Sulphur, Halogens and Noble Gases – NDA Chemistry Notes

This chapter covers three groups of elements: sulphur, the halogens (fluorine, chlorine, bromine, iodine), and the noble gases (helium, neon, argon, krypton, xenon, radon). These elements and their compounds appear regularly in NDA papers.

The chapter rewards students who know the specific properties of each element, know the trends within the halogen group, and know the uses of key compounds.

1. Sulphur

Sulphur is a yellow, solid non-metal. It is odourless in its pure form. It does not conduct electricity.

Sulphur burns in air to produce sulphur dioxide gas (SO₂). [NDA 2011-I] When sulphur is heated in a spatula and the gas collected, the gas X is SO₂, sulphur dioxide. SO₂ turns moist litmus red. It is an acidic oxide. [NDA 2011-I]

Sulphur is used in hair-setting chemicals in beauty parlours. [NDA 2009-I] Hair contains keratin protein, which has sulphur-containing amino acids. Permanent wave (perm) solutions break and reform sulphur-sulphur (disulphide) bonds in hair protein to set the shape.

Sulphur is used in vulcanisation of rubber.  In vulcanisation, sulphur is added to rubber and heated. Sulphur forms cross-links between rubber polymer chains. This makes rubber stronger, more elastic, and more durable. Car tyres are made of vulcanised rubber.

Allotropes of Sulphur

Sulphur exists in several allotropic forms. The two most important for the exam are rhombic sulphur and monoclinic sulphur.

Property	Rhombic Sulphur	Monoclinic Sulphur
Shape	Rhombic (octahedral) crystals	Needle-shaped crystals
Colour	Yellow	Pale yellow
Stability	Stable below 96°C	Stable above 96°C
Also called	α-sulphur	β-sulphur
Melting point	113°C	119°C

The transition temperature between the two forms is 96°C. Below 96°C, rhombic sulphur is the stable form. Above 96°C, monoclinic sulphur is stable.

Both allotropes contain S₈ molecules, eight sulphur atoms arranged in a puckered ring.

When molten sulphur is poured into cold water, a rubber-like amorphous form called plastic sulphur is formed. Plastic sulphur is not a true allotrope.

Sulphur Dioxide (SO₂)

Sulphur dioxide is a colourless gas with a sharp, pungent smell. It is produced when sulphur burns in air.

S + O₂ → SO₂

Properties of SO₂:

• Acidic oxide: dissolves in water to form sulphurous acid (H₂SO₃): SO₂ + H₂O → H₂SO₃
• Turns moist blue litmus red [NDA 2011-I]
• Bleaching agent: bleaches coloured organic matter by reduction
• Acts as both oxidising agent and reducing agent

SO₂ is a major air pollutant. It dissolves in rain to form sulphurous and sulphuric acids, contributing to acid rain.

CFC gases cause ozone depletion: chlorofluorocarbons release chlorine radicals that destroy the ozone layer. SO₂ → acid rain | Lead compounds → harmful to nervous system | CO₂ → greenhouse effect.

Sulphuric Acid (H₂SO₄)

Sulphuric acid is one of the most important industrial chemicals in the world. It is a colourless, oily, dense liquid.

Manufacture (Contact Process):

Step 1: S + O₂ → SO₂ (sulphur burned in air). Step 2: 2SO₂ + O₂ → 2SO₃ (SO₂ oxidised to SO₃, over vanadium pentoxide V₂O₅ catalyst at 450°C). Step 3: SO₃ + H₂SO₄ → H₂S₂O₇ (oleum, SO₃ absorbed in concentrated H₂SO₄). Step 4: H₂S₂O₇ + H₂O → 2H₂SO₄ (oleum diluted with water to give H₂SO₄).

SO₃ is NOT directly dissolved in water because it produces a mist of fine H₂SO₄ droplets that are difficult to handle. Instead, SO₃ is first absorbed in concentrated H₂SO₄ to form oleum, then diluted.

