P.A. Cox - Inorganic chemistry (793955), страница 43
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Aluminum and theelements of groups 1 and 2 are classed as pre-transition metals, theremaining ones as post-transition metals.Formation of compounds with positive ions depends on a balancebetween ionization energies and lattice or solvation energies. Posttransition metals have higher ionization energies and are lesselectropositive than pre-transition metals.Trends down groups 1 and 2 are dominated by increasing ionic size. Inlater groups the structural and bonding trends are less regular, and thereis an increased tendency to lower oxidation states, especially in period6.Many of the elements can form anionic species.
Compounds withcovalent bonding are also known: these include organometalliccompounds and (especially with post-transition metals) compoundscontaining metal-metal bonds.The periodic table (A4)Trends in atomic propertiesChemical periodicity (B2)(A5)Lattice energies (D6)ScopeThe transition metals and the lanthanides and actinides have characteristic patterns of chemistry and are treated inSections H and I. The remaining non-transition metals include the elements of group 12 although they are formallypart of the d-block, as the d orbitals in these atoms are too tightly bound to be involved in chemical bonding and theelements do not show characteristic transition metal properties (see Topic G4).Figure 1 shows the position of non-transition metals in the periodic table. They fall into two classes with significantlydifferent chemistry. The pre-transition metals comprise groups 1 and 2 and aluminum in group 13.
They are‘typical’ metals, very electropositive in character and almost invariably found in oxidation states expected for ions in anoble-gas configuration (e.g. Na+, Mg2+, Al3+). In nature they occur widely in silicate minerals, although weatheringprocesses give rise to concentrated deposits of other compounds such as halides (e.g.
NaCl, CaF2) carbonates (CaCO3)and hydroxides (AlO(OH)) (see Topic J2).Metallic elements from periods 4–6 in groups following the transition series are post-transition metals. They areless electropositive than the pre-transition metals and are typically found in nature as sulfides rather than silicates.
TheyG1—INTRODUCTION TO NON-TRANSITION METALS189Fig. 1. Position of non-transition metals in the periodic table, with post-transition metals shaded.form compounds with oxidation states corresponding to d10 ions where s and p electrons have been ionized (e.g. Cd2+,In3+, Sn4+) but these are less ionic in character than corresponding compounds of pre-transition metals. In solution,post-transition metals form stronger complexes than with pre-transition metals.
Lower oxidation states (e.g. Tl+, Sn2+)are also common.Positive ionsThe formation of ionic compounds depends on a balance of energies as illustrated for NaCl in Topic D6, Fig. 1. Energyinput required to form ions must be compensated by the lattice energy of the compound. For ions in solution, a similarcycle could be drawn, including the solvation energy rather than the lattice energy. For group 1 atoms with the (ns)1configuration, the second ionization energy involves an electron from an inner shell and is so large that the extra latticeor solvation energy obtainable with M2+ cannot compensate for it. For group 2 elements with the (ns)2 configuration thesecond ionization energy is more than compensated by extra lattice energy.
Thus M2+ compounds are expected, a solidsuch as CaF(s) having a strong tendency to disproportionate.Figure 2 gives some data for groups 2 and 12 that are relevant in understanding the trends in pre- and post-transitionmetal groups. Ionization energies decrease, and ion sizes increase, down group 2 (see Topic A5). Increasing size givessmaller lattice energies, and so a decrease in ionization energy is also required if the electropositive character is to beretained. This happens in groups 1 and 2, and the electrode potentials shown in Fig.
2 become slowly more negative forthe lower elements.Group 12 atoms have the electron configuration ((n−1)d)10 (ns)2 and also form positive ions M2+ by removal of the selectrons. Filling the d shell from Ca to Zn involves an increase of effective nuclear charge that raises the ionizationenergy and reduces the ionic radius. Lattice energies for Zn2+ are expected to be somewhat larger than for Ca2+, andthe formation of Zn2+ is also assisted by the slightly lower sublimation energy of metallic zinc. Nevertheless, thesefactors do not compensate fully for the increased ionization energy, and so zinc is less electropositive (less negativevalue) than calcium. On descending group 12, ionization energies do not decrease to compensate for smaller latticeenergies as they do in group 12, andvalues increase down the group.
