P-block elements III A elements (Group 13) P-block elements- III A P-block elements- III A
General Trend
The p-block elements The outermost electron enters one of the p-orbitals. There are six groups of p-block elements (Groups 13, 14, 15, 16, 17 and 18). The general outer electronic configuration is ns2 np1-6.
The covalent radii and metallic character increase on moving down the group and decrease on moving across a period. The ionization enthalpy, electronegativity and oxidizing power increase across a period and decrease down the group. Unlike the s-block elements, which are all reactive metals, the p- block elements comprise of both metals and non-metals. P-block elements- III A
General Trend
** Since the chemical behaviour of metals and non-metals vary, a regular gradation of properties is not observed in p-block elements. Nevertheless some generalizations may be drawn. P-block elements- III A
Difference in Chemical Behaviour of the First Element
The first member of each group differs in many respects from the other members.
These differences are quite striking in Groups 13-16.
The effects of small size, high electro-negativity and non-availability of d- orbitals for the first member are responsible for these differences.
Due to non-availability of d-orbitals, the first member can display a maximum coordination number of 4, whereas the others can display higher coordination numbers. P-block elements- III A
Difference in Chemical Behaviour of the First Element
Hence we come across species like [SiF6]2-, PCl5, PF5, SF6, but analogous species for carbon, nitrogen and oxygen are not known.
The first member, having small size and high electronegativity, can form pπ - pπ bonds with itself or other elements e.g. C = C, N = N, C = O, C = N, N = O etc.
The heavier members do not display pπ - pπ multiple bonding but can show pπ - dπ bonding. P-block elements- III A
Inert Pair Effect
The p-block elements display two oxidation states. the s-block elements display only one oxidation state, the group number.
The higher oxidation state is equal to the group number minus 10
(i.e. number of s and p electrons in the valence shell)
The lower one is two units less than the group number
(i.e. number of p-electron in the valence shell).
Example: Al has 13 electron, the higher oxidation state is 13-10= 3;
13Al: [Ne]10 3s23p1, the lower oxidation is 1. P-block elements- III A
Inert Pair Effect
The higher oxidation state is displayed only when both the ns and np electrons are involved in bond-formation.
The lower oxidation state is observed when only the np electron(s) participate in bond formation.
On moving down the group the ns electrons tend to remain inert and do not participate in bond formation.
This unwillingness of the outermost s orbital electron pair to participate in bond formation is called inert pair effect. P-block elements- III A Inert Pair Effect
The reason for this effect is explained in terms of bond energy.
Eenergy is needed to uncouple the s-electrons and on the other hand energy is released during bond formation.
If the energy released is sufficient to unpair the s-electrons, then they participate in bond formation, otherwise they do not.
The bond energy decreases down the group and hence inert pair effect is prominent for the lower members.
The lower oxidation state becomes more stable on descending the group (the inert pair effect). P-block elements- III A
Group-13 The elements in this group are: boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). The general electronic configuration is ns2np1. Atoms in this group have 3 valence electrons a full s orbital and one electron in the p orbital This group includes a metalloid (boron), and the rest are metals This family includes the most abundant metal in the earth’s crust (aluminum) P-block elements- III A
Group-13 The Group III elements are the first to really distinguish non-metallic and metallic character in the group. Boron, with an electronegativity of 2.0, very much forms covalent bonds. And given its odd number of electrons and inability to form four bonds to achieve an octet configuration, is involved in forming some remarkable compounds. Al is by far the most important of the elements in the row for two reasons, i. it is third only to Si and O in the earth’s crust, and ii. it is a smallest non-reactive metal, which makes it important in manufacturing. P-block elements- III A
Oxidation states and Bond Type
The common oxidation states are; +3 and +1. The +3 oxidation states are favorable except for the heavier elements. Heavier elements such as Tl prefers +1 oxidation state due to its stability. The inert pair effect
The stability of the +1 oxidation state, increases down the group. P-block elements- III A Oxidation states and Bond Type
Compounds of Ga (I), In (I) and Tl (I) are known.