Oleum (Fuming Sulphuric Acid): Oleum = H₂SO₄ + dissolved SO₃. Also written as H₂S₂O₇ (pyrosulphuric acid).

Property	Detail
Colour	Colourless, oily liquid
Density	Very dense, about 1.84 g/cm³
Hygroscopic	Absorbs water from surroundings
Dehydrating agent	Removes water from substances: charcoal reacts with conc. H₂SO₄
Oxidising agent	Strong oxidising agent when hot and concentrated
Dilution	ALWAYS add acid to water; NEVER water to acid (generates enormous heat)

Sulphuric acid is used in explosives.  Sulphuric acid is used in paper manufacturing for degumming.

Equivalent weight of H₂SO₄ = 98 ÷ 2 = 49. [NDA 2012-II]

Hydrogen Sulphide (H₂S)

Hydrogen sulphide is a colourless gas with the characteristic smell of rotten eggs.

• Produced when sulphur compounds decompose: rotten eggs, sewage
• Slightly soluble in water, forms a weakly acidic solution
• Reducing agent
• Toxic gas
• Used as a laboratory reagent for precipitating metal sulphides

2. Halogens (Group 17)

Halogens are a group of non-metallic elements in Group 17 (Group VIIA) of the periodic table. The word “halogen” means “salt former”. They react with metals to form salts.

Members: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At).

General Electronic Configuration: ns²np⁵, one electron short of a noble gas configuration. This is why halogens are highly reactive. They strongly want to gain one electron.

Reactivity Order

Halogens become less reactive going down the group. This is directly tested in NDA.

Increasing order of reactivity (least to most reactive): Iodine < Bromine < Chlorine < Fluorine. [NDA 2006-II]

Fluorine is the most reactive of all halogens, and the most electronegative element in the entire periodic table.

Iodine is the least reactive halogen (among the common four).

Reason for the trend: Going down the group, atomic size increases. The nucleus is farther from the outermost electrons. The attraction for an incoming electron decreases. So reactivity decreases.

Physical States at Room Temperature

Halogen	Symbol	State at Room Temp	Colour
Fluorine	F₂	Gas	Pale yellow
Chlorine	Cl₂	Gas	Greenish-yellow
Bromine	Br₂	Liquid	Reddish-brown
Iodine	I₂	Solid	Dark grey/violet, lustrous

Bromine and mercury are the only two elements that are liquid at room temperature.

Iodine is the only non-metal that is lustrous (shiny).

Iodine sublimes, converting directly from solid to gas when heated. [NDA 2010-II]

Fluorine (F)

Fluorine is the most electronegative element in the entire periodic table. It is the most reactive of all halogens. It is a pale yellow gas.

Fluoride and dental health: Deficiency of fluorine causes dental caries (tooth decay). [NDA 2008-II] Excess fluorine causes fluorosis, discolouration and damage of teeth and bones.

Fluoride is added to drinking water and toothpaste in small amounts to prevent tooth decay. This is called water fluoridation.

Chlorine (Cl)

Chlorine is a greenish-yellow gas with a sharp, irritating smell. It is highly toxic. It is denser than air.

Key properties:

• Strong oxidising agent
• Bleaching action: bleaches coloured matter by oxidation (via nascent oxygen)
• Dissolves in water: Cl₂ + H₂O → HCl + HOCl (hypochlorous acid)
• HOCl releases nascent oxygen. This is the bleaching agent
• Disinfects water: kills bacteria

Bleaching Powder: Made by passing chlorine gas over slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Bleaching powder formula: CaOCl₂ (calcium oxychloride). Used to bleach cotton, linen, paper; disinfect drinking water and swimming pools.

Chlorine and ozone depletion: Chlorofluorocarbons (CFCs), used in refrigerants and aerosols, release chlorine radicals in the stratosphere. Chlorine radicals destroy ozone in a chain reaction.

