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: 11
Grade code: 2.2.1.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 2.2.1.CS.2
Indicator code: 2.2.1.LI.2
Theme: SYSTEMATIC CHEMISTRY OF THE ELEMENTS
Subtheme: PERIODICITY
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This lesson focuses on Group 17 of the Periodic Table – the Halogens. While we know them as elements, their compounds are all around us in Ghana. From the common salt (sodium chloride) we use for cooking, sourced from places like Ada Foah, to the bleach (like Parazone) used in our homes for cleaning, and the chlorine that the Ghana Water Company Limited uses to make our tap water safe to drink. In this lesson, we will explore the fascinating chemistry that explains why these elements and their compounds behave the way they do. We will investigate how their properties change as we move down the group, a key concept in periodicity.
A. The Halides and Their Reducing Power Halogens: The elements in Group 17 of the periodic table (Fluorine, Chlorine, Bromine, Iodine). Halide Ions (X⁻): When a halogen atom gains one electron, it forms a negative ion called a halide ion (e.g., F⁻, Cl⁻, Br⁻, I⁻). Reducing Agent: A substance that donates electrons in a redox reaction and gets oxidized in the process.
The main trend to remember is that the reducing power of the halide ions increases down the group.
Trend: I⁻ > Br⁻ > Cl⁻ > F⁻ (Strongest reducing agent) > (Weakest reducing agent)
Explanation: Atomic/Ionic Radius: As you go down Group 17, each element has an additional electron shell. This means the ionic radius increases (I⁻ is the largest, F⁻ is the smallest). Electron Shielding: The outermost electron in a larger ion (like I⁻) is further away from the positively charged nucleus. It is also "shielded" from the nucleus's pull by more inner electron shells. Ease of Electron Loss: Because the outermost electron is held less tightly in larger ions, it is easier to remove or donate. Conclusion: Since a reducing agent's job is to donate electrons, the ion that can donate its electron most easily is the strongest reducing agent. Therefore, the iodide ion (I⁻) is the strongest reducing agent among the halides.