Group 13 Elements ( Boron Family)

Size: px

Start display at page:

Download "Group 13 Elements ( Boron Family)"

Marshall Byrd
6 years ago
Views:

1 Unit -11 The p- Block Elements Total no. of groups- 6 ( Group 13-18) General electronic configuration- ns 2 np 1-6 ( except for He) The first member of p-block differs from remaining members of their corresponding groups in two major respects- a. Small size b. Non availability of d- orbitals e.g. B forms [BF 4 ] - while Al forms [AlF 6 ] 3- Ability to form pπ pπ multiple bonds to itself ( C=C, C C, N N) and to other second row elements ( C=O, C N, N=O) B ( Non metal); Al, Ga, In, Tl ( Metals) Outer Electronic configuration - ns 2 np 1 Group 13 Elements ( Boron Family) Atomic Radii- On moving down the group, atomic radius increases due to addition of extra shell. Deviation- Atomic radius of Ga < Al due to poor shielding effect of 3d electrons. Ionisation Enthalpy: 13 Al [Ne] 3s 2 3p 1 31Ga [Ar] 3d 10 4s 2 4p 1 On moving down the group, IE should decrease but there is deviation between Al and Ga (Al<Ga due to poor shielding effect of 3d electrons, and between In and Tl ( In <Tl due to poor shielding effect of 5d and 4f electrons) 49 In [Kr] 4d 10 5s 2 5p 1 81Tl [Xe] 4f 14 5d 10 6s 2 6p 1 Electronegativity- On moving down the group, Electronegativity should decrease.

2 It decreases from B to Al and then increases due to discrepancies in atomic sizes. Density - On moving down the group, it increases. OXIDATION STATE a.max. oxidation state/ Group oxidation state = total no. of valence electrons = 3 b. B does not form +3 ions because sum of first three IE is very high. So, it forms covalent compounds. c. Al forms Al +3. d. Inert Pair Effect- On moving down the group, due to poor shielding of d and f electrons, the increased effective nuclear charge holds ns electrons tightly, restricting their participation in bonding. This is inert pair effect. (The occurrence of oxidation state two unit less than the group oxidation state.) So, Ga, In, Tl show both +1 and +3 oxidation state. On moving down the group, from Ga to Tl, stability of +1 oxidation state increases. e. In +3 oxidation state, electron deficient compounds are formed ( 6 electrons around central atom). So, they have tendency to accept a pair of electrons and behave as Lewis acids. The tendency to behave as Lewis acids decreases down the group. Boron fluorides and chlorides easily accept a lone pair of electrons from NH 3. BF 3 + NH 3 BF 3.NH 3 BCl 3 + NH 3 BCl 3.NH 3 REACTIVITY TOWARDS AIR- Crystalline Boron is unreactive. Amorphous boron and aluminium form their oxides and nitrides on heating. 2E (s) + 3O 2 (g) Δ 2E 2 O 3 (s) 2E (s) + N 2 (g) Δ 2EN(s) Nature of oxides- Oxide of B ( acidic)

3 Oxide of Al and Ga ( amphoteric) Oxide of In and Tl ( basic) REACTIVITY TOWARDS Acids and Alkalies- Boron does not react. Al reacts with both and shows amphoteric character. 2Al (s) + 6HCl (aq) 2Al (s) + 2NaOH (aq) + 6H 2 O (l) 2Al 3+ (aq) + 6Cl - (aq) + 3H 2 (g) 2Na + [Al(OH) 4 ] - (aq) +3H 2 (g) Sod. Tetrahydroxoaluminate(III) With conc. HNO 3, Al forms a protective oxide layer on the surface. REACTIVITY TOWARDS HALOGENS- 1.They react with halogens to form trihalides except TlI 3 ( inert pair effect) 2E(s) + 3X 2 (g) 2EX 3 (s) ( X=F, Cl, Br,I) 2. Trihalides of BORON are strong Lewis acids. BF 3 + NH 3 BF 3.NH 3 BCl 3 + NH 3 BCl 3.NH 3 ANOMALOUS PROPERTIES OF BORON- 1. The trichlorides, bromides and iodides of Gp 13 elements are covalent in nature [ e.g. BCl 3, AlCl 3, GaCl 3, AlBr 3 etc.]. They are hydrolysed in water and form [M(OH) 4 ] - (sp 3 ), [M(H 2 O 6 ] 3+ (sp 3 d 2 ). BORON does not form [M(H 2 O 6 ] Halides of both B and Al are Lewis acids but only Aluminium halide exist as dimmers whereas Boron halide exist as monomer. 3. Maximum covalency of BORON is 4 due to absence of d orbitals e.g. B forms [BF 4 ] - (covalency is 4) Al forms [AlF 6 ] 3- (covalency is 6) Boron forms only covalent compounds while other members of the group form both ionic and covalent compounds. E.g. BF 3 : covalent

4 AlF 3 : ionic SOME IMPORTANT COMPOUNDS OF BORON 1. BORAX White crystalline solid Formulae Na 2 B 4 O 7.10H 2 O On heating, Na 2 B 4 O 7.10H 2 O Na 2 B 4 O 7 NaBO 2 + B 2 O 3 Boric anhydride 2. ORTHOBORIC ACID Sod. Metaborate Borax bead White crystalline solid Formulae H 3 BO 3 On heating, H 3 BO 3 HBO 2 B 2 O 3 Metaboric acid Boric anhydride Boric acid is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion It does not release H + ions on its own. It releases H + ions from water by accepting OH - from it. B(OH) 3 + 2HOH [B(OH) 4 ] - + H 3 O + 3. DIBORANE Colourless, highly toxic gas Formula B 2 H 6 Lab method of preparation: 2NaBH 4 + I 2 B 2 H 6 + 2NaI + H 2 Industrial method of preparation: 2BF 3 + 6NaH B 2 H 6 + 6NaF Structure of Diborane: Each Boron atom is sp 3 hybridised. Out of the four sp 3 hybrid orbitals, one is without an electron. The four terminal H atoms and the two B atoms lie in one plane. Above and below this plane are two bridge bonds. The terminal B-H bonds are normal 2- centre-2-electron bonds but the two bridge bonds are 3- centre-2-electron bonds also referred as banana bonds.

