p- Block Elements

p- Block Elements


This was discoved in 1985. The real name of Fullerene is buckminster fullerene. Fullerene is third allotrope form of carbon material after graphite and diamond whose molecule consists of 20 hexagonal and 12 pentagonal rings as the basis of an icosohedral symmetry closed cage structure. Each carbon atom is bonded to three others and is sp2 hybridised.
The most stable and common of fullerenes is the C60 molecule. C60 molecule has two bond lengths - the 6:6 ring bonds can be considered "double bonds" and are shorter than the 6:5 bonds. C60 is not "superaromatic" as it tends to avoid double bonds in the pentagonal rings, resulting in poor electron delocalisation. As a result, C60 behaves like an electron deficient alkene, and reacts readily with electron rich species. Fullerene is harder than diamond. Also, the ductility is 100 times stronger than steel. Fullerene has amazing electrical conductivity and has more stronger conductivity than copper. Fullere's weighs only 1/6th of copper material. The geodesic and electronic bonding factors in the structure account for the stability of the molecule. Theoratically, an infinite number of fullerenes can exist, their structure based on pentagonal and hexagonal rings, constructed according to rules for making icosahedra.
One of the most important uses of Fullerene is medicine.
Fullerenes are active molecules.
Fullerene molecule can be used as an antioxidant because it can easily react with radicals due to the high affinity of the electron.
It is used as an anti-aging and anti-damage agent in the cosmetic sector.
Fullerenes are used as antiviral agents. This use is provided by its unique molecular structure, antioxidant effect and biological compatibility.
May be used for osteoporosis treatment because of its preferential localization.

Inert Pair Effect

The reluctance of two ns-electrons to take part in bond formation due to the high energy needed for unpairing them is called inert pair effect. This was proposed by Sidgwick.
Some p-block elements, such as thullium(Tl), polonium(Po), tin( Sn ), lead(Pb), bismuth( Bi ) etc exhibit Inert pair effect. The '5s' electron of tin and '6s' electrons of lead and bismuth have a tendency to remain inert due to inert pair effect.
From their electronic arrangement, we see that these elements contain inner 'd' or 'f' electrons and due to poor shielding effect of 'd' or 'f' orbitals, the effective nuclear charge on outer most 's'-electrons act strongly. So, the nucleus pull the 's'-electrons strongly. Consequently, these two 's'-electrons show a less tendency to participate in chemical reaction and these two 's'-electrons remain inert.

Consequences of Inert Pair Effect

# Due to inert pair effect, many physical and chemical properties of the relating elements become changes.
SnCl2 is more stable than SnCl4 because, in the formation of SnCl2, 'Sn' metal utilize two '5p'-electrons only. While, in the formation of SnCl4, 'Sn' metal used both '5s' and '5p' electrons. But, '5s' electron of 'Sn' is not available for formation of molecule due to inert pair effect. Hence, SnCl4 becomes unstable.
# Inert pair effect cause the variable valency of elements.
Sn & Pb shows +2 & +4 oxidation states while Tl shows +3 & +1 oxidation state due to inert pair effect.
# Inert pair effect, affect melting and boiling point of elements. For example, the melting and boiling point of 'Po' is less than that of 'Te'. Generally, the melting and boiling point of elements increase down the group.
Now, 'Po'-element contain maximum number of 'd' and 'f' electrons. So, the inert pair effect becomes more effective for 'Po' than 'Te'. As a result, the '6s'-electron of 'Po' is less available than '5s'-electron of 'Te'. Hence, in case of Po, the inter-atomic Vander waal's force of attraction is less than Te. Consequently, the melting and boiling point of Po is less than Te.
# Inert pair effect, influence the oxidizing and reducing properties of compounds. For example, +5 oxidation state of Bi is less stable than +3 oxidation state. Due to this, BiF5 act as a strong oxidizing agent. It oxidized the other reducing substance readily, and changes it's oxidation state from +5 to more stable +3.

Structure of Peroxydisulphuric Acid

Peroxydisulfuric acid is the inorganic compound with the chemical formula H2S2O8 also called Marshall's acid after its inventor Professor Hugh Marshall. Oxidation state of Sulfur is +6. It is a powerful oxidizing agents.
Structure of Peroxydisulphuric Acid Structure of Peroxydisulphuric Acid

Which is more acidic H2SO4 or H2SO3 ?

