NEET Chemistry Notes p-Block Elements – Group 13 Elements: Boron Family
Group 13 Elements: Boron Family
Group 13 Elements: Boron Family
Boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl) are the members of group 13. Boron is non-metal while rest of the members are metals.
Important Properties
- General Configuration ns1, np1
- Oxidation State +3,+1
Stability of +3 oxidation state decreases down the group and that of 1 oxidation state increases down the group due to inerteffect. Fourth period and onwards effective nuclear charge increases due to poor shielding effect, therefore the radius of gallium (135 pm) is less than that of aluminium (143 pm) and down the group, inert pair effect increases.
- Hydrides
Boron hydrides exist in dimeric or polymeric form. The simplest boron hydride is diborane,B2H6 Aluminium forms only one colourless, solid polymeric hydride (AlH3)n, , known as alane. B, Al and Ga form complex anionic hydrides, e.g. NaBH4, LiAlH4 and LiGaH 4.
All are strong reducing agents
LiAlH4 >NaBH4 (reducing power)
- Halides
Halides have incomplete octet, therefore, have a high tendency to accept electrons and behave as Lewis acid.
BI3 > BBr3 > BC13 > BF3 (acidic character)
Due to back bonding and resonance, the B-F bond of BF3 gets a bond order of 1.33.
- Oxides
Down the group, their basic character increases, e.g. B203 is weakly acidic, A1203 is amphoteric and other oxides are basic in nature.
Preparation, Properties and Uses of Boron
Occurrence
The important minerals of boron are borax (Na2B407 10H2O), orthoboric acid (H3B03) and kernite (Na2B407 4H2 O)
Preparation
Now a days boron is obtained by electrolysis of a fused mixture containing boric anhydride, magnesium oxide and magnesium fluoride at 1100°C.
Crystalline boron is obtained by the reduction of B203 with aluminium powder.
Amorphous boron of low purity is called Moissan boron. It is black in colour.
Physical Properties
Boron exists in two allotropic forms, i.e. amorphous and crystalline. Crystalline boron is chemically inert while amorphous boron is chemically active. It is a bad conductor of heat and electricity.
Chemical Properties
- Amorphous boron on heating with air at 700°C forms a mixture of oxide and nitride.
- Boron is attacked by oxidising acids like conc H2S04 and HN03.
It dissolves in fused alkalies and liberates hydrogen.
It reacts with strong electropositive metals at high temperature and forms borides such as Mg3B2.- It is a powerful reducing agent.
Uses
It is used as control rods in atomic reactors and as a deoxidiser.
Preparation, Properties. and Uses of Aluminium
Occurrence
The important ores of aluminium are bauxite A10x(OH)3_2x (where 0<x<l) and kaolinite [Al2(OH)4Si2Os]
Preparation
Bauxite contains Si02, iron oxides and titanium oxide (Ti02) as impurities. Leaching of bauxite ore is carried out with cone. NaOH solution.
Hall-Heroult Process
Aluminium is obtained by the electrolysis of A1203 mixed with Na3AlF6 or CaF2. Overall reaction is as follows
Physical Properties
It is bluish white lustrous metal and loses its lustre due to the formation of protective oxide film A1203 . It is light, malleable, ductile, good conductor of electricity and heat.
Chemical Properties
With Alkalies Al readily dissolves in alkalies evolving H2 and forming the corresponding aluminates.
With Acids The metal dissolves in dil. HCl and dil. H2S04 solutions evolving H2. Conc. HN03 has no reaction, the metal rendered passive due to surface oxidation.
Uses
It is used for making house hold utensils, frames, bodies of aircraft automobiles etc. It is used in making paints, as a mordant in dyeing and calico printing. Ammonal (mixture of A1 powder and ammonium nitrate) is used as explosive. Magnalium, duralumin and alnico are important alloys of aluminium.