P.A. Cox - Inorganic chemistry (793955), страница 35
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The species listed in Table 1 illustrate the wide variety ofisoelectronic relationships that exist between the compounds formed by elements in different groups and periods.Species with SN=4 are found throughout the p block, but ones with lower steric numbers and/or multiple bonding arecommon only in period 2. In analogous compounds with heavier elements the coordination and steric numbers are oftenincreased by polymerization (compare CO2 and SiO2,and) or by a change of stoichiometry (e.g.). Species with steric numbers higher than four require octet expansion and are not found in period 2.
Many of thespecies listed in Table 1 are referred to in Topics F2–F10 dealing with the appropriate elements.Ionic chemistrySimple monatomic anions are formed by only the most electronegative elements, in groups 16 and 17 (e.g. O2−,Cl−). Although C and N form some compounds that could be formulated in this way (e.g. Li3N and Al4C3), the ionicmodel is not very appropriate for these.
There are often structural differences between oxides or fluorides and thecorresponding compounds from later periods. These are partly due to the larger size and polarizability of ions, butcompounds of S, Se and Te are also much less ionic than oxides (see Topics D4, F7, F8 and F9).and); onesMany polyanions are known. Those with multiple bonding are characteristic of period 2 (e.g.with single bonding are often more stable for heavier elements (e.g.), and some form polymerized structures (seeTopic D5).
Simple cations are not a feature of nonmetal chemistry but some polycations such asandcan beformed under strongly oxidizing conditions. Complex cations and anions are discussed below.F1—INTRODUCTION TO NONMETALS151Table 1. A selection of molecules and ions (including polymeric forms) classified according to the valence electron count (VE) and the steric number(SN) of the central atom shown in bold typeAcid-base chemistryMany nonmetal oxides and halides are Lewis acids (see Topic C9). This is not so when an element has its maximumpossible steric number (e.g.
CF4, NF3 or SF6) but otherwise acidity generally increases with oxidation state. Suchcompounds react with water to give oxoacids, which together with the salts derived from them are commoncompounds of many nonmetals (see Topics D5 and F7). Compounds with lone-pairs are potential Lewis bases, basestrength declining with group number (15>16>17). In combination with ‘hard’ acceptors the donor strength decreasesdown a group (e.g. N≫ P>As) but with ‘soft’ acceptors the trend may be reversed.Ion-transfer reactions give a wide variety of complex ions, including ones formed from proton transfer (e.g.and OH−), halide complexes (e.g.
[PC14]+, [SF5]−), and oxoanions and cations (e.g.).Such ions are formed in appropriate polar solvents (see