Ch 3 · Group VII Properties · IGCSE Chemistry 0620
Topic 3 of 5 in The Periodic Table — IGCSE Chemistry 0620 — 48 lessons.
Introduction
The halogens of Group VII are the most reactive non-metals and the mirror image of Group I: the alkali metals get more reactive down the group, the halogens less reactive. Displacement reactions between halogens and halide solutions are a standard practical and a reliable source of marks for observations, equations and, for Extended candidates, an explanation in terms of electrons.
Key ideas — The halogens and their trends
Chlorine, bromine and iodine are non-metals that exist as diatomic molecules: Cl_2, Br_2 and I_2. Each atom has seven outer-shell electrons and gains one to form a halide ion, X^-, or shares one in a covalent bond.
🧑🏫 Teacher's explanation
Imagine a group of people trying to grab a single ball from a table. The person closest to the ball (the top of the group) has a very easy time reaching it because they are small and agile. As you move further away (down the group), the people get much larger and bulkier; they can still grab the ball, but it takes much more effort and they are less 'reactive' or quick to react.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass (M_r) of bromine, Br_2, given A_r(Br) = 80.
🔍 Worked example — Standard
Predict the products and state of observation when chlorine gas is added to a solution of potassium bromide, KBr_(aq).
🔍 Worked example — Exam-style
A sample of iodine (I_2) has a mass of 0.50 g. Calculate the number of moles present. (A_r(I) = 127).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Displacement reactions
A more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide. Adding chlorine water to potassium bromide solution turns the solution orange because bromine is formed; adding it to potassium iodide gives a brown solution of iodine. Bromine displaces iodide but cannot displace chloride, and iodine displaces neither.
🧑🏫 Teacher's explanation
Think of a displacement reaction like a game of musical chairs where the most 'aggressive' or 'hungry' person takes the seat from someone less aggressive. In Group VII, the halogens are non-metals that are desperate to gain one electron to become stable. The 'hungrier' the halogen, the more it wants that electron.
📖 Definitions
🔍 Worked example — Easy
Predict the products and state the observation when chlorine gas is bubbled through a solution of potassium bromide.
🔍 Worked example — Standard
Write the ionic equation for the reaction between bromine and potassium iodide.
🔍 Worked example — Exam-style
A student adds iodine to a solution of sodium chloride. What is the result and why?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State whether a reaction occurs. [1] (b) If a reaction occurs, write the balanced chemical equation for the reaction. [2] (c) Explain your answer to part (a) in terms of reactivity. [2] [5 marks]
Key ideas — Explaining the trend in reactivity
🧑🏫 Teacher's explanation
Imagine you are trying to catch a ball (the electron) being thrown toward you. If you are standing right next to the person throwing it, you can grab it easily. But if you move further away, or if there are several people standing between you and the thrower, it becomes much harder to catch that ball.
📖 Definitions
🔍 Worked example — Easy
State the physical state and color of Chlorine (Cl_2) and Iodine (I_2) at room temperature.
🔍 Worked example — Standard
Write a balanced equation for the reaction between Chlorine (Cl_2) and Potassium Bromide (KBr) in aqueous solution.
🔍 Worked example — Exam-style
Explain why the reactivity of halogens decreases as you move from Chlorine to Iodine.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State whether a reaction occurs. [1] (b) If a reaction occurs, write the balanced chemical equation for the reaction. [2] (c) Explain, in terms of electron attraction, why bromine is less reactive than chlorine. [3] [6 marks]
Key ideas — Predicting the properties of other halogens
Fluorine sits above chlorine and astatine below iodine. Extending the trends:
🧑🏫 Teacher's explanation
Think of Group VII elements like a group of people trying to win a game of 'Capture the Electron'. The halogens are non-metals that are very hungry for one extra electron to achieve a stable, full outer shell.
📖 Definitions
🔍 Worked example — Easy
Predict the state and color of the elements above Fluorine and below Iodine in Group VII.
🔍 Worked example — Standard
Write a balanced equation for the reaction between Chlorine gas and Potassium Bromide solution.
🔍 Worked example — Exam-style
A solution of Sodium Iodide (NaI) is reacted with Chlorine gas (Cl_2). State the observations and write the balanced equation.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State whether a reaction occurs. [1] (b) Explain your answer to part (a) in terms of reactivity. [1] (c) Write a balanced chemical equation for the reaction if it occurs. [2] [4 marks]
Worked example
Question. A student adds bromine water to separate solutions of potassium chloride and potassium iodide.
🔍 Worked example — Easy
Identify the state and color of Chlorine, Bromine, and Iodine at room temperature.
🔍 Worked example — Standard
A solution of Potassium Chloride (KCl) is reacted with Chlorine gas (Cl_2). Predict the observation.
🔍 Worked example — Exam-style
Write the balanced ionic equation for the reaction between Bromine (Br_2) and Potassium Iodide (KI).
⚠️ Common errors
Common mistakes
Practice
Answers Bromine is a red-brown liquid [B1]; iodine is a grey-black solid [B1]. Density increases down the group [B1]; colour becomes more intense or darker down the group [B1]. Solution turns brown [B1]; Cl_2 + 2NaI arrow 2NaCl + I_2 [M1 formulae, A1 balanced]. Solid [B1]; iodine would displace astatine because iodine is more reactive than astatine [B1]. Chlorine atoms are smaller so the outer shell is closer to the nucleus [B1]; there is less shielding by inner shells [B1]; so an electron is attracted and gained more easily [B1].