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Group 13 Metals

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Group 13 Metals 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.
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