5 Properties of Diborane: 1. Colourless, highly toxic gas 2. Reaction of ammonia with diborane gives Borazine [ B 3 N 3 H 6 ] also known as inorganic benzene. Group 14 Elements ( Carbon Family) C 6 ( non metal), Si 14 ( non metal), Ge 32 (metalloid), Sn 50 (metal), Pb 82 (metal) Outer Electronic configuration - ns 2 np 2 Atomic Radii- On moving down the group, atomic radius increases due to addition of extra shell. Ionisation Enthalpy: On moving down the group, IE should decrease but there is deviation between Sn and Pb (Sn<Pb due to poor shielding effect of 5d and 4f electrons) Sn 50 [Kr] 4d 10 5s 2 5p 2 Pb 82 [Xe] 4f 14 5d 10 6s 2 6p 2 Electronegativity- On moving down the group, Electronegativity should decrease. But, Si to Pb are almost the same. They are more electronegative than Gp 13 because they are smaller in size. Density - On moving down the group, it increases.

6 OXIDATION STATE a. Common oxidation states = +4 and +2 b. Because sum of first four IE is very high. So, compounds in +4 oxidation state are generally covalent in nature. c. Inert Pair Effect- On moving down the group, due to poor shielding of d and f electrons, the increased effective nuclear charge holds ns electrons tightly, restricting their participation in bonding. This is inert pair effect. (The occurrence of oxidation state two unit less than the group oxidation state.) On moving down the group, stability of +2 oxidation state increases. So, Pb compounds in +2 state are stable and in +4 oxidation state are strong oxidizing agents. GeCl 2 and SnCl 2 are good reducing agents. PbO 2 and (CH 3 COO) 4 Pb are good oxidizing agents. REACTIVITY TOWARDS OXYGEN- All members when heated in oxygen form oxides of formula MO and MO 2. Oxides in higher oxidation states of elements are generally more acidic than those in lower oxidation states. CO 2, SiO 2 and GeO 2 SnO 2, PbO 2 CO GeO SnO, PbO Acidic Amphoteric Neutral Acidic Amphoteric REACTIVITY TOWARDS WATER- C, Si, Ge, Pb are not affected by water. Sn decomposes steam. Sn + 2H 2 O SnO 2 + 2H 2 REACTIVITY TOWARDS HALOGEN- They form halides of formula MX 2 and MX 4 ( where X= F, Cl, Br, I)

7 Elements Stability of MX 2 Stability of MX 4 C Si Ge increases decreases Sn Pb PbX 2 is more stable than PbX 4.( inert pair effect) ANOMALOUS BEHAVIOUR OF CARBON- 1. Due to non availability of d orbital, it shows maximum covalence of four. 2. It has unique ability to form pπ pπ multiple bonds to itself ( C=C, C Ξ C) and to other atoms of small size and high electronegativity. E.g. ( C=O, C Ξ N) 3. Catenation It has tendency to link with one another through covalent bonds to form chains or rings.this is because C C bonds are very strong. SOME IMPORTANT COMPOUNDS OF CARBON- A. CARBON MONOXIDE Preparation- Commercially it is prepared by passing steam over hot coke. The mixture of CO and H 2 thus produced is known as water gas or synthesis gas. C (s) + H 2 O (g) K CO (g) + H 2 (g) Water gas When air is used instead of steam, a mixture of CO and N 2 is produced, which is called producer gas. C (s) + O 2 (g)+ 4N 2 (g) 1273 K 2 CO (g) + 4N 2 (g) Producer gas

8 Properties and Uses- 1. CO undergoes combustion forming CO 2 with the liberation of heat. So, water gas and producer gas are important industrial fuels. 2. CO is a powerful reducing agent. So, it is used in the extraction of many metals from their oxides ores. Fe 2 O 3 (s) +3CO(g) Δ 2Fe (s) + 3CO 2 (g) ZnO (s) +3CO(g) Δ Zn (s) + CO 2 (g) 3. In CO molecule, there are one sigma and two pi bonds between C and O. Due to lone pair of electrons on C, CO molecule acts as a donor and reacts with certain metals to form METAL CARBONYLS. 4. POISONOUS NATURE- CO forms a stable complex with haemoglobin and prevents haemoglobin in the RBCs from carrying oxygen round the body and ultimately resulting in death. B. CARBON DIOXIDE- Preparation- By complete combustion of C and C containing fuels C (s) +O 2 (g) CO 2 (g) CH 4 (g) +2O 2 (g) Δ CO 2 (g) + H 2 O (g) Properties and Uses - 1. PHOTOSYNTHESISh 6 CO H 2 O C 6 H 12 O O H 2 O Chlorophyll 2. GREEN HOUSE EFFECT- As CO 2 content increases, temperature of the atmosphere increases. 3. DRY ICE- Solid CO 2 is dry ice. It is used as a refrigerant for ice cream and frozen food. 4. FIRE EXTINGUISHER- Being heavy and non supporter of combustion, it is used as fire extinguisher.

Subject: Chemistry Chapter 11: P-Block Element

One mark questions Subject: Chemistry Chapter 11: P-Block Element 1. What are p-block Elements? p-block elements are those in which the lost electron enters the p orbital of the outermost shell. 2. Is