Lesson Notes By Weeks and Term v4 - SHS 2

Lesson Video

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Performance objectives

• Describe properties of hydrides, oxides, hydroxides, and chlorides of period 3 elements.
• Predict how these compounds react with water and air.
• Discuss thermal stability of some carbonates and nitrates.
• Write balanced chemical equations for these reactions.

Lesson summary

Welcome, students! Today, we are exploring one of the most beautiful and predictive ideas in Chemistry: Periodicity. The Periodic Table is not just a catalogue of elements; it is a map that reveals fascinating patterns. We will focus on the Period 3 elements (from Sodium to Argon) because they show a perfect transition from metals on the left to non-metals on the right. By studying their common compounds—like the oxides (think of sand or rust), chlorides (like our common table salt), and hydrides—we can understand and even predict how they will behave.

Lesson notes

The foundation for understanding the trends in Period 3 is the change in fundamental properties of the elements themselves as you move from left (Na) to right (Cl): Metallic Character: Decreases (Na, Mg, Al are metals; Si is a metalloid; P, S, Cl are non-metals). Electronegativity: Increases (The ability to attract bonding electrons increases). Atomic Radius: Decreases.

This change in electronegativity is the main reason for the trends we will observe. The larger the difference in electronegativity between the Period 3 element and oxygen, hydrogen, or chlorine, the more ionic the bond will be. A. Hydrides of Period 3 (NaH, MgH₂, AlH₃, SiH₄, PH₃, H₂S, HCl)

| Compound | Formula | Bonding & Structure | Acid-Base Character | Reaction with Water | | :--- | :--- | :--- | :--- | :--- | | Sodium Hydride | NaH | Ionic Lattice. Large electronegativity difference between Na (0.9) and H (2.1). Contains H⁻ (hydride) ions. | Strongly Basic. The H⁻ ion is a very strong base. | Reacts vigorously: NaH(s) + H₂O(l) → NaOH(aq) + H₂(g) | | Magnesium Hydride | MgH₂ | Ionic Lattice. Similar to NaH. | Basic. | Reacts with water: MgH₂(s) + 2H₂O(l) → Mg(OH)₂(aq) + 2H₂(g) | | Aluminium Hydride | (AlH₃)n | Polymeric Covalent. Electronegativity difference is smaller. | Neutral | Reacts slowly with water. | | Silane | SiH₄ | Simple Covalent Molecule. Tetrahedral shape. Non-polar. | Neutral. | Does not react with pure water. | | Phosphine | PH₃ | Simple Covalent Molecule. Trigonal pyramidal shape. Weakly polar. | Very Weakly Basic. Can accept a proton to form PH₄⁺. | Insoluble in water. | | Hydrogen Sulphide | H₂S | Simple Covalent Molecule. Bent shape. Polar. | Weakly Acidic. Dissolves in water to form a weak acidic solution. H₂S ⇌ H⁺ + HS⁻ | Dissolves to form hydro-sulphuric acid. | | Hydrogen Chloride | HCl | Simple Covalent Molecule. Linear. Highly polar. | Strongly Acidic. | Dissolves readily and ionizes completely to form hydrochloric acid. HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq) |

Key Trend: The bonding changes from ionic to covalent. The acid-base character changes from basic to neutral to acidic.

Evaluation guide

• Describe the patterns in physical and chemical properties of compound (hydrides,
• oxides, hydroxides and chlorides) of period 3 elements .
• Talk for Learning Approach: Using think- pair-share approach,
• predict and describe the physical properties of the compounds of the period three elements.