We shall learn in this article about Chemistry 11th p- block elements: Notes of group 13. In p-block elements the last electrons enter the outermost p- orbitals. As we know that a set of p-orbitals can accommodate only six electrons as maximum hence there are six groups from 13 to 18. Boron, carbon, nitrogen, oxygen, fluorine and helium are the first element of these groups. Their valence shell electronic Configuration is ns2np1-6 (except He).
A lot of variation in properties of elements in a group of p-block is observed due to differences in inner core of electrons. The maximum oxidation state shown by a p-block elements is equal to the total number of valence electrons (i.e., the sum of the s and p electrons) and known as group oxidation state.
The heavier elements of this block show inert pair effect which increases the stability of their oxidation state that is two unit less than the group oxidation state. It is interesting to note that the non- metals and metalloids exist only in p-block.
Chemistry 11th p-block elements: Notes of group 13
Boron is a typical non metal, aluminum is a metal but shows many chemical similarities to boron, gallium and thallium are almost metallic in nature. Boron is a fairly rare element, mainly occurs as orthoboric acid (H3BO3), borax (Na2B4O7.10H2O) and kernite (Na2B4O7.4H2O). There are two isotopic forms of boron 10B(19%) and 11B(81%).
Occurrence of group 13 elements:
Aluminium is the most abundant metal of p-block elements and the third most abundant element in the earth crust (8.33%) by mass. Bauxite (Al2O3.2H2O) and cryolite Na3AlF6 are the important minerals of aluminium. Gallium, indium and thallium are less abundant in nature.
The atomic and physical properties of group 13 elements:
(1) Electronic Configuration: The outer electronic Configuration of these elements is ns2np1. In this group boron and aluminium have noble gas Core, gallium and indium have noble gas plus 10 d-electrons and thallium has 14 f- and 10 d- electrons in p-block elements.
(2) Atomic radii: In this group, atomic radius is expected to increase. However, a deviation can be seen. Atomic radius of Ga is less than that of Al due to presence of 10 d-electrons which offer only poor screening effects for the outer electrons from the increased nuclear change. The order of atomic radius is B< Ga < Al <I n < Tl
(3) Ionisation enthalpy: The Ionisation enthalpy values as expected from the general trends do not decrease smoothly down the group. These variations observed due to inability of d- and f-electrons which have low screening effects to compensate the increase in nuclear charge. The order of Ionisation enthalpy is as B > Tl > Ga > Al > In.
(4) Electronegativity: Down the group, Electronegativity first decreases from B to Al and then decrease marginally. This is due to discrepancies in atomic size of the elements.
(5) Physical properties: Boron is non metallic in nature. It is extremely hard and black coloured solid. Boron has unusually high m.p. Rest of the members are soft metals with low m.p. and high electrical conductivity. It is worthwhile to note that gallium with unusually low melting point (303K) could exist in liquid state during summer. Density of the elements increases down the group.
Chemical properties of group 13 elements:
(1) Oxidation state: Due to small size of boron, the sum of its first three Ionisation enthalpy is very high which forces it to form covalent compounds in +3 oxidation state. But as we move from B to Al, the sum of the first three Ionisation enthalpy of Al considerably decreases and is therefore able to form Al3+ ions. In fact, Al is a highly electropositive metal.
However, down the group, due poor screening effects of intervening d and f orbitals, the increased effective nuclear charge holds ns2 electrons tightly and causes inter pair effect. As a result only p-orbitals electrons may be involved in bonding. In fact in Ga, In and Tl, both +1 and +3 oxidation states are observed.
The relative stability of +1 oxidation state progressively increases for heavier elements. Al < Ga < In < Tl. In trivalent state, the number of electrons around the central atom in a molecule of the compound of these elements will be only six and these compounds are electron deficient and behave as a Lewis acid. In trivalent state, most of these compounds being covalent are hydrolysed in water and form tetravalent species [M(OH)4]. While AlCl3 in acidified aqueous solution forms octahedral species [Al(H2O)6].
(2) Reactivity towards air: Boron is unreactive in crystalline form. Al forms a very thin oxide layer on the surface which protects the metal from further attack. Amorphous B and Al on heating in air form B2O3 and Al2O3 respectively. With N2 at high temperature they form nitrides.
2E(s) + 3O2 (g) → 2E2O3(s)
2E(s) + N2(g) → 2EN(s)
The oxides of boron is acidic. The oxides of Al and Ga are amphoteric while those of In and Tl are basic.
(3) Reactivity towards acid and alkalies: Boron does not react with acids and alkalies. Aluminium dissolves in mineral acids and aqueous alkalies and show amphoteric character.
2Al(s) + 6HCl(aq) → 2Al3+(aq) + 6Cl–(aq) + 3H2(g)
However, Conc. HNO3 renders aluminium passive by forming a protective oxide layer on the surface. Aluminium also reacts with aqueous alkalies and liberates dihydrogen gas.
2Al(s) + 2NaOH(aq) + 6H2O (l) → 2Na+[Al(OH)4]–(aq) + 3H2(g)
(4) Reactivity towards halogens: The elements of group 13 reacts with halogens to form trihalides except TlI3.
