p-BLOCK ELEMENTS - HALOGENS

GENERAL CHARACTERISTICS

The group 17 of periodic table consists of five elements fluorine, chlorine, bromine, iodine and astatine and are known as halogens (sea salt forming elements). Astatine is artificially prepared by radioactive element.

ELECTRONIC CONFIGURATION

Element	Symbol	At No.	Valence shell electronic configuration
Fluorine	F	9
Chlorine	Cl	17
Bromine	Br	35
Iodine	I	53
Astatine	At	85

PHYSICAL STATE

Intermolecular forces in halogens are weak and increase down the group. Thus F2, Cl2 are gases, Br2 volatile liquid and I2 volatile solid.

ATOMICITY

All halogens are diatomic in nature.

ABUNDANCE

Being very reactive in nature they are not found free in nature. Their presence in earth’s crust follows the order.

F2 > Cl2 > Br2 > I2 > At

COLOUR

They absorb light in the visible range forming excited states and thus they are coloured in nature.

F2 (pale yellow)

Cl2 (yellowish green)

Br2 (reddish brown)

I2 (deep violet)

METALLIC CHARACTER

All the elements are non metals and metallic character increases down the group.

OXIDATION STATE

F2 Cl2 Br2 I2 At

-1 -1 to + 6 -1 to +6 -1 to +7 -1,+1,+5

BOND ENERGY AND BOND LENGTH

The bond length increases from fluorine to iodine

	F-F	Cl-Cl	Br-Br	I-I
Bond length (Å)	1.42	1.99	2.28	2.67
Bond energy (kJ mol-1)	158.8	242.6	192.8	151.1

Due to small size the interelectronic repulsions between non bonding electrons are high in case of fluorine which results in weakening of F-F bond.

DENSITY

It increases down the group in a regular fashion and follows the order

F > Cl > Br > I

Atomic radii
Ionic radii
Atomic volume
Density
Electronegativity
Oxidising power
Reactivity
Affinity for hydrogen
Reduction potential
Solubility, all follow the above given order

IONISATION POTENTIAL

The ionisation potential of halogens is very high and value decreases down the group. Thus iodine also forms I+ and I3+ and forms compounds like ICl, ICN, IPO4. In molten state these compounds conduct electricity and show ionic character.

ELECTRON AFFINITY

The halogens have the high values for electron affinity following the order.

Cl > F > Br > I

Due to small size of fluorine (hence high electron density) the extra electron to be added feels more electron-electron repulsion. Therefore fluorine has less value for electron affinity than chlorine.

SOLUBILITY

Halogens are soluble in water which follow the order

F2 > Cl2 > Br2 > I2

The solubility of iodine in water is enhanced in presence of KI

KI + I2 ⇌ KI3 ⇌ K+ +

In organic solvents like CS2, CHCl3 and CCl4 the Cl2, Br2 and I2 are more soluble and give coloured solutions. Thus Cl2 gives yellow, Br2 gives brown and I2 gives violet solution.

PROPERTIES OF HALIDE IONS (X^-)

Some properties of halide ions follow the following order

F– > Cl– > Br– > I–

Basic character

Reducing character

Heat of hydration

NATURE OF BONDS WITH OTHER ELEMENTS

Halogens with metals of low ionisation potentials form ionic bonds. The ionic character follows the order

M - F > M - Cl > M - Br > M - I

The more the ionic character, the more is the mp and bp of halides.

Halogens with metals of high ionisation potentials (transition metals) and nonmetals form covalent bond.

COMPOUNDS OF HALOGENS

HYDRACIDS (HX)

All halogens directly combine with hydrogen to form HX known as hydracid.

H2 + X2

2HX

HF and HCl can be obtained by heating fluorides and chlorides with conc. H2SO4

CaF2 + H2SO4

CaSO4 + 2HF

2 NaCl + H2SO4

Na2SO4 + 2HCl

HBr and HI are strong reducing agents in nature and reduce conc. H2SO4 and cannot be prepared by this method.

2HBr (or 2HI) + H2SO4

Br2 (or I2) + 2H2O + SO2

Hence they are prepared by hydrolysis of phosphorus halides

PX3 + 3H2O

H3PO3 + 3HX (X = Br, I)

PROPERTIES

HF is a low boiling liquid due to intermolecular hydrogen bonding, while HCl, HBr, HI are gases. The bpt follows the trend HF > HI > HBr > HCl

Some other properties observe the following trend : HI > HBr > HCl > HF

Acid Strength

Reducing character

Bond length

Trend for following properties : HI < HBr < HCl < HF

Thermal stability

Dipole moment

Bond strength

Stability

Itching of glass : Glass contains silica which reacts with HF.

OXY ACIDS

All form oxyacids of the type HOX, HXO2, HXO3 and HXO4 as shown below

Name	O. N. of X	Fluorine	Chlorine	Bromine	Iodine	Salt’s name
Hypohalous acid, HXO	+1	HOF	HOCl	HOBr	HOI	Hypohalite
Halous acid, HXO2	+3	-	HClO2	-	-	Halite
Halic acid, HXO3	+5	-	HClO3	HBrO3	HIO3	Halate
Perhalic acid, HXO4	+7	-	HClO4	HBrO4	HIO4	Perhalate

Trend for following properties

HXO4 > HXO3 > HXO2 > HXO

Thermal stability

Acid strength

Oxidising nature

The conjugate bases obtained from above acids are

. Their stability increases with the increase of oxygen atoms due to greater chances of dispersal of negative charge. The more is the stability of conjugate base, the more is the acid character.