Tl (I) compounds are more stable than Tl (III) which are oxidizing in nature.
Ga (I) compounds are reducing indicating that Ga (III) is more stable.
The higher oxidation state is generally covalent.
Boron is always covalent and does not form B3+ ion
The M3+ ions are associated with high hydration energies and hydrated cations are known.
However, some compounds of Al and Ga like AlCl3 and GaCl3 are covalent in the anhydrous state. P-block elements- III A
Physical properties
The elements of Group 13 have; 1) smaller atomic radii and 2) higher electronegativities compared to s-block elements. However, these properties do not vary in a regular manner. The atomic radius of Ga (135 pm) is slightly less than that of Al (143 pm). The electronegativity and ionization energy consequently are higher than expected. Ga contains ten ‘d’electrons Similarly, the inclusion of fourteen ƒ’ electrons on the inner core affect the size and ionization energy of Tl. P-block elements- III A Physical properties
The elements at top of the group are hard , covalent materials. those at bottom are soft metals, as reflected in their enthalpies of atomization and their melting points.
Also, they are hard and soft in terms of their Lewis acidity in the same order (elements at top are hard and those at bottom are soft ). The latter is correlated with polarizability of the atomic orbitals.
The First I.E. decreases down the group, but there is a minor hiccup at Ga. Similarly, the electronegativities are do not decrease smoothly. hard and soft in terms of Lewis acid and base for explaining stability of compound. 'Hard' applies to species which are small, have high charge states, and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable. The soft acids react faster and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds with hard bases hard and soft in terms of Lewis acid and base
Comparing tendencies of hard acids and bases vs. soft acids and bases
Property Hard acids and bases Soft acids and bases
atomic/ionic radius small large
Oxidation state high low or zero
Polarization low high
affinity Ionic bonding Covalent bonding P-block elements- III A Physical properties Some important physical constants of the Group 13 elements are shown in table below: P-block elements- III A
Chemical Properties
Elements of Group 13 are quite reactive
Much of the important chemistry of the group 13 elements can be understood on the basis of their electronic structure.
Since the elements have a [core]ns2 np1 electron configuration, neutral group
13 compounds can form up to three bonds.
This only provides for 6 electrons (not a complete octet) around the group 13 atom so such compounds are called “electron-deficient”. P-block elements- III A Chemical Properties
Boron’s chemistry is so different from that of the other elements.
Chemically boron is a non-metal, it has a tendency to form covalent bonds and displays similarities with silicon, which will be discussed later.
Boron combines with many metals to form borides e.g. MgB2, and Fe2B where it displays negative oxidation state. All elements except Tl, when treated with halogens, oxygen or sulphur form halides (MX3) oxides (M2O3) and sulphides (M2S3).
+1 Thallium forms TlX, Tl2O and Tl2S. ( as Tl ) P-block elements- III A Chemical Properties B and Al form nitrides by direct combination with nitrogen at very high temperature.
B and Al form carbides on heating with carbon.Aluminium carbide (Al4C3) on hydrolysis given methane.
Al4C3 + 12H2O 4Al(OH) 3+ 3 CH4
• Boron carbide (B12C3) is a hard, high melting, inert compound. Al has a very high affinity for oxygen
-1 (enthalpy of formation of Al2O3 is – 1676 KJ mol ) and is used to remove oxygen from other metal oxides. • This forms the basis of the Thermite process for extracting many metals from their oxides. 3 MnO2 + 4Al 2Al2O3 + 3Mn
Fe2O3+ 2Al Al2O3 + 2Fe P-block elements- III A
Chemical Properties
The reactions of the elements with acids differ. Boron reacts only with oxidizing acids to form boric acid
2B + 3H2SO4 2H3BO3 + 3SO2
B + 3HNO3 H3BO3 + 3 NO2
Boric acid is better represented as B(OH)3 and does not contain replaceable hydrogen.
The other elements react with dilute mineral acids to evolve hydrogen
2M + 6HCl 2MCl3 + 3H2 Al is render passive with concentrated nitric acid. P-block elements- III A
Chemical Properties
Boron liberates hydrogen when fused with alkali.
