Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Halogens
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Halogens
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 2
Theme: Chemistry And Environment
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Halogens are elements in Group 17 of the periodic table. The word "halogen" means "salt-former" because they readily react with metals to form salts. They are highly reactive non-metals and exist as diatomic molecules (F2, Cl2, Br2, I2) in their elemental state.
All halogens possess seven valence electrons, specifically an electron configuration of ns2np5 in their outermost electron shell. This configuration signifies their strong tendency to gain one electron to achieve a stable octet (ns2np6), mimicking the electron configuration of noble gases. This electron affinity is responsible for their high electronegativity and strong oxidizing capabilities.
Chlorine (Cl): Atomic number (Z) = 17 Full electronic configuration: 1s2 2s2 2p6 3s2 3p5 Shell arrangement: 2, 8, 7 Valence electrons: 7 (in the 3rd principal energy level)
Bromine (Br): Atomic number (Z) = 35 Full electronic configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 Condensed configuration: [Ar] 3d10 4s2 4p5 Shell arrangement: 2, 8, 18, 7 Valence electrons: 7 (in the 4th principal energy level)
Iodine (I): Atomic number (Z) = 53 Full electronic configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 Condensed configuration: [Kr] 4d10 5s2 5p5 Shell arrangement: 2, 8, 18, 18, 7 Valence electrons: 7 (in the 5th principal energy level)
Explanation of Reactivity: The presence of seven valence electrons (one electron short of a stable octet) drives their reactivity. They readily accept an electron to form a uninegative ion (X−), or share electrons by forming a single covalent bond. Halogens have seven valence electrons, specifically an electron configuration of ns2np5 in their outermost shell. This configuration makes them highly reactive as they tend to gain one electron to achieve a stable octet (ns2np6), resembling the noble gases.
Chlorine (Cl): Atomic number (Z) = 17 Electronic configuration: 1s2 2s2 2p6 3s2 3p5 Shell configuration: 2, 8, 7 Valence electrons: 7 (in the 3s and 3p orbitals)
Bromine (Br): Atomic number (Z) = 35 Electronic configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 (or [Ar] 3d10 4s2 4p5)
Shell configuration: 2, 8, 18, 7 Valence electrons: 7 (in the 4s and 4p orbitals)
Iodine (I): Atomic number (Z) = 53 Electronic configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 (or [Kr] 4d10 5s2 5p5)
Shell configuration: 2, 8, 18, 18, 7 Valence electrons: 7 (in the 5s and 5p orbitals)
Note: The tendency to gain an electron to achieve a stable octet explains their high electronegativity and oxidizing power. The physical properties of halogens exhibit clear trends as one descends Group
1
7. These trends are primarily influenced by increasing atomic size and molecular mass, which in turn affect intermolecular forces. | Property | Fluorine (F2) | Chlorine (Cl2) | Bromine (Br2) | Iodine (I2) | Gradation/Trend (Explanation) | | :--------------------- | :----------------------- | :----------------------- | :----------------------- | :----------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Physical State (at R.T.) | Gas (pale yellow) | Gas (greenish-yellow) | Liquid (reddish-brown) | Solid (greyish-black) | Increases in intermolecular (van der Waals) forces: As molecular mass and size increase down the group, the strength of van der Waals forces between molecules increases, requiring more energy to separate them, hence the change in state. | | Colour | Pale yellow | Greenish-yellow | Reddish-brown | Greyish-black (sublimes to purple vapour) | Deepens: This is due to the larger electron clouds down the group, which become more easily excited. They absorb lower energy (longer wavelength) light, leading to the transmission of higher energy (shorter wavelength) light, which gives the observed colours. | | Melting Point (°C) | -219.6 | -101.5 | -7.2 | 113.7 | Increases: Stronger van der Waals forces require more thermal energy to overcome, leading to higher melting points. | | Boiling Point (°C) | -188.1 | -34.0 | 58.8 | 184.3 | Increases: Same reason as melting point. | | Density (g/cm3 at R.T.) | 1.69 g/L (gas) | 3.21 g/L (gas) | 3.10 g/cm3 (liquid) | 4.93 g/cm3 (solid) | Increases: As atomic mass increases, the mass per unit volume increases. | | Solubility in Water| Reacts vigorously (forms HF, O2) | Sparingly soluble (forms HCl, HClO) | Sparingly soluble (forms HBr, HBrO) | Very