Quick Notes

Quick Notes

Chemistry Chapter 4: Periodic Table of Elements
4.5 Elements in Group 17
4.5 Elements in Group 17
CHEMISTRY • Form 4 • Chapter 4: The Periodic Table of Elements

4.5Elements in Group 17

Group 17 elements are known as halogens. They exist as diatomic molecules and have seven valence electrons. Discover their physical trends, chemical reactions and reactivity changes down the group.

Learning Objectives

  • Generalise the changes in the physical properties of elements when going down Group 17.
  • Summarise the chemical properties of Group 17 elements.
  • Generalise the changes in the reactivity of elements when going down Group 17.
  • Reason out the physical and chemical properties of other elements in Group 17.

Trend of Physical Properties and Reactivity Down Group 17

Observe how molecular size, physical state, colour and reactivity change from chlorine to iodine.

Brief Explanation

Understand the key ideas about Group 17 halogens.

Halogens are Group 17 elements that have seven valence electrons. In chemical reactions, a halogen atom receives one electron to form a halide ion with a −1 charge: \( \text{X} + e^- \rightarrow \text{X}^- \).

Physical Properties

Down Group 17, the molecular size increases. This strengthens the intermolecular forces, so melting points, boiling points and density increase. The physical state changes from gas to liquid to solid.

Chemical Properties

All halogens react with water to form acidic solutions, with metals to form metal halides, and with alkalis to form metal halides, metal halates and water.

With water

Chlorine reacts with water to produce hydrochloric acid and hypochlorous acid:

\( \mathrm{Cl_2(g) + H_2O(l) \rightleftharpoons HCl(aq) + HOCl(aq)} \)

With metals

Iron reacts with bromine to produce iron(III) bromide:

\( \mathrm{2Fe(s) + 3Br_2(l) \rightarrow 2FeBr_3(s)} \)

With alkalis

Iodine reacts with sodium hydroxide to produce sodium iodide, sodium iodate(I) and water:

\( \mathrm{I_2(s) + 2NaOH(aq) \rightarrow NaI(aq) + NaOI(aq) + H_2O(l)} \)

Halogen Physical state at room temperature Colour Melting point (°C) Boiling point (°C)
Chlorine, Cl₂ Gas Greenish yellow −101 −34
Bromine, Br₂ Liquid Reddish brown −7 59
Iodine, I₂ Solid Purplish black 114 184

Try to Answer First

Answer mentally, then check the explanation.

1

Why do the melting and boiling points of halogens increase down Group 17?

Answer: Going down the group, the molecular size increases, causing the attraction force between molecules (intermolecular forces) to become stronger. Therefore, more heat energy is required to overcome these forces.
2

What is the colour and physical state of bromine at room temperature?

Answer: Bromine is a reddish-brown liquid at room temperature.
3

Explain why chlorine is more reactive than bromine.

Answer: The atomic size of chlorine is smaller than bromine, so its valence shell is closer to the nucleus. The nuclear attraction force towards electrons is stronger, making it easier for chlorine to attract an electron to achieve a stable octet arrangement.

Common Mistakes

Avoid these errors when studying Group 17 halogens.

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Confusing intermolecular forces with covalent bonds: stating that covalent bonds break during melting or boiling instead of weak intermolecular forces between molecules.
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Confusing the reactivity trend of Group 1 with Group 17: thinking reactivity increases down Group 17, whereas reactivity actually decreases down Group 17 because non-metals need to attract, not lose, electrons.
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Writing halogen elements as monoatomic atoms in chemical equations, for example writing Cl instead of Cl₂ or Br instead of Br₂.
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Writing incorrect oxidation states or formulas when halogens react with iron, for example writing FeBr₂ instead of FeBr₃.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Group 17 halogens lose electrons to achieve an octet state.

Correct Fact

Halogens have 7 valence electrons, so they gain/receive 1 electron to form an anion with a −1 charge (X⁻).

Misconception

The darkening colour down the group is because of metallic properties.

Correct Fact

Halogens remain non-metals that do not conduct heat or electricity; colour depth is a characteristic physical trait related to their molecular structure.

Misconception

All halogens react with equal vigour with iron.

Correct Fact

Reactivity decreases down the group; chlorine reacts most vigorously with iron, followed by bromine, and iodine reacts least vigorously.

Summary

Key facts to remember.

  • Group 17 consists of non-metals known as halogens (F₂, Cl₂, Br₂, I₂, At, Ts) that exist as diatomic molecules.
  • Down the group, molecular size increases, strengthening intermolecular forces and leading to increases in melting points, boiling points and densities.
  • The physical state changes from gas (Cl₂) to liquid (Br₂) to solid (I₂).
  • All halogens react with water, metals and alkalis to form characteristic products.
  • Reactivity decreases down the group because a larger atomic radius weakens the nucleus's ability to attract an incoming valence electron.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following statements correctly explains why the reactivity of halogens decreases down Group 17?

2

What are the products formed when iodine reacts with sodium hydroxide solution?

B. Fill in the Blanks

3

At room temperature, bromine exists as a reddish-brown _______.

4

When chlorine gas dissolves in water, it forms hydrochloric acid and _______ acid.

C. Match the Halogen with Its Physical Colour / Description

Drag each halogen on desktop. On mobile, tap a halogen and then tap the matching box.

Answer Choices

Fluorine Chlorine Bromine Iodine

Physical Colour / Description

1Light-yellow poisonous gas
2Greenish-yellow gas
3Reddish-brown liquid
4Purplish-black solid
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Halogen Diatomic molecule Intermolecular forces Metal halide Hypochlorous acid Electron attraction