This is particularly marked withmercury, where especially high ionization energies result from the extra nuclear charge consequent on filling the 4fshell in the sixth period, combined with relativistic effects (see Topic A5).190SECTION G—CHEMISTRY OF NON-TRANSITION METALSFig. 2. Data for formation of M2+ ions of groups 2 and 12, showing (a) ionic radii, (b) sublimation enthalpies of the elements, (c) sum of the firsttwo ionization energies, and (d) standard electrode potentials.G1—INTRODUCTION TO NON-TRANSITION METALS191Group trendsThe above analysis shows how electropositive character remains strong throughout pre-transition groups. The majorvertical trends in the stability and structure of compounds result from the changing ionic size. The small radius of Li+and Be2+ gives some peculiarities, which are sometimes described as diagonal relationships. Thus the solubilitiesand thermal stabilities of lithium compounds are often closer to those of magnesium than to those of other group 1 elements.Beryllium has even more marked differences from the rest of group 2, showing similarities with its diagonal neighboraluminum.
These relationships can be related to the size/charge ratio of ions. The small ion Li+ gives lattice andsolvation energies more similar to Mg2+ than to Na+. The very small Be2+ is comparable with Al3+ in its polarizingpower, which produces deviations from ionic character in solid-state and solution chemistry.Size also increases down post-transition metal groups but the chemical trends are less regular.
Solid compounds oftenhave lower coordination numbers than expected by comparison with pre-transition metal ions of similar size, and havepatterns of stability and solubility that suggest an appreciable degree of covalent bonding. The changing balance betweenionization and lattice (or solvation) energies also has the consequence that lower oxidation states become morefavorable. These tendencies are especially marked in period 6 (Hg, Tl, Pb, Bi). Thus many TlI and PbII compounds areknown, the states TlIII and PbIV being strongly oxidizing (see further discussion in Topic G5). The inert-pair effect isa somewhat misleading term for this phenomenon, implying the existence of an electron pair (ns)2 too tightly bound tobe involved in bonding.
In fact, the ‘inert pair’ can have important structural consequences (see Topic G6). Thediscussion above also emphasizes that the relative stability of oxidation states always depends on a balance of factors, noton ionization energies alone.Non-cationic chemistryAlthough cationic chemistry has been emphasized above, other types of bonding are possible with the elements of allgroups in this Section. These include the following.• Covalent compounds. Compounds with predominantly covalent character include organometallic compounds.• Anionic compounds. Under unusual conditions, group 1 elements can form anions such as Na−. Some posttransition elements form polyatomic ions.• Metal-metal bonding.
This is especially a feature of post-transition groups and can accompany many ‘unusual’oxidation states, of which Hg1 (in fact) is the commonest example.Section G—Chemistry of non-transition metalsG2GROUP 1: ALKALI METALSKey NotesThe elementsSolution chemistrySolid compoundsOrganometallic compoundsRelated topicAll elements are found in silicates; sodium and potassium aremore abundant and occur in chloride deposits. The elements arevery electropositive and reactive.M+ aqua ions show only weak complexing properties except withmacrocyclic ligands. The elements form strongly reducingsolutions in liquid ammonia.Very ionic compounds are formed with halides, oxides and manycomplex ions.
The heavier elements form superoxides, peroxidesand some sub-oxides. Alkalides (containing M− ions) andelectrides can be made.Lithium alkyls such as Li4(CH3)4 are oligomeric compounds withmulticenter bonding. Organometallic compounds of the heavierelements are more ionic and less stable.Introduction to non-transition metals (G1)The elementsThe elements of group 1 are collectively known as alkali metals after the alkaline properties of their hydroxides suchas NaOH.
The atoms have the (ns)1 electron configuration and the M+ ions are therefore easily formed. Alkali metals arethe most electropositive of all elements, and their compounds among the most ionic. Some group trends are shown inTable 1. Roughly constant electropositive character is maintained down the group by parallel fall in atomization,ionization, and lattice or hydration energies (see Topic G1).
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