2B + 6NaOH 2Na3BO3 + 3H2 Al and Ga dissolve in alkali to form tetrahydroxoaluminate (III) and tetrahydroxogallate (III) respectively.
M + 4NaOH Na[M(OH) 4] + 2H2 P-block elements- III A Chemical Properties Because of their electron-deficient nature, group 13 compounds containing the element (M) in the (+3) oxidation state have a formally vacant npz orbital and usually act as Lewis acids (electron acceptors).
R R R Base + R M Base M or Base M R R R R R This is probably the most important feature of group 13 reagents and they are used in organic synthesis (e.g. Friedel-Crafts alkylation) and as catalysts or co-catalysts for many different kinds of chemical processes.
Me C2H4 + Zr + B(C6F5)3 Zr Me [MeB(C6F5)3] Me
polyethylene P-block elements- III A Chemical Properties group 13 compounds can also form “partial” multiple bonds with terminal atoms that contain lone pairs of electrons. The extent to which this happens depends on the energies of the AO’s involved
(the empty npz orbital and those of the lone pairs) and as you would expect from MO theory it happens mostly for boron. R R X M X M R R R O Where, for example, X = F N R R P-block elements- III A Chemical Properties For the heavier elements, “bridging” is often observed if there are no other electron donors to provide electron density to the vacant orbital. If the substituents contain lone pairs of electrons, the bridges can be formed from two -electron donor-acceptor bonds:
X R M M R R R X P-block elements- III A Chemical Properties
When the substituents do not have any lone pairs of electrons, the bridges can be formed from three-center-two-electron bonds Such bonds are readily explained by MO theory or a combination of VB and MO theory:
H H H B B H H H Diborane
Instead of using pure AO’s , MO two sp3 hybrids from each B and the two 1s AO’s for the bridging H atoms. P-block elements- III A Boron and Aluminum
There are a variety if allotropes of boron, the most famous being B12 All of them are formed in odd ways to achieve an octet configuration.
Boron is actually extracted from the earth as Na2B4O7 (borax). The red color of the material is due to an iron contaminant.
Al is present in thousands of minearals, however, the only important ore for Al is bauxite, which contains hydrated oxides such as
Al2O3.H2O. Bauxite is usually reddish-brown, but can also be white, tan, and yellow, depending on the type and concentration of iron minerals present. P-block elements- III A Applications of B and Al
B(OH)3 is a Lewis acid, boric acid “organic” way to kill pests. Boron also forms some very interesting compounds with nitrogen called boron nitrides, - have the same electronic configuration as graphite and C60 and consequently have prompted a lot of interest in them for new materials.
2B + 2NH3 2BN + 3 H2
Al is used in many manufacturing processes, most famously, the production of paper. Aluminum sulfate is known as papermaker’s alum.
