Class: Senior Secondary School 2 (SS2 / SSS2)
Term: Second Term
Week: 6
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Elements and Periodicity
Previous Lesson: Oxygen: Meaning, Preparation, Properties and Uses.
Topic: HALOGENS
Subject Matter: Meaning of halogens (group VII elements that form salts), electronic configuration of halogens, physical properties of halogens (gradation down the group), chemical properties of halogens (gradation down the group)

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define halogens and state their position in the periodic table.
Describe the general electronic configuration of halogens.
Explain the trends in physical properties of halogens down the group.
Discuss the trends in chemical properties of halogens down the group.

Affective Domain:

Appreciate the importance of halogens in everyday life.
Recognize the need for safety precautions when handling halogens in the laboratory.
Show willingness to participate in group discussions about halogen properties.

Psychomotor Domain:

Write the electronic configurations for selected halogens (e.g., F, Cl, Br).
Identify apparatus used for the laboratory preparation of chlorine.
Observe and describe physical and chemical properties of halogens through simple experiments.

Collaborate with peers to discuss the trends observed in halogen properties.
Share findings and observations related to halogen characteristics.

Reference Materials

The following resources were used in planning this lesson:

Senior Secondary Schools Education Curriculum
State Unified Scheme of Work
New School Chemistry for Senior Secondary Schools by Osei Yaw Ababio
https://www.britannica.com/science/halogen-element
https://byjus.com/chemistry/halogens/

Instructional Materials

The teacher will teach this lesson with the aid of:

Periodic table
Chart/table showing physical properties and similarities among halogens
Red litmus paper
Blue litmus paper
Samples of halogens (if available and safe, e.g., iodine crystals, bromine water)
Apparatus for laboratory preparation of chlorine (e.g., round-bottom flask, dropping funnel, delivery tube, gas jar, manganese (IV) oxide, concentrated hydrochloric acid)

Rationale for the Lesson

This lesson helps pupils understand the characteristics of halogens, which are important elements with various applications in daily life and industries. It enables pupils to grasp the concept of periodicity and how properties change within a group in the periodic table.

Prerequisite/Previous Knowledge

Pupils should have prior knowledge of the periodic table, groups and periods, atomic structure, valency, and basic chemical bonding.

Lesson Content/Board Summary

HALOGENS

Meaning of Halogens

Halogens are the elements found in Group 17 (or VII) of the periodic table. The word “halogen” means “salt-forming” because these elements readily react with metals to form salts. They are highly reactive non-metals.

The halogens include:

Fluorine (F)
Chlorine (Cl)
Bromine (Br)
Iodine (I)
Astatine (At) – a radioactive element

Electronic Configuration of Halogens

Halogens have seven electrons in their outermost shell, giving them a general electronic configuration of ns²np⁵. This electron configuration makes them highly electronegative and strong oxidizing agents, as they readily gain one electron to achieve a stable noble gas configuration.

Examples:

Fluorine (F): 1s²2s²2p⁵
Chlorine (Cl): 1s²2s²2p⁶3s²3p⁵
Bromine (Br): [Ar]3d¹⁰4s²4p⁵
Iodine (I): [Kr]4d¹⁰5s²5p⁵

Physical Properties of Halogens

The physical properties of halogens show a clear gradation down the group:

State at Room Temperature: Fluorine (gas), Chlorine (gas), Bromine (liquid), Iodine (solid).
Colour: Fluorine (pale yellow), Chlorine (yellowish-green), Bromine (reddish-brown), Iodine (greyish-black solid, purple vapour).
Melting and Boiling Points: Increase down the group due to increasing Van der Waals forces with increasing atomic size.
Density: Increases down the group.
Solubility in Water: Sparingly soluble in water but dissolve readily in organic solvents like CCl₄.
Electrical Conductivity: They are poor conductors of electricity.

Chemical Properties of Halogens

Halogens are very reactive elements due to their strong tendency to gain an electron. Their reactivity generally decreases down the group.