The oxidation state of S is +6 in H2SO4 and +4 in H2SO3. Greater is the oxidation number of central atom, more it attracts the electrons towards itself and also, more it stabilises the conjugate base. So, H2SO4 is stronger acid than H2SO3

Borazine or Borazole (Inorganic Benzene)

Borazine, also known as borazole, is a polar inorganic compound with the chemical formula B3N3H6. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene.
When B2H6 and NH3 in the ratio of 1:3 are heated borazole is obtained.
                  B2H6 + 6NH3 -----> 2B3N3H6 + 12H2

Properties of Borazine

1. It is colorless liquid
2. It shows unsaturated reaction.
                  B3N3H6 + 3HCl → B3N3H9Cl3
                  B3N3H6 + 3Br2 → B3N3H6Br6
                  B3N3H6 + 3H2O → B3N3H12O3
3. It forms two series of methyl derivatives-
a. N- methyl derivatives
b. B- methyl derivatives
4. It is isoelectronic with benzene.
5. When borazine is pyrolysed above 340°C, B3N6H10 and B5N5H8 are produced. These products are boron- nitrogen analogues of diphenyl and naphthalene respectively.
6. Borazine gets slowly hydrolysed by water to produce boric acid, ammonia and Hydrogen.Hydrolysis is favoured by the increase in temperature.
                  B3N3H6 + H2O -----> H3BO3 + NH3 + H2

Struture of Borazine

In Borazine both Boron and Nitrogen are sp2 hybridised. Each N-atom has one lone pair of electrons, while each B-atom has an empty p-orbital. (B-N) bond in borazine is a Dative bond, which arises from the sidewise overlapping between the filled p-orbitals of N-atom and empty p-orbitals of B-atom. In this cyclic compound, the three BH units and three NH units are alternate to each-other.

Preparation and Preperties of Diborane

Preparation of Diborane

1. It is prepared by the action of H3PO4 on Mg-boride-
                  H3PO4 + Mg3B2 = B2H6 + Mg3(PO4)2
2. When BF3 is reduced by sodium borohydride or LiH, diborane is formed.
                  4BF3 + 3NaBH4 = 2B2H6 + 3NaBF4

Properties of Diborane

Physical Propeeties:
* It is air sensitive, volatile and reactive gas with repelling smell.
* It is fairly stable in absence of moisture and grease.

Chemical Propeeties of Diborane

1. Reaction with dry HCl: it react with dry HCl in presence of Al2X6, forms B2H5Cl
                  B2H6 + HCl = B2H5Cl + H2
2. Reaction with concentrated KOH solution:
With concentrated KOH solution, it forms potassium hypoborite.
                  B2H6 + 2KOH = K2(BH2OH)2 + H2

3. Hydrolysis: On hydrolysis, diborane forms boric acid.
                  B2H6 + 6H2O = 2H3BO3 + 6H2

4. Burn in Air: It burns in air release large amount of heat.
                  B2H6 + 3O2 = B2O3 + 3 H2O

5. Reaction with chlorine: it violently react with chlorine.
                  B2H6 + 6Cl2 = 2BCl3 + 6HCl

6. Reaction with Ammonia:
Diborane react with ammonia but the product is depend upon the experimental condition.
                  B2H6 + 2NH3 ---L.T.----> B2H6.2NH3
                  B2H6 + 2NH3 ---H.T.---->2BN + 6H2
                  3B2H6 + 6NH3 ---H.T.---->2B3N3H6+ 12H2

Why diamond is a non-conductor and graphite fairly a good conductor of electricity ?

In diamond, unit cell is tetrahedral. Each carbon atom is sp3 hybridized and forms four sigma bonds with four other carbon atoms and this trend extends throughout the crystal. All the four electrons of each carbon atom are tightly bound in these covalent bonds. As a result, no free electron are available in diamond so it is a nonconductor of heat and electricity.
However, in Graphite, each carbon atom is sp2 hybridized and bonded to three neighbor carbon atoms so one of its four valance electrons is relatively free to move along the layers. Hence, graphite is a conductor parallel to the layers and non-conductor perpendicular to the layer.

Why PbCl2 is ionic but PbCl4 is fairly covalent compound ?

When a metal forms more than one halide, the halides in which metal has lower oxidation state tends to be ionic while that in higher oxidation state is covalent. Similarly, high polarizing power of Pb+4 as compare to Pb+2 makes PbCl2 mainly ionic but, PbCl4 fairly covalent.

Sulphur forms SF4 and SF6 but oxygen does not form OF4 and OF6 why ?

Due to absence of d-orbitals in the valence shell, oxygen cannot show an oxidation state of +4 and +6 and hence does not form OF4 and OF6.
However, S contains d-orbitals in the valence shell and hence can show a maximum oxidation state of +6. Since F2 is a very strong oxidising agent, it oxidises sulphur to its higher oxidation state of +4 and +6 and thus forms SF4 and SF6.