HYPOHALOUS ACIDS (HOX)

They are formed in aqueous solution by disproportionation of halogens.

X2 + H2O ⇌ HOX + HX

Acid character and thermal stability follows the order

HClO > HBrO > HIO

Hypohalites disproportionate in aqs. solution to halides and halates.

3– OX ⇌ 2X– + XO3–

Rate of disproportionation

The structure of HClO and HFO are

Bleaching powder CaOCl2 is a mixed salt of HOCl and HCl

HALIC ACIDS

HClO3 and HBrO3 exist in aqueous solution and HIO3 is a white solid. The stability follows the order

They are strong oxidising in nature

Their salts, NaClO3 is a powerful weed killer and KClO3 (Berthelot’s salt) is used in fireworks and matches as oxidising agent.

PERHALIC ACIDS
Perhalates are obtained by electrolytic oxidation of halates

Perborate is obtained by oxidation with F2 in basic solution

They are strong oxidising agents following the order

STRUCTURE OF OXYACIDS

Hypochlorous acid (HOCl)

Chlorous acid (HClO2)

Chloric acid (HClO3)

Perchloric acid (HClO4)

OXIDES

All form oxides of different types. Halogens do not combine directly with oxygen hence they are prepared by indirect methods.

STRUCTURE OF SOME OXIDES

All oxides are powerful oxidising agents and decompose with explosion when heated or hit.

INTERHALOGEN COMPOUNDS

Halogens combine among themselves to form compounds known as interhalogens or interhalogen compounds of the type AX, AX3, AX5 and AX7. Their structure and hybridisation is as follows:

AX type sp3 AX3 sp3d AX5 sp3d2 AX7 sp3d3

Linear T-shaped Square pyramidal Pentagonal bipyramidal

A - X

Examples - IBr, BrCl,ClF3, BrF3, ICl3BrF5, IF5, IF7 ClF, BrF, ICl

PROPERTIES

They are covalent, more reactive, strong oxidising and diamagnetic in nature.

PREPARATION
By direct combination of halogens

By the action of halogens on lower interhalogen

Polyhalide ions - The ions containing more than two halogen atoms are known as polyhalide ions eg.

⇌

Other examples are

Iodine Cation - Iodine also forms I+ and I3+ cations due to less ionisation energy as (CH3COO)3I, I (ClO4)3 and IPO4 have been isolated.

Pseudohalogens and Pseudohalides - The substances behaving like halogens are known as pseudohalides.

Some examples are-

FLUORINE (SUPER HALOGEN)

Discovered by Moisson 1886

OCCURRENCE

In combined state it occurs as

Fluorspar CaF2
Cryolite Na3AlF6
Fluorapatite CaF2.3Ca3(PO4)2

It is present in small amounts in soil, river water, bones and teeth of animals.

PREPARATION

Dennis method - By electrolysis of fused sodium or potassium hydrogen fluoride (dry) between graphite electrodes . (KHF2 is known as Fremy salt)

(At cathode)

(At anode)

Whytlaw Gray’s method - Electrolysis of fused KHF2 is carried out in Cu cell (electrically heated) which serves the purpose of cathode, Anode is of graphite
Modern method - Electrolysis of fused mixture of KF and HF is carried out in steel vessel (cathode). Anode is of graphite

PROPERTIES

It is pale greenish yellow gas can be condensed to pale yellow liquid and then pale yellow solid. It is very reactive.
Most active - It directly combines with metals and non metals eg Al, Mg, C, P, S, As, Sb, Br2, I2 etc to from fluorides, Cu and Hg form a protective coating of fluoride.
Reaction with Xe - With Xenon it forms three definite halides

XeF2, XeF4 and XeF6 having the following structure

XeF2 hybridisation sp3d

Linear symmetrical

XeF4 hybridisation sp3d2

Square planar

XeF6 sp3d3 hybridisation

Distorted octahedral

With hydrogen even in dark -
With water

Oxidising nature

With alkali
Dil.

Conc.

With NH3
(not explosive)
With H2S
With hydrocarbons - It reacts with hydrocarbons violently eg. CH4

Hence fluorination is carried out in presence of N2 (it dilutes F2) and catalyst copper gauge.

USES

Fluoro derivatives are solvents, lubricants, refrigerants, fire extinguishers, fungicides, germicides, dyes and plastics etc.
For separation of U235 by forming UF6 from natural uranium
Preparation of Teflon (C2F4)n
FREONS - Chlorofluoro compounds of methane and ethane are known as freons. They are extremely, unreactive, non corrosive, easily liquefiable compounds. Freon-12 (CCl2F2) is most common and prepared as

They are used as refrigerants and propellants .

Magic acid - FSO3H SbF5 is strongest acid and known as magic acid.

CHLORINE

It was discovered by Scheele by heating HCl acid (muriatic acid) with MnO2 and named it oxymuriatic gas. Davy established its nature and called it chlorine.