Oxidation state (Monovalency trivalency), inert pair effect and causes of inert pair effect.

Inert pair effect.

General valence shell electronic configuration is

=ns^₂np¹_.=

In heavier P-block elements the ns electron pair do not tend to participate in bonding.The resultance of ns electron pair of heavy P-block elements to participate in bonding is called inert pair effect. The consequence of this effect is that heavy P-block elements exhibit lower oxidation state(valence) I,e their oxidation state decreased by 2.Inert pair effect increases on descending a group as elements become heavier.

Cause of inert pair effect.

Trivalence and monovalence of group IIIA elements are consequences of inert pair effect. The elements which exhibit inert pair effect exhibit mono valency and an element which do not exhibit inert pair effect exhibit tri valency.

In order to exhibit trivalent, the ns electron pair most be excited.If enthalpy of formation (energy released when a bond is formed) is greater than the energy required to unpair the ns electron, the ns electron paired become excited to produced three unpaired electrons.Consequently, all the three valence electron participate in bonding resulting oxidation state +3.

However, if the energy required to unpaired the ns electron is greater than the enthalpy of formation(energy released when bonds are formed) than ns electron pair does not undergo excitation and remains the pair.Consequently, the ns electron, the pair does not participate in bonding rather only then np electron participate in bonding resulting oxidation state.(This is the cause of inert pair effect).

Note:

When ΔHf>E_ex exists tri valency

When ΔHf <E_exexist mono valency.

Oxidation states (Valency).

General valence electronic configuration =ns² np^1.

These elements exhibit oxidation state +3(tri valency) and +1 ( mono valency). If all the valence electrons (ns²,np¹) participate in bonding than oxidation state +3 results.However, if only np1 electron participate in bonding(due to the inert pair effect) than the oxidation state +1 results.

Oxidation state +3 Tri Valency.

This is the most common and the most stable oxidation state for these elements.Stability of oxidation state +3 decrease from B to Tl.

Stability of oxidation state is given below.

$$B^{+3}>Al^{+3}+>Ga^{+3}>In^{+3}>Tl^{+3}$$

$$B^{+3}>Al^{+3}+>Ga^{+3}>In^{+3}=most\, common$$

$$ Tl^{+3}=Unstable$$

Except, Thallium all these element exhibit oxidation states +3

The compound of boron is always covalent.

A compound of boron is always covalent this is because boron has extremely high ionisation energy (I¹+I²+I³) which is not exceeded by either lattice energy or hydration energy. Hence, the formation of B³⁺ ion is not possible.

Other elements in the group also form a compound.However, compounds of these elements are covalent only when they are anhydrous. In solution, a covalent compound of these elements ionises to produce M³⁺ ion (Al³⁺,Ga³⁺, In³⁺and Tl³⁺⁾. The change from covalent to ionic compound take place because the energy required to produce M3+ ion is provided by the high enthalpy of hydration of these ions.

The structure of AlCl₃ differ in anhydrous form and when to dissolve in water.

AlCl₃ is covalent and AlCl3.H₂O is ionic.

That's why AlCl₃ ionise in solution .

For example.

$$AlCl_3+6H_2O→[Al(H_2)_6^{3+}]+3Cl^-$$

$$AlCl_3=Covalent$$

$$[Al(H_2O)_6^{3+}]=hydrated\, cation$$.

A structure of the AlCl₃when anhydrous.

AlCl3 is covalent in nature when anhydrous since Al³⁺ is small-sized and highly charged AlCl₃ in anhydrous gas phase exist as dimeric form.But Al form metal ion (Al⁺³) in water. This changes from covalent to ionic in nature this is because the ions are hydrated [Al(H₂O)6]³⁺, and the amount of hydration energy involved exceeds the ionisation energy. In a case of AlCl₃, ionisation energy require to convertAl to Al_³⁺ is 513kJ/mol and the total hydration energy is 580kj/mol.Here, hydration energy is greater than the ionisation energy. So AlCl₃ is ionised in solution.

Hydrated Al³⁺ ion has octahedral structure.

In hydrated Al3+ ion due to the formation of strong Al³⁺→OH₂ bonds, the O-H bonds in water becomes weak.Hence, there is the tendency of hydrogen to dissociated as H⁺. In other words presence of hydronium ion in solution represent that aqueous solution in AlCl₃ is acidic. Hydrated Al³⁺ ion further hydrolysed as,

$$[Al(H_2O)_6^{3+}]+H_2O\rightleftharpoons Al(H_2O)_5[OH^{2+}]+H_3O^+$$

Oxidation state +1.

Except, Tl all elements in group IIIA exhibit oxidation state +3. Since, inert pair effect increase from top to bottom in a group, stability of oxidation state +1 increase from B to Tl.

Stability of oxidation state +1 is given below.

$$B^{+1} <Al^{+1}<Ga^{+1} <In^{+1}Tl^{+1}$$

$$B^{+1}=unstable$$

$$Tl^{+1}= most\, stable$$

Oxidation state +1 is most stable in thallium this is explained as that Group IIIA In heavier P-block elements the ns electron pair do not tend to participate in bonding.The resultance of ns electron pair of heavy P-block elements to participate in bonding is called inert pair effect.Thallium is the lowermost element of group IIIA and we know there is increasing the tendency of s-electron to be inert on descending down the group.So, 's' electrons must likely to be inert in Tl hence, +1 oxidation state is stable in thallium.The reason that the twos- electrons do not take part in bonding is energy. If the energy required to unpair them exceed the energy involved when they form bonds, then electrons will remain pained.

However, compounds with Ga(I) and In (I) are well known, but such compounds are relatively less stable than oxidation state.

Oxidation state +2.

Gallium has apparently divalent in some compounds like GaCl_2.However, it is not actual divalent as its structure has been shown as Ga[GaCl]4. In this compounds, Ga is in oxidation state +1 and +3 .Hence, oxidation state is+2 does not exist.

$$2GaCl_2→Ga[GaCl_4]$$

$$Ga=[I]$$

$$[GaCl_4]=[III]$$

A reason for trivalency and mono valency of group IIIA. Group IIIA elements have outer electronic configuration ns2np1 and they should be trivalent due to the loss of three electrons. But in some cases, the elements show monovalent.Monovalency is explained by the 's' electron in the outer shell remaining paired and not participate in bonding.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.