p - Block Elements

1. Halogens

2. Inert Gases

1) Halogens

The group comprises of : a) fluorine , b) chlorine , c) bromine ,d) iodine and e) astatine.

The last element is radioactive. They are collectively known as halogens (Greek, halos + gen, salt former).

The most common compound containing a halogen is sodium chloride (common salt), known since antiquity.

Due to high reactivity, halogens do not occur free in nature but in form of salts (halides).

- The elements have seven electrons in their outermost shell (ns2 np5 ).

- They acquire noble gas configuration by either gaining an electron to form halide ion, or acquiring a share in one electron by forming a covalent bond.

- All natural halogens are diatomic.

Physical properties of Halogen group elements

1.1) General Group Trends

- The halogens are all coloured. Fluorine and chlorine are pale –yellow and yellowish –green gases respectively; bromine is a dark-red liquid and iodine a black, lustrous solid that readily sublimes giving a purple vapour.

- The halogens are the smallest atoms in the respective periods of the periodic table and the size increases with increase in atomic number. The halogens have very high electronegativity values, which decrease with increase in size. Fluorine is the most electronegative element in the periodic table.

- Halogens need one electron to complete the octet, thus the electron affinity values are high. The electron affinity decreases from chlorine to iodine. The electron affinity of fluorine is less than that of chlorine; this is because of the small size of the fluorine atom, which makes the incoming electron encounter a lot of electron – electron repulsion.

- The X-X bond dissociation enthalpy decreases from Cl2 to I2. The value for F2 is abnormally low and it is likely that the weakness of the F-F bond is largely a consequence of repulsions between the nonbonding electron pairs. The low bond enthalpy is responsible for the high reactivity of fluorine.

1.2) Oxidation States

Fluorine is the most electronegative element and always shows an oxidation state of -1.

This oxidation state is the most common and stable state for all halogens.

The other members display positive oxidation states (+1) with oxygen or fluorine.

They can also display oxidation states of +3, +5 and +7 by utilizing vacant d orbitals.

Positive oxidation states are displayed in halogen oxides and interhalogens.

Oxidation states of +4 and +6 are less common and displayed in ClO2, BrO2, I2O4, Cl2O6 and BrO3.

a) Oxidizing Power

The halogens are good oxidizing agents.

Generally speaking, a higher member displaces a lower from the halide.

The following reactions illustrate this point :

F2 + 2X- → 2F- + X2 (X=Cl, Br, I)

Cl2 + 2X- → 2Cl- + X2 (X=Br, I)

Br2 + 2I- → 2 Br- + I2

The oxidizing power decreases down the group, which is reflected in their decreasing reduction potential values.

The strength of an oxidizing agent depends on several energy terms.

The reaction represents the oxidizing action of a halogen and is best represented in the form of a Born - Haber Cycle.

(1/2) X2 (standard state) + e- → X- (aq)

If the standard state of the halogen is solid i.e. for I2, ∆H is the sum of enthalpy of fusion and enthalpy of vaporization.

For a liquid halogen, i.e. Br2, ∆H is enthalpy of vaporization.

Energy is absorbed in steps II and III and released in IV and V.

From Hess's law ∆E is given by:

∆E = ∆H + ∆Hd + EA + ∆H hyd

∆Hd = enthalpy of dissociation

EA = electron affinity

∆Hhyd = enthalpy of hydration

For F2 and Cl2 which exist as gases, ∆H is omitted.

The enthalpy changes associated with the reaction (I) decrease from fluorine to iodine .

The high oxidizing power of fluorine is attributed to low enthalpy of dissociation and high enthalpy of hydration.

Some important reactions of halogens are listed in Table below :

1.3) Basic Properties of Halogens

Metallic character increases down the group and decreases across a period.

This is not very apparent in halogens as very little is known about the last element, astatine.

However, there are certain evidences to show existence of halogen cations.

Fluorine is the most electronegative element; it has no tendency to display positive oxidation state.

ClF ionizes to form Cl+ and F- Br+ exist in complexes like [Br (Pyridine)]+ NO3-.

ICN on electrolysis liberates iodine at anode, indicating formation of I+.

Many pyridine complexes of I+ are known

Example : [I (py)]+ NO3-, [I (py)2]+ ClO4- and [I(py)]+ CH3COO-, (py = pyridine).

Iodine dissolves in oleum to give a blue solution that contains I2+ and I3+

2I2/3I2 + 6H2S2O7 → 2I2+/2I3+ + 2HS3O10- + 5H2SO4 + SO2

Molten iodine conducts electricity and it has been shown that I3+ and I3- are present.

Many other compounds containing Cl3+, Br3+, I3+, I5+ have been prepared

ICl is an electrophilic iodinating agent, believed to contain I+

a) Hydrogen Halides

Binary compounds of hydrogen and halogen are formulated as HX and in the anhydrous state are referred to as hydrogen halides. Their aqueous solutions are called hydrohalic acids.

The halogens react with hydrogen forming HX.

The reaction with fluorine is violent and explosive while that with iodine is slow illustrating decrease in reactivity on moving down the group.

At room tempera HF is a liquid due to strong intermolecular hydrogen bonding, while others are gases.

The melting and boiling points shows a gradual increase in the order :

HCl < HBr < HI

The hydrogen halides are covalent in the gaseous state, but in aqueous solution they ionize and behave as strong acids (except HF, which is a weak acid, pKa = 3.2 at 250 C).

The ionization may be represented as

HX + H2O → H3O+ + X- (X= Cl, Br, I)

The aqueous solutions form isotopes with maximum boiling points (negative deviation from Raoult's Law).