Al2O3 + H2SO4 Al2 (SO4)3 + 3H2O P-block elements- III A Applications of B and Al
Al are well known as it is widely used in every day life, for example; Al foil, cooking pans, Al window sashes, etc. Al metal usually exceeds 99% purity, and the metal itself and its alloys are widely used. P-block elements- III A
Aluminum (Al)
Third most abundant element in the earth’s crust, which is found in compounds with oxygen and often with silicon. Al exists as aluminosilicates in the Earth’s crust. Relatively weak metal so it is often alloyed with Cu, Mg, Mn, etc. to produce a strong, but light materials (high strength to weight ratio) Easily oxidized. The thin, tough oxide layer produced provides a protective barrier. Al is used to produce silver and white flames and sparks. • It is a common component of sparklers and is often alloyed with Mg into magnalium for extra-bright fireworks. P-block elements- III A
Production of Al Initially, pure Al was very expensive because of the difficulty of extracting the metal from its oxide. Al metal is obtained by electrolysis of bauxite
(Al2O3) in Cryolite Na3AlF6. Al is recovered by the electrochemical Hall- Héroult process (relatively inexpensive way). The electrochemical process following overall
+3 -2 reaction : 4Al + 6O + 3C 4Al + 3CO2 Production of Al metal is Still very energy intensive, requiring large amounts of electricity. P-block elements- III A
Chemistry of Al Al metal dissolves in mineral acids, except concentrated nitric acid. Al metal dissolves in aqueous solutions of alkali metal hydroxides evolving hydrogen. Al forms compounds with most non-metallic elements and shows a rich chemistry, but unlike boron, no cluster hydrides are known. As oxide and halides, Al have already been described. Organo-aluminum compounds. P-block elements- III A
Compounds of Aluminum
Al2O3 (Aluminum oxide commonly know as alumina) Variety of crystal structures Many forms are important ceramic materials Some impure forms of alumina are ruby (Cr+3), sapphire (Fe+3 and Ti+4), and topaz (Fe+3 ) Amphoteric
The trace metals added to get the color: Cr+3 Fe+3 and Ti+4 Fe+3 P-block elements- III A Organoaluminum compounds Organo-Al compounds are used in large quantities for olefin polymerization. Olefin polymerization are industrially manufactured from Al metal, hydrogen, and an olefin as follows.
2 Al + 3H2 + 6 CH2 CHR →Al2(CH2CH2R)6 They are dimers except those with bulky hydrocarbyl groups.
. For example, trimethylaluminum, Al2(CH3)6, is a dimer in which methyl groups bridge aluminum atoms by electron deficient bonds. P-block elements- III A
Organoaluminum compounds
The Ziegler-Natta catalyst is olefin polymerization catalyst . devolped 1950s and the Nobel prize award in 1963. .Consists of an organoaluminium compound and a metal compound.
Organnoaluminum compounds are very reactive and burn spontaneously in air. They react violently with water and form saturated hydrocarbons, with aluminium changing to aluminium hydroxide as follows;
Al(CH2CH2)3 + 3H2O 3C2H6 +Al(OH)3 Therefore, they should be handled in the laboratory under a perfectly inert atmosphere. P-block elements- III A Gallium (Ga) The anomalous position of Ga affect its chemistry, and is a consequence of the Scandide contraction (d- block contraction). ** Scandide contraction the effect of having full d orbitals (d10) on the period 4 elements (Ga, Ge, As, Se and Br). This is reflected in the electron configuration of the element: It is the first in the group to have a set of filled d orbitals preceding the valence p orbitals. The very poor shielding of the d electrons results in a higher-than-expected effective nuclear charge on the valence electrons of Ga, and hence its anomalous behaviour.
We have already seen that Ga2H6 is more stable than the corresponding Al hydride, and in some ways Ga can act closer to boron than Al does. P-block elements- III A Thallium (Tl) Thallium, the first element in Group 13 ( A) to have a filled f orbital preceding the valence orbitals. This is called the Lanthanide contraction. . A similar effect as the Scandide contraction Another important factor, the primary influence of which is to greatly stabilize the +1 oxidation state of Tl compared to the group oxidation state of +3, is the inert-pair effect. This is caused by the greater separation in the energy of the ns and np orbitals on going down the group. The inert pair effect operates for all the heavy p-block elements, whereas the scandide and lanthanide contractions lose their importance with increasing group number (i.e. with the addition of more valence electrons within the p- subshell). P-block elements- III A
The composition of fireworks The main part of fireworks is concerned with pyrotechnics – it is a mixture of substances – produce an effect by heat, light, sound, gas, smoke or a combination of these – exothermic reactions that do not rely on oxygen from external sources. P-block elements- III A
5 basic ingredients 1. A fuel typically based on metal or metalloid powders, or black powder; 2. An oxidiser that produces oxygen to support the combustion of the fuel.
perchlorates (ClO4 - ), chlorates (ClO3 - ) or nitrates (NO3 - ); 3. Colourants, usually chloride salts of suitable metals such as Sr, Na or Cu; 4. A binder that holds the pellet together; 5. A chlorine donor to react with the colour-imparting metals, which will enhance the colour intensity.