2E(s) + 3X2(g) → 2EX3(s)
Where X= F, Cl, Br, I
Anomalous behaviour of Boron:
The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water and form tetrahedral and octahedral species except in boron exist in aqueous medium. The maximum covalency of B is 4 due to the absence of d-orbitals while in case of other elements, the maximum covalency can be expected beyond 4. Most of the other metals halides are dimerised through halogen bridging (Al2Cl6) while boron trihalides are monomeric and are strong Lewis acid.
Some important compounds of Boron:
Borax: Na2B4O7.10H2O
It is a white crystalline solid. Its formula is Na2B4O7.10H2O. Really, it is a tetranuclear unit and formula is Na2[B4O5(OH)4].8H2O. It dissolves in water to give an alkaline solution.
Na2B4O7 + 7H2O → 2NaOH + 4H3BO3 (orthoboric acid)
On heating, borax first losses water molecules and swells up. On further heating, it turns into a transparent liquid, which solidifies into glass like material known as borax bead.
Na2B4O7.10H2O → Na2B4O7 → 2NaBO2 + B2O3
The metaborate of many transition metals have characteristic Colour and therefore borax bead test can be used to identify them.
Orthoboric acid: H3BO3
This acid, H3BO3 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 in aqueous solution of borax.
Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4B(OH)3
It can be also formed by hydrolysis of most boron compounds like halides, Hydrides etc.
Structure of orthoboric acid:
It has a layer structure in which planar BO3 units are joined by H-bond.
Properties of Boric acid:
(1) It is a weak monobasic acid and acts as a Lewis acid by accepting electrons from a hydroxyl ion.
B(OH)3 + 2NaOH → [B(OH)4]– + H3O +
(2) On heating, orthoboric acid above 370K forms metaboric acid HBO2 which on further heating yields boric oxide (B2O3)
H3BO3 → HBO2 → B2O3
Diborane (B2H6)
Preparation of Diborane:
(1) It is prepared by treating boron trifluoride with LiAlH4 in diethyl ether.
4BF3 + 3LiAlH4 → 2B2H6 + 3LiF + 3AlF3
(2) In laboratory, Diborane can be prepared by the oxidation of Sodium borohydride with Iodine
2NaBH2 + I2 → B2H6 + 2NaI + H2
(3) On industrial scale, Diborane is produced by the reaction of BF3 with Sodium hydride.
2BF3 + 6NaH → B2H6 + 6NaF
Properties of Diborane:
(1) Diborane is a colourless, highly toxic gas with a b.p. of 180K. It can catches fire spontaneously upon exposure to air. It burns in oxygen releasing an enormous amount of energy.
4B2H6 + 3O2 → B2O3 + 3H2O
ΔfHo= -1976 KJmol-1
(2) Boranes are readily hydrolysed by water to give boric acid.
B2H6 + 6H2O → 2B(OH)3(aq) + 6H2(g)
(3) Diborane undergoes cleavage reactions with Lewis base to give borane adducts.
B2H6 + 2NMe3 → 2BH3.NMe3
B2H6 + 2CO → 2BH3.CO
(4) Borane reacts with ammonia to give B2H6.2NH3 which further heating on gives borazine (B3N3H6) known as inorganic benzene.
3B2H6 + 6NH3 → 3[BH2(NH3)4]+[BH4]– → 2B3N3H6 + 12H2
Structure of Diborane:
In the structure of Diborane, the four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. In this structure, the four terminal H-bonds are regular two centre – two electron bonds while the two bridge (B-H-B) bonds are three centre – two electron bonds known as banana bonds.
Summary of Chemistry 11th p- block elements
In this chapter, we have discussed about elements of this group 13 of p-block. Boron is a typical non- metal and the other members are metals. The availability of 3 valence electrons leads to the so called electron deficiency in boron compounds. Due to this deficiency, boron compounds behave as a Lewis acid. The important compounds of boron with dioxygen are boric acid and borax. Boric acid is a weak monobasic acid. The borax bead test gives characteristi- c colours of transition metals. Aluminium exhibits +3 oxidation state. With heavier elements +1 oxidation state gets progressively stabilized on going down the group. This is a consequence of the so called inert pair effect.
FAQ of Chemistry 11th p- block elements
Q.No 1. Why white fumes does appear around the bottles of anhydrous aluminium chloride ?
Ans:- Anhydrous aluminium chloride is partially hydrolysed with atmospheric moisture to liberate HCl gas. Moist HCl appears white in colour.
Q.No 2. Why Boron is unable to form BF63- ion ?
Ans:- Due to non availability of d-orbitals, boron is unable to expand its octet. Therefore, the maximum covalency of boron is 4.
Q.No 3 why is boric acid considered as a weak acid?
Ans:- Because it is not able to release H+ ions on its own. It receives OH– ions from water molecules to complete its octet and in turn releases H+ ions.
Q.No 4. What happens when aluminium is treated with dilute NaOH ?
Ans:- Aluminium reacts with dilute NaOH and liberates dihydrogen gas.