As fluorine is the most electronegative element, the H-F bond has maximum ionic character and HF should be the strongest acid, however the reverse is true.

The paradox arises due to the fact that acid strength which is given by the following reaction

HX (hydrated) + H2O → H3O+ + X- (hydrated)

b) Halogen Oxides

A large number of halogen oxides are known , some of which are unstable.

Compounds of Halogens with Oxygen

The binary compounds of fluorine and oxygen are referred to as fluorides.

The most stable oxide is I2O5 .

The oxides of chlorine and bromine decompose readily.

The bonds are largely covalent due to small electronegativity difference, however some ionic character is noted in iodine oxides.

ClO2 and Cl2O find some practical importance as bleaching agents and germicides, while I2O5 is used in estimation of carbon monoxide.

c) Oxoacids of Halogens

Four series of oxoacids of halogens are known :

The structures of the ions formed are shown in figure :

The halogen atom is sp3 hybridized, there is significant pл – dл -bonding involving filled 2p orbital of oxygen and vacant d orbitals on halogens.

Many of the oxoacids are known in solution, or in form of salts.

Fluorine being more electronegative than oxygen does not form any oxoacid except the unstable HOF, which is obtained by passing F2 over ice.

It is difficult to prepare and isolate because of its reactivity towards H2O and F2

HOF + H2O → HF + H2O2

HOF + F2 → HF + OF2

The oxoacids of chlorine are well known and arise from disproportionation of chlorine and related reactions:

In basic solution chlorine dioxide disproportionate to form chlorate and chlorite and the latter is used to form the free acid.

2ClO2 + 2OH- → ClO2- + ClO3- + H2O

Ba(ClO2)2 + H2SO4 → 2HClO2 + BaSO4 (s)

The acid strength of oxoacids increases with increase in oxidation number of the order of acid strength is

HOCl < HOClO < HOClO2 < HOClO3

- As the oxidation state of the halogen increases, the number of oxygen atoms attached to the halogen increases.

- As oxygen is more electronegative, the halogen – oxygen bond is polarized leaving a small positive charge on the halogen.

This now attracts the electrons of O-H bond towards itself, facilitating loss of hydrogen as proton.

Greater the number of oxygen atoms, more is the acidity

- Acid strength of oxoacids with halogen in the same oxidation state, decreases with decrease in electro-negativity of the halogen.

HOI < HOBr < HOCl

Sodium hypochlorite (NaOCl) is used in bleaching cotton fabrics, wood pulp and is a disinfectant.

Anhydrous perchloric acid (HClO4) is a very powerful oxidizing agent.

Periodic acid (HIO4) is used to oxidize alkenes to glycols.

d) Interhalogens

Halogens react with each other to form interhalogens.

These are subdivided into four types :

AX, AX3, AX5, AX7

The total number of halogen atoms is even as this gives rise to diamagnetic species.

They are generally more reactive than the parent halogens (except fluorine) as A-X bond is weaker than X-X bond.

The interhalogens can be prepared in a variety of ways, including direct reaction of the elements (the favoured product often depends on ratio of halogens used) and reaction of metal halides or other halogenating agents.

Examples include :

Some interhalogens may serve as intermediates in synthesis of other interhalogens .

ClF + F2 → ClF3 (T = 200 – 3000 C)

ClF3 + F2 → ClF5 (hv, Room Temperature)

Several interhalogens undergo auto ionization in the liquid state and are used as non-aqueous solvents

The interhalogens are hydrolyzed to give halide and oxohalide; the larger halogen forms the oxohalide

Shapes of interhalogens on basis of VSEPR Theory

The structure can be derived by VSEPR theory.

As expected interhalogens of the types AX are linear.

Interhalogens of the type AX3 (ClF3) are T-shaped, where the central halogens 'A' (Cl) is sp3d hybridized with two lone pairs of electrons .

ICl3 exists as a dimer, (ICl3)2, in the solid state in which two T-shaped ICl3 units are joined together.

Interhalogens of the type AX5 and AX7 have square pyramidal and pentagonal bipyramidal structures respectively.

In AX5 the central halogen is sp3d2 hybridized and there is one lone pair of electrons whereas in AX7 the central halogen has sp3d3 hybridization.

Ternary interhalogens have recently been reported e.g. IFCl2, IFCl

e) Polyhalides and polyhalonium Ions

Halide ions associate with molecules of halogens or interhalogens to form polyhalides.

The solubility of iodine is enhanced in potassium iodide due to formation of triiodide ion, I3-

KI + I2 → KI3

More complex ions like I5-, I7- are known.

Polyhalides containing two or three different halogens are known eg [ICl2]-, [BrF4]-, [BrICl]- etc.

Polyhalonium Cations example : [ICl2]+, [BrF2]+ are obtained as a result of auto ionization of ICl3 and BrF3.

Other Cations example : Br3+, I3+ [ClF6]+ etc. are also known

f) Pseudohalogens

Parallels have been observed between the chemistry of the halogens and other dimeric species. Such molecules are called pseudohalogens.

Example: Cyanogens (CN)2, thiocyanogen (SCN)2 and selenocyanogen (SeCN)2

A pseudohalogen is a univalent chemical aggregate, comprising of two or more electronegative atoms, which in the free state show properties similar to halogens.

They combine with hydrogen to form an acid and with silver, a salt insoluble in water.

The anions of pseudohalogens are called pseudohalides.

The most important pseudohalide is CN- and its similarity with Cl- is shown below :

Comparison of a Pseudohalogen with a Halogen