Lesson Notes By Weeks and Term v4 - SHS 2
PERIODICITY
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PERIODICITY
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Subject: Chemistry
Class: SHS 2
Term: 2nd Term
Week: 8
Grade code: 2.2.1.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 2.2.1.CS.1
Indicator code: 2.2.1.LI.2
Theme: SYSTEMATIC CHEMISTRY OF THE ELEMENTS
Subtheme: PERIODICITY
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Welcome, learners! Today, we are embarking on a journey across Period 3 of the Periodic Table, from Sodium (Na) to Argon (Ar). We will not be looking at the elements themselves, but at the families of compounds they form: their hydrides, oxides, hydroxides, and chlorides. Understanding the trends in these compounds is fundamental to chemistry. It allows us to predict how substances will behave without having to memorise thousands of individual facts. In Ghana, these compounds are all around us.
Foundation: The Trend Across Period 3 Before we look at the compounds, let's remember the trend in the elements themselves: Na, Mg, Al, Si, P, S, Cl, Ar. Metallic Character: Decreases from left to right. Na, Mg, Al are metals. Si is a metalloid. P, S, Cl are non-metals. Electronegativity: Increases from left to right. This is the ability of an atom to attract bonding electrons. Chlorine is very electronegative; Sodium is not.
This change from metallic to non-metallic character, driven by increasing electronegativity, is the master key to understanding all the trends we will discuss. A. Hydrides of Period 3 (NaH to HCl)
These are compounds of Period 3 elements with hydrogen.
| Hydride | Element Type | Bonding | Structure | Reaction with Water | Acid-Base Character | | :--- | :--- | :--- | :--- | :--- | :--- | | NaH, MgH₂ | Metal | Ionic | Giant Ionic Lattice | Reacts vigorously to form H₂ gas and the metal hydroxide. `NaH(s) + H₂O(l) → NaOH(aq) + H₂(g)` | Basic (H⁻ ion is a strong proton acceptor) | | AlH₃ | Metal | Covalent | Polymeric | Reacts with water. | Amphoteric | | SiH₄, PH₃ | Metalloid/Non-metal | Covalent | Simple Molecular | SiH₄ is stable in water. PH₃ is sparingly soluble. | SiH₄ (Neutral), PH₃ (Weakly Basic) | | H₂S, HCl | Non-metal | Covalent | Simple Molecular | Dissolve and ionize in water to form acidic solutions. `HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)` | Weakly Acidic (H₂S), Strongly Acidic (HCl) |