Chapter 11 : The p-Block Elements
Boron is a fairly rare element, mainly occurs as orthoboric acid, (H3
), borax, Na2
O, and kernite, Na2
Topics covered in this snack-sized chapter:
The outer electronic configuration of these
elements is ns2
11.1.1 Electronic Configuration
On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase.
The ionisation enthalpy values as expected
from the general trends do not decrease
smoothly down the group.
The decrease from B to Al is associated with increase in size.
11.1.3 Ionization Enthalpy
Down the group, electronegativity first decreases from B to Al and then increases.
Boron is non-metallic in nature. It is extremely hard and black coloured solid.
11.1.5 Physical Properties
Reactivity towards acids and alkalies: Boron does not react with acids and alkalies even at moderate temperature; but aluminum dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character.
11.1.6 Chemical Properties
The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as NH3
to complete octet around boron.
B + :NH3
Some useful compounds of boron are borax, orthoboric acid and diborane.
11.2 Important Trends and Anomalous Properties of Boron arrow_upward
It is the most important compound of boron.
It is a white crystalline solid of formula Na2
In fact it contains the tetra nuclear units and correct formula; therefore, is Na2
O. Borax dissolves in water to give an alkaline solution.
Orthoboric acid, H3
is a white crystalline solid, with soapy touch. It is sparingly soluble in water but highly soluble in hot water.
It can be prepared by acidifying an aqueous solution of borax.
Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion:
The simplest boron hydride know, is diborane.
It is prepared by treating boron trifluoride with LiAlH4
in diethyl ether.
The structure of diborane, B2
Boron being extremely hard refractory solid of high melting point, low density and very low electrical conductivity.
Boron fibres are used in making bullet-proof vest and light composite material for aircraft.
The main industrial application of manufacture of heat resistant glasses.
11.4 Uses of Boron and Aluminum and their Compounds arrow_upward
The valence shell electronic configuration of
these elements is ns2
11.5.1 Electronic Configuration
There is a considerable increase in covalent
radius from C to Si, thereafter from Si to Pb a small increase in radius is observed.
The first ionization enthalpy of group 14 members is higher than the corresponding members of group 13.
11.5.3 Ionization Enthalpy
Due to small size, the elements of this group are slightly more electronegative than group 13 elements.
All group 14 members are solids. carbon and silicon are non-metals, germanium is a metalloid, tin and lead are soft metals with low melting points.
11.5.6 Physical Properties
Reactivity towards water: Carbon, silicon and germanium are not affected by water. Tin decomposes steam to form dioxide and dihydrogen gas.
11.5.7 Chemical Properties
Carbon also has unique ability to form pπ– pπ multiple bonds with itself and with other atoms of small size and high electronegativity.
Few examples of multiple bonding are: C=C, C ≡ C, C = O, C = S, and C ≡ N. Heavier elements do not form pπ– pπ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.
Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings.
Carbon exhibits many allotropic forms; both
crystalline as well as amorphous.
11.6 Important Trends and Anomalous Properties of Carbon arrow_upward
In diamond each carbon atom undergoes sp
3 hybridization and linked to four other carbon atoms by using hybridized orbitals in tetrahedral fashion, The C–C bond length is 154 pm.
Graphite has layered structure and distance between two layer is 340pm.
Each layer is composed of planar hexagonal rings of carbon atoms.
Graphite conducts electricity along the sheet.
11.8 Some Important Compounds of Carbon and Silicon arrow_upward
Oxides of Carbon
Two important oxides of carbon are carbon monoxide, CO and carbon dioxide, CO2
Direct oxidation of C in limited supply of oxygen or air yields carbon monoxide.
When air is used instead of steam, a mixture of CO and N2 is produced, which is called producer gas.
It is prepared by complete combustion of carbon and carbon containing fuels in excess of air.
- In photosynthesis process green plant convert atmospheric CO2
into carbohydrates such as glucose.
Silicon dioxide, commonly known as silica, occurs in several crystallographic forms.
Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms
Three-dimensional structure of SiO2
11.8.3 Silicon Dioxide, SiO2
They are a group of organosilicon polymers.
The manufacture of silicones is alkyl or aryl substituted silicon chlorides.
A large number of silicates minerals exist in nature. Some of the examples are feldspar, zeolites, mica and asbestos.
The basic structural unit of silicates is SiO4
Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation, e.g., ZSM-5.