The following are chemical properties of halogens:

Oxidizing Power: Halogens are strong oxidizing agents. Their oxidizing power decreases down the group (Fluorine > Chlorine > Bromine > Iodine). A more reactive halogen can displace a less reactive halide ion from its solution.
- Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
- Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)
Reaction with Metals: They react with most metals to form metal halides (salts).
- 2Na(s) + Cl₂(g) → 2NaCl(s)
- 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)
Reaction with Hydrogen: They react with hydrogen to form hydrogen halides.
- H₂(g) + F₂(g) → 2HF(g) (explosive in dark)
- H₂(g) + Cl₂(g) → 2HCl(g) (explosive in sunlight)
- H₂(g) + Br₂(g) → 2HBr(g) (requires heating)
- H₂(g) + I₂(g) → 2HI(g) (reversible, requires strong heating)
Reaction with Water: Chlorine and bromine react with water to form a mixture of hydrohalic acid and hypohalous acid.
- Cl₂(aq) + H₂O(l) ⇌ HCl(aq) + HOCl(aq)
Bleaching Action: Chlorine and bromine water act as bleaching agents due to the formation of hypohalous acid, which releases nascent oxygen.
- HOCl → HCl + [O] (nascent oxygen)

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks them to recall their knowledge of the periodic table, specifically naming elements in Group 17.
Pupils’ Activity: Pupils respond to the teacher’s questions by naming elements in Group 17.
Learning Point: Pupils activate prior knowledge of the periodic table and elements.

Step 2: Meaning and Electronic Configuration of Halogens

Time: 7 minutes
Teaching Skill: Explanation/Questioning
Teacher’s Activity: The teacher defines halogens, explains their position in the periodic table, and guides pupils to write the general and specific electronic configurations for fluorine and chlorine.
Pupils’ Activity: Pupils listen, define halogens, and write electronic configurations in their notes.
Learning Point: Pupils understand the definition and electronic structure of halogens.

Step 3: Physical Properties of Halogens

Time: 8 minutes
Teaching Skill: Explanation/Observation
Teacher’s Activity: The teacher uses a chart to illustrate the physical properties of halogens (state, colour, melting/boiling points, density) and explains the trends down the group.
Pupils’ Activity: Pupils observe the chart, note the trends, and ask questions for clarification.
Learning Point: Pupils understand how physical properties vary down the halogen group.

Step 4: Chemical Properties of Halogens

Time: 8 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains the chemical properties of halogens, focusing on their oxidizing power, reactivity with metals and hydrogen, and bleaching action, providing relevant examples.
Pupils’ Activity: Pupils listen, copy notes, and discuss the reactivity trends.
Learning Point: Pupils grasp the chemical behaviour and reactivity trends of halogens.

Step 5: Demonstration and Discussion

Time: 7 minutes
Teaching Skill: Demonstration/Discussion
Teacher’s Activity: The teacher demonstrates the effect of chlorine water on red and blue litmus paper and discusses the laboratory preparation of chlorine, emphasizing safety rules. If actual preparation is not feasible, the teacher describes the process with diagrams.
Pupils’ Activity: Pupils observe the demonstration, describe the changes in litmus paper, and discuss the implications of the observations and safety rules.
Learning Point: Pupils observe practical aspects of halogen properties and understand safety in the lab.

Step 6: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define halogens and state their general electronic configuration.
List three physical properties of halogens and describe their trend down the group.
Explain why halogens are strong oxidizing agents.
Write a balanced chemical equation for the reaction between chlorine gas and potassium bromide solution.

Pupils’ Activity: Pupils answer orally and in writing.
Learning Point: Pupils demonstrate understanding of the lesson.

Step 7: Conclusion

Time: 5 minutes
Teaching Skill: Summarization/Homework Assignment
Teacher’s Activity: The teacher summarizes the key points of the lesson on halogens and assigns homework for further practice.
Pupils’ Activity: Pupils listen to the summary and copy down the homework.
Learning Point: Pupils consolidate their learning and prepare for independent study.

Lesson Keywords

Halogens – Elements in Group 17 of the periodic table, known as salt-formers.
Electronic Configuration – The arrangement of electrons in an atom’s orbitals, for halogens, it is ns²np⁵.
Oxidizing Agent – A substance that accepts electrons from another substance.
Gradation – A gradual change in properties down a group in the periodic table.
Displacement Reaction – A reaction where a more reactive element displaces a less reactive element from its compound.

Differentiation

For struggling learners, the teacher will provide simplified notes and focus on identifying halogens and their most common properties. Advanced learners will be encouraged to research specific uses of different halogens and present their findings to the class.

Note for teachers using this lesson plan

Ensure that safety precautions are strictly followed when discussing or demonstrating experiments involving halogens, especially with chlorine and bromine, due to their toxic and corrosive nature. Use visual aids like the periodic table and property charts to enhance understanding. Encourage pupils to participate actively and ask questions.