Plus One Chemistry Notes Chapter 11 The p Block Elements is part of Plus One Chemistry Notes. Here we have given Kerala Plus One Chemistry Notes Chapter 11 The p Block Elements.
|Text Book||NCERT Based|
|Chapter Name||The p Block Elements|
|Category||Plus One Kerala|
Kerala Plus One Chemistry Notes Chapter 11 The p Block Elements
p-block of the periodic table is unique in terms of having all types of elements; metals, non-metals and metalloids. There are six groups (group 13 to 18) of p block elements in the periodic table. Their valence shell electronic configuration is ns2 np1-6.
Inert Pair Effect
The reluctance of ns electron to take part in the bond formation is known as inert pair effect.
Some of the elements of p block particularly heavier metals like Sn, Pb etc. show different oxidation states due to inert pair effect. This lowest oxidation states of such elements are more predominant due to non participation of s electrons in the formation of bonds.
The first element of every group departs from the general group behavoiur is called ‘anomalous behavoiur’.
Group 13 Elements: The Boron Family
The outer electronic configuration of these elements is ns2 np6. This difference in electronic structures affects the other properties and consequently the chemistry of all the elements of this group.
Atomic and Ionic Radii.
It increases on moving down the group due to the addition of new shells. It is lesser than s-block elements. Atomic radius of Ga (Z = 31, r = 135 pm) is slightly smaller than that of Al (Z = 13, r = 143 pm) because the effective nuclear charge of Ga is more than Al as electrons filled in the d orbitals do not screen the nucleus effectively because of their shapes and the poor penetration power.
The ionisation enthalpy of p block elements increases along a period and decrases down the group. In group 13 elements, the ionisation enthalpy values do not decrease smoothly down the group. The decrease from B to Al is associated with increase in size.
Greater the effective nuclear charge and smaller the atomic radius, greater will be the electronegativity. Down the group, electronegativity first decreases from B to Al and then increases due to the discrepancies in atomic size of the elements.
Boron is non-metallic in nature. The allotropic forms of boron are amorphous boron and crystalline boron. It is extremely hard and black coloured solid.
Melting and Boiling Points.
Melting point and boiling point are higher than those of group 12 due to stronger interatomic attraction. Boron has very high melting point than others due to network structure of boron.
In trivalent state, the number of electrons around the central atom in a molecule of the compounds of these elements (Boron in BF3) will be only 6. Such molecules are considered as electron deficient. Such type of molecules can accept a lone pair of electrons to achieve a stable configuration and thus behave as Lewis acid.
Important Trends and Anomalous Properties of Boron
- Very small size
- High I.E and electronegativity
- +3 oxidation state
- Covalent nature
- Non-availability of d orbitals
- Non-metalic character
- Maximum covalency = 4
Compounds of Boron
The series of compounds of boron with hydrogen is known as boranes. Two series of boranes, BnHn+4 and BnHn+6 are particularly important. The simplest form of borane is diborane (B2H6) which is used as reducing agent.
Diborane does not exist because it is eletron deficient. It exists in the form of dimer B2H6 .The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below the planar, there are two bridging H-atoms. The terminal B-H bonds are regular bonds but the bridge B-H bonds are different and can be described in terms of three centre two electron bonds shown below:
B5 = 1s2 2s2 2p1
Uses of Diborane
- Both LiBH4 and NaBH4 are used as reducing agents.
- They are used for preparing other metal borohydrides.
- As fuel for supersonic aeroplanes.
Group 14 elements : Carbon Family
The valence shell electronic configuration of group 14 elements is ns2 np2 .
There is a considerable increase in covalent radius from C to Si, there after from Si to Pb a small increase in radius is observed. This is due to the presence of completely filled d and f-orbitals in heavier members.
There is a sharp decrease in I.E values from carbon to silicon. The I.E of lead is expected to be less than that of tin but it is slightly more.
Carbon is the most electronegative element while then remaining elements have almost same value.
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.
C >> Si > Ge ~ Sn >> Pb
Catenation α.M-M bond enthalpy
Catenation is phenomenon of an atom to form a strong covalent bond with the atoms of itself. Carbon shares the property of catenation of maximum extent because it is small in size and can form pπ-pπ multiple bonds to itself.
Important Trends and Anomalous Behaviour of Carbon
- Small atomic size
- High I.E and electronegativity
- Absence of vacant d-orbitals
- Tendency to form pπ-pπ multiple bonds
Allotropes of Carbon
Purest form of carbon. In diamond, each carbon atom is in sp3 hybridised state and is tetrahedrally bonded to 4 other carbon atom. The C-C bond length is 154 pm and C- C-C bond angle is 109°28. This 3 dimensional net work structure makes diamond harder and a bad conductor of electricity.
Graphite has layered structure. Distance between 2 layers is 340 pm. C-C bond length within the layer is 141.5 pm. Each carbon atom in hexagonal ring undergoes sp2 hybridisation with 3 neighbouring carbon atoms. The 4,h electron present in the unhybridised p- orbital is used for forming a n bond.
Fullerenes are large cage like spheroidal molecules with the formula C60, C70, C84 etc. C60 has been named as Buckminister fullerence.
It contains 20 hexagonal rings and 12 pentagonal rings of sp2 hybridised carbon atoms. The diameter of ball cage is about 70 pm (superconduction).
It is formed by the incomplete combustion of carbon or carbon containing fuels. It is present in automobile exhaust gases.
The pure carbon monoxide is prepared on smaller scale by dehydrating (removing water) pure formic acid with cone. H2SO4.
CO is commercially produced by the passage of steam over red hot coke. We get mixture of CO and H2. It is called water gas or synthesis gas.
Preparation of Producer gas.
When air is passed instead of steam over heated coke, a mixture of CO and N2 are produced which is called producer gas.
Methods of preparation.
- It is formed by the combustion of carbon and other fossil fuels in an excess of oxygen.
C (s) + O2 (g) → CO (g)
C5H12 (I) +8O2(g) → 5CO2 (g) + 6H2O (I)
- Commercially CO2 is produced by heating limestone.
In laboratory, CO2 is produced by the action of acids on carbonates.
CaCO3 (s) + 2HCI (aq) → CaCI2 (aq) + H2O (I) + CO2(g)
Carbon dioxide is also formed during fermentation.
Uses of CO2.
It is used as refrigerant. Solid CO2 is known as dry ice which is used as coolant for preserving icecream and food.
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