Disproportionation of chlorine compounds: Some chlorine oxyanions can undergo disproportionation, where the same element is both oxidised and reduced simultaneously. ClO⁻, ClO₂⁻, ClO₃⁻ can show disproportionation. ClO₄⁻ (perchlorate) CANNOT. It has chlorine in its highest oxidation state (+7) and cannot be further oxidised. [NDA 2016-II]

Tincture of Iodine: Tincture of iodine is a dilute solution of elemental iodine used as an antiseptic for wounds. It does NOT contain acetone.

Bromine (Br)

Bromine is the only non-metallic element that is liquid at room temperature. It is a reddish-brown liquid with an unpleasant smell.

Bromine vapour is toxic and irritating.

Bromine is less reactive than chlorine but more reactive than iodine.

Iodine (I)

Iodine is a dark grey solid that sublimes to give a violet vapour. It is the least reactive of the common halogens.

Iodine and the thyroid gland: Iodine is essential for the production of thyroid hormones. Deficiency of iodine causes goitre, enlargement of the thyroid gland.

Table salt is iodised: potassium iodide (KI) is added to common salt, to prevent iodine deficiency in the population. Iodised salt is a MIXTURE of KI and NaCl. [NDA 2010-II]

Iodine test for starch: Iodine turns starch blue-black. This is the standard laboratory test for starch.

Iodine as antiseptic: Iodine solution is used as an antiseptic.

Tincture of iodine is a dilute solution of iodine in alcohol. It is used to disinfect fresh wounds. It does not contain acetone.

3. Noble Gases (Group 18)

Noble gases are in Group 18 (Group VIII or Group 0) of the periodic table. They are also called inert gases because they are chemically almost completely unreactive.

Members: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn).

Why are noble gases inert? Noble gases have completely filled outermost electron shells: He has 2 electrons (full), others have 8 electrons in outermost shell (ns²np⁶). This filled configuration is extremely stable. They do not need to gain, lose, or share electrons. So they do not react. [NDA 2008-I]

Valency of noble gases = 0. They do not combine with other elements under normal conditions. [NDA 2016-II], Radon has zero valency.

Properties and Uses:

Noble Gas	Symbol	Key Property / Use
Helium	He	Lightest noble gas
Neon	Ne	Used in neon signs (glows orange-red when electricity passed through)
Argon	Ar	Most abundant noble gas in atmosphere (about 0.93%)
Krypton	Kr	Used in special photographic flash lamps
Xenon	Xe	Used in powerful lighthouse lamps and flash photography
Radon	Rn	Radioactive

Which noble gas is NOT found in the atmosphere? Radon is NOT found in the atmosphere in significant amounts. [NDA 2007-I] It is produced by radioactive decay of radium and seeps out of the earth’s crust. The other noble gases, argon, krypton, xenon, are all present in the atmosphere.

Neon signs: When electricity is passed through neon gas in a sealed tube, neon glows with a characteristic orange-red light. Different noble gases produce different colours when electrically excited.

Helium for airships: Helium is preferred over hydrogen for airships because helium is chemically less reactive. [NDA 2006-I] (Covered in more detail in Chapter 12.)

Scuba diving: Scuba diving tanks contain air diluted with helium. At high pressure underwater, nitrogen in normal air dissolves into blood and causes nitrogen narcosis. Helium is less soluble and less narcotic, so it replaces nitrogen in diving mixtures.

Valence electronic configuration of noble gases = ns²np⁶ (except He = 1s²). [NDA 2008-I]

4. Fluorosis vs Dental Caries

Condition	Cause	Effect
Dental caries (tooth decay)	Deficiency of fluorine	Teeth decay easily
Fluorosis	Excess fluorine	Discolouration, mottling, brittleness of teeth and bones

Fluoride is like a double-edged sword: too little causes caries, too much causes fluorosis. Small controlled amounts in water and toothpaste are beneficial.

Arsenic causes melanosis (skin darkening). Fluoride causes fluorosis.

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