanide (an ionic compound) to dissolve gold, while aluminum smelting at VALCO (Tema) harnesses the electrical conductivity of molten cryolite (Na₃AlF₆) to produce metallic aluminum.
Key Syllabus Learning Objectives
- 1Explain ionic, covalent, and metallic bonding using dot-and-cross Lewis diagrams
- 2Compare the physical properties of ionic, simple molecular, giant covalent, and metallic substances
- 3Write chemical formulas of binary compounds and radical compounds using valency rules
- 4Balance chemical equations and state symbols (s, l, g, aq) accurately
Ionic vs Covalent Bonding: Formation & Dot-and-Cross Diagrams
- Formation of NaCl: Sodium (2,8,1) transfers 1 electron to Chlorine (2,8,7). Forms Na⁺ [2,8] and Cl⁻ [2,8,8]. Both attain noble gas structures held by strong ionic attraction.
- Formation of H₂O: Oxygen (2,6) shares two pairs of electrons with two Hydrogen atoms (1), forming two single covalent O-H bonds with two lone pairs on oxygen.
- Double and Triple Bonds: O₂ shares two pairs (O=O); N₂ shares three pairs (N≡N), giving molecular nitrogen immense chemical stability.
Table salt (NaCl) mined at the Ada Songor lagoon forms hard crystalline cubes because of the rigid alternating 3D ionic lattice of Na⁺ and Cl⁻ ions.
Metals lose electrons to form cations; non-metals gain electrons to form anions in ionic bonds. Non-metals share electrons in covalent bonds.
Balancing Chemical Equations & Radicals
- Common Polyatomic Radicals: Ammonium (NH₄⁺), Hydroxide (OH⁻), Nitrate (NO₃⁻), Carbonate (CO₃²⁻), Sulfate (SO₄²⁻), Phosphate (PO₄³⁻).
- Rules for balancing: Never change subscripts in a chemical formula; only alter stoichiometric balancing coefficients in front of formulas.
- Always inspect and balance elements in order: Metals first, then non-metals, then hydrogen, and oxygen last.
When baking soda (NaHCO₃) is heated in bread making in bakeries, it decomposes into sodium carbonate, water vapor, and carbon dioxide gas, causing dough to rise.
Chemical coefficients balance atom counts; chemical subscripts define molecular identities and must never be altered.
- In dot-and-cross diagrams, use dots (•) for electrons of one atom and crosses (×) for the other to distinguish their origins clearly.
- Always include state symbols [ (s), (l), (g), (aq) ] when requested by WAEC questions; omitting them costs A1 marks.
- Remember that graphite conducts electricity because each carbon atom forms only 3 covalent bonds, leaving 1 delocalized electron per carbon atom.
- Attempting to balance equations by changing chemical formula subscripts (e.g. changing H₂O to H₂O₂).
- Stating that solid NaCl conducts electricity (in solids, ions are held rigidly in the lattice and cannot move).
- Forgetting parentheses when multiplying polyatomic radicals (e.g. writing CaOH₂ instead of Ca(OH)₂).
- Can I draw dot-and-cross diagrams for NaCl, MgO, H₂O, and CH₄?
- Can I write the correct formula for Aluminum Sulfate [Al₂(SO₄)₃] using valency?
- Can I balance the combustion equation of propane: C₃H₈ + O₂ → CO₂ + H₂O?
- Can I explain why diamond does not conduct electricity whereas graphite does?
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