Chapter 5: The Halogens and Noble Gases – Chemistry 2nd Year Notes

This chapter covers The Halogens and Noble Gases from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). The halogens of Group VIIA (F, Cl, Br, I, At) have the configuration ns2 np5 and are the most reactive non-metals, while the noble gases of Group 0 (He, Ne, Ar, Kr, Xe, Rn) have completely filled shells and are very unreactive.

Here you will learn the properties and trends of the halogens, the acid strength of the hydrogen halides, the bleaching action of chlorine, why noble gases are inert, and how xenon still forms compounds.

Learning Objectives

• List the halogens and noble gases with their electronic configurations.
• Explain why halogens are strong oxidising agents and how reactivity changes down the group.
• Describe the trend in acid strength of the hydrogen halides.
• Explain the bleaching and germicidal action of chlorine.
• Explain why noble gases are unreactive yet xenon can form compounds.
• State the uses of the halogens and noble gases.

Key Concepts

The Halogens

The halogens exist as diatomic molecules (F2, Cl2, Br2, I2). At room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid, so melting and boiling points increase down the group. Because each atom needs just one more electron to complete its octet, halogens are the most reactive non-metals and strong oxidising agents. Going down the group, size increases and electronegativity decreases, so oxidising power falls in the order F greater than Cl greater than Br greater than I. A higher halogen can therefore displace a lower one, for example Cl2 + 2KBr gives 2KCl + Br2.

Hydrogen Halides

The halogens form hydrogen halides (HF, HCl, HBr, HI) which dissolve in water to give acids. Their acid strength increases down the group (HF less than HCl less than HBr less than HI) because the H to X bond becomes weaker and easier to break as the halogen gets larger. HF is the weakest acid because of its strong bond and hydrogen bonding, while HI is the strongest.

Chlorine, Bleaching and Water Treatment

With water chlorine forms hydrochloric and hypochlorous acids; the hypochlorous acid releases nascent oxygen, which bleaches coloured materials and kills germs. This is why chlorine and bleaching powder are used to bleach cloth and paper and to disinfect drinking water and swimming pools.

The Noble Gases and Xenon Compounds

The noble gases have completely filled outer shells, giving very high ionization energies and almost zero electron affinity, so they are chemically inert and exist as single atoms. For a long time they were thought to form no compounds. However, xenon, the largest common noble gas with a low ionization energy, reacts with the strongly electronegative fluorine to form real compounds: xenon difluoride (XeF2), xenon tetrafluoride (XeF4) and xenon hexafluoride (XeF6). Helium, neon and argon remain inert.

Important Definitions

Formulas & Rules

Item	Fact
Halogens	ns2 np5, diatomic, strong oxidisers; power F greater than Cl greater than Br greater than I
States	F2, Cl2 gas; Br2 liquid; I2 solid
Acid strength of HX	HF less than HCl less than HBr less than HI
Displacement	Cl2 + 2KBr gives 2KCl + Br2
Xenon compounds	XeF2, XeF4, XeF6

Diagrams & Illustrations

Position of the halogens and noble gases: a periodic-table schematic showing Group VIIA (halogens, ns2 np5) and Group 0 (noble gases, filled shells) on the far right.

Halogen trends: graphs showing electronegativity and oxidising power decreasing from fluorine to iodine.

Noble gas uses: a chart of uses (helium in balloons and diving gas, neon in signs, argon in bulbs and welding, krypton and xenon in lamps) with a note that xenon forms XeF2, XeF4 and XeF6.

Solved Examples & Numericals

Example 1: Displacement

Will bromine displace chlorine from sodium chloride? No. Chlorine is a stronger oxidising agent than bromine, so bromine cannot displace it; the reverse happens.

Example 2: Acid strength

Arrange HF, HCl, HBr and HI in order of increasing acid strength. HF less than HCl less than HBr less than HI, because down the group the H to X bond weakens and breaks more easily in water.

Example 3: Xenon

Why can xenon form compounds while helium cannot? Xenon is much larger with a low ionization energy, so its outer electrons can bond with fluorine; helium holds its two electrons too tightly.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the general characteristics and trends of the halogens.

The halogens are the non-metals of Group VIIA, with the outer configuration ns2 np5. They exist as diatomic molecules and, because each atom needs only one more electron to complete its octet, they are the most reactive non-metals and powerful oxidising agents. Going down the group, atomic size increases and electronegativity decreases, so reactivity and oxidising power fall in the order F greater than Cl greater than Br greater than I, and a higher halogen can displace a lower one from its salt. Their physical states change from gases (F2, Cl2) through a liquid (Br2) to a solid (I2) as melting and boiling points rise. The hydrogen halides they form are acids whose strength increases down the group. Because of their reactivity, halogens are never found free in nature.

Q2: Why are noble gases inert, and how does xenon form compounds?

The noble gases are inert because their outermost shells are completely filled (helium 1s2, the others ns2 np6). This gives very high ionization energies and almost zero electron affinity, so they neither lose, gain nor share electrons easily and exist as single atoms. For years they were believed to form no compounds. However, xenon is the largest common noble gas and its outer electrons are far from the nucleus with a low ionization energy; when combined with the extremely electronegative fluorine, xenon forms real compounds such as XeF2, XeF4 and XeF6. The lighter noble gases hold their electrons too tightly and remain inert.

Q3: Discuss the uses of the halogens and noble gases.

Fluorine compounds are used in toothpaste, non-stick coatings and refrigerants; chlorine bleaches cloth and paper, disinfects water and makes PVC and bleaching powder; bromine is used in photography and flame retardants; and iodine is an antiseptic. The noble gases are valued for their inertness and light: helium fills balloons and is used in diving gas and as a coolant, neon glows in advertising signs, argon provides an inert atmosphere in bulbs and welding, and krypton and xenon are used in high-power and flash lamps.

MCQs with Answers

Quick Revision Summary

• Halogens (F,Cl,Br,I,At) = ns2 np5, diatomic, strongest non-metals and oxidisers.
• Oxidising power and electronegativity decrease down: F greater than Cl greater than Br greater than I.
• States: F, Cl gas; Br liquid; I solid. Acid strength HF less than HCl less than HBr less than HI.
• Chlorine bleaches and disinfects via nascent oxygen from hypochlorous acid.
• Noble gases have filled shells, are inert and monatomic.
• Xenon forms XeF2, XeF4, XeF6; He, Ne, Ar stay inert.

Exam Tips

• Remember the trend directions: reactivity and oxidising power decrease down halogens, but HX acid strength increases down.
• HF is the weakest acid, HI the strongest; a common MCQ.
• Learn one displacement equation to prove the oxidising-power order.
• Explain noble-gas inertness with completely filled shell and high ionization energy.
• Only xenon forms fluorides; He, Ne, Ar do not.