The p-Block Elements General trends: 1. Valence shell electronic configuration : ns2 np1 2. Atomic radius increases on moving down the group Exception: The atomic radius of Ga is less than that of Al. This is because of poor shielding of d-electrons in Ga. 3. Ionisation enthalpy: Decreases (not smoothly) on moving down the group
Exceptions : Al < Ga (Due to poor shielding by d-electron) In < Tl (Due to poor shielding by f-electrons)
4. Electronegativity: Decreases from B to Al and then increases on moving down the group Physical properties (a) Boron is a non-metal and is an extremely hard and has high melting point due to a very strong crystalline lattice. (b) Ga has low melting point (303 K). Therefore, it can exist in liquid state during summer. (c) Density of these elements increases down the group. Chemical properties (i) BCl3 , AlCl3 : Electron deficient molecules BCl3 behaves as Lewis acid.
AlCl3 becomes stable by dimerisation.
(ii) Reactivity towards air: Boron is un-reactive in crystalline form.
(iii) Reactivity towards acids and alkalies: Boron does not react with acids and alkalis and aluminium shows amphoteric character. (iv) Reactivity towards halogens
Anamalous properties of boron: (i) Tri-chlorides, bromides, and iodides of group 13 elements being covalent in nature are hydrolysed in water. (ii) The maximum covalence of B is 4. This is due to the absence of d-orbitals. On the other hand, for aluminium and other elements, the maximum covalence can be expected beyond 4. Some important compounds of boron:
(i) Borax: It is a white crystalline solid of formula . Borax is used in the manufacture of heat resistant glasses, glass wool, and fibre glass.
Mixture of sodium metaborate and boric anhydride is called Borax bead and it is used in flame test
(ii) Orthoboric acid (H3BO 3): It is a white crystalline solid with soapy touch. An aqueous solution of orthoboric acid is used as mild antiseptic.
Structure of boric acid
(iii) Diborane, B 2H6 Prepared by treating boron trifluoride (BF3 ) with LiAlH4 in diethyl ether. 4BF3 + 3LiAlH4 → 2B2 H6 + 3LiF + 3AlF3
It contains two bridging hydrogen atoms between two boron atoms. The bridging bonds are three-centre, two-electron bonds.
On reaction with ammonia it forms borazine. 3B2 H6 + 6NH3 → 2B3 N3 H6 + 12H2
Group 14 elements (carbon family): C, Si, Ge, Sn, Pb
Carbon is the most versatile element in the world. Valence shell electronic configuration: ns2 np2 Covalent radius: Increases from C to Si, but small increase from Si to Pb (due to presence of completely filled d- and f-orbitals) Ionisation enthalpy: Higher than those of corresponding group 13 elements
Electronegativity: Slightly more electronegative than corresponding group 13 elements
Physical properties: (i) C, Si are non-metals; Ge is metalloid; Sn, Pb are soft metals. (ii) Melting and boiling points of these elements are higher than those of group 13 elements.
Chemical properties (i) Reactivity towards oxygen: Two types of oxides – monoxide (MO) and dioxide (MO 2 ) (ii) Reactivity towards water: C, Si, Ge, Pb do not react with water.
(iii) Reactivity towards halogen: Form halides of formula MX2 and MX4 PbI4 does not exist Except CCl4, other tetrahalides are easily hydrolysed by water.
Carbon shows anamalous behaviour due to following reasons:
(i) smaller size, (ii) higher electronegativity (iii) higher ionisation enthalpy (iv) unavailability of d orbitals
Catenation: The tendency to link with one another through covalent bonds to form chains and rings; this property is called catenation. The order of catenation is C >> Si > Ge » Sn. Pb does not undergo catenation
Allotropes of carbon:
Diamond: 1. sp3 hybridisation 2. Four directional covalent bonds 3. Used for sharpening tools, ornaments, making dyes, manufacturing tungsten filament, etc.
Graphite: 1. sp2 hybridisation 2. three sigma bonds and one p bond 3. layered structure 4. conductor of electricity so it is used for making the electrodes present in batteries and in industrial electrolysis.
Fullerenes: 1. Cage-like molecules 2. C60 is called Buckminsterfullerene 3. 20 six-membered rings 4. 12 five-membered rings 5. sp2 hybridisation
Other Forms of Carbon
Carbon black is used as 1. a black pigment ink. 2. a filler in automobile. tyres
Coke is used as 1. fuel. 2. reducing agent in metallurgy.
Charcoal (amorphous form): Activated charcoal is used 1. for absorbing poisonous gas. 2. as water filters to remove organic contaminators. 3. in air condition systems, to remove odour.
Some important compounds of carbon and silicon:
(i) Carbon monoxide (CO): It is a colourless, odourless, and almost water insoluble gas. CO is a powerful reducing agent. Carbon in limited supply of air or oxygen yields carbon monoxide.
Mixture of CO(g) and H2 (g) → Water gas or synthesis gas
CO is a highly poisonous gas as it forms a complex with haemoglobin. It prevents haemoglobin in the red blood corpuscles from carrying oxygen, leading to death.
(ii) Carbon dioxide:
(i) Photosynthesis
(ii) Greenhouse gas (iii) Colourless and odourless gas and has low solubility in water. With water, it forms carbonic acid (H2 CO 3 ). (iv) Solid CO 2 is called dry ice. (v) sp hybridisation (vi) Resonance structure
Some important compounds of silicon: Silicon dioxide (SiO 2) (i) Known as silica. (ii) Occurs in several crystallographic forms such quartz, cristobalite, tridymite.
Three dimensional structure of SiO 2
Quartz is used as piezoelectric material.
Silicones Group of Organosilicon polymer
Silicates (i) The basic structural unit of silicate is
.
(ii) Man made silicates – Glass and cement Zeolites (i) These are aluminosilicate minerals. E.g. ZSM–5. (ii) Widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation.