sparingly soluble | Decreases: Fluorine reacts violently. Chlorine and bromine are slightly soluble, forming oxyacids. Iodine is almost insoluble. All halogens are significantly more soluble in non-polar organic solvents (e.g., carbon tetrachloride, hexane) because they are non-polar molecules. | | Property | Fluorine (F2) | Chlorine (Cl2) | Bromine (Br2) | Iodine (I2) | Trend Down the Group (F to I) | | :--------------------- | :----------------- | :----------------- | :----------------- | :----------------- | :------------------------------------------------------------------------------------------------------------------------------ | | Physical State | Gas | Gas | Liquid | Solid | Changes from gas to liquid to solid. This is due to increasing strength of intermolecular (van der Waals) forces with increasing atomic size/molecular mass. | | Colour | Pale yellow | Greenish-yellow | Reddish-brown | Greyish-black | Colour deepens. This is due to the absorption of light at lower energies as the electron cloud becomes larger and more diffuse, leading to different wavelengths of light being reflected. | | Odour | Pungent | Pungent, suffocating | Pungent, irritating | Pungent | All have pungent, irritating odours. | | Solubility in water| Reacts vigorously | Slightly soluble (reacts to form HCl and HClO) | Slightly soluble | Sparingly soluble (reacts slowly) | Decreases slightly, but all are more soluble in organic solvents (e.g., CCl4, hexane, ethanol) due to their non-polar nature. | | Melting Point (°C) | -220 | -101 | -7.2 | 113.7 | Increases. Higher molecular mass leads to stronger van der Waals forces, requiring more energy to overcome them. | | Boiling Point (°C) | -188 | -34 | 58.8 | 184 | Increases. Same reason as melting point. | | Density (g/cm3) | 1.7 (g/L) | 3.2 (g/L) | 3.1 (g/cm3) | 4.93 (g/cm3) | Increases. Due to increasing atomic mass and stronger intermolecular forces leading to closer packing of molecules. | Summary of Gradation: Physical State: Gas → Liquid → Solid Colour: Pale yellow → Greenish-yellow → Reddish-brown → Greyish-black (deepens)
Melting and Boiling Points: Increase Density: Increases Solubility: Generally sparingly soluble in water but more soluble in non-polar organic solvents.
Water Purification and Public Health (Nigeria): Application: Chlorine is the primary chemical used by municipal water treatment plants (e.g., those managed by state water corporations like the Lagos Water Corporation or Enugu State Water Corporation) to disinfect public drinking water supplies across Nigeria. It effectively kills bacteria, viruses, and other microorganisms that cause waterborne diseases such as cholera, typhoid, and dysentery.
Integration: Students can relate this directly to the safety of the water they drink daily. Teachers can ask students to observe if their local water has a faint "chlorine smell" (indicating disinfection) and discuss the importance of water treatment in preventing epidemics, especially during rainy seasons when water sources are more prone to contamination. This links to public health, environmental sanitation, and local governance. Goitre Prevention and Nutritional Health (Nigeria): Application: Iodine, in the form of potassium iodide or iodate, is routinely added to table salt in Nigeria. This "iodized salt" is a crucial public health intervention to combat iodine deficiency disorders (IDD), the most visible symptom of which is goitre (enlargement of the thyroid gland).
Integration: Students can be encouraged to check the packaging of salt used at home for the "iodized" label or the NAFDAC registration number, which often indicates adherence to food standards. This application connects chemistry to nutrition, health policy, and the economic impact of disease prevention on productivity in communities. Industrial Production and Economic Development (Nigeria): Application: Halogens are vital in the chemical industry for manufacturing a range of products essential to modern life and economic development. For example, chlorine is a key raw material for polyvinyl chloride (PVC) production. PVC is extensively used in Nigeria for plumbing pipes, electrical insulation, window frames, and flooring in the construction industry.
Integration: The teacher can prompt students to identify items in their homes, schools, or communities made of PVC (e.g., plastic chairs, water pipes, electrical cables). This highlights how chemistry provides the foundational materials for infrastructure and contributes to employment and industrial growth in Nigeria, linking to vocational training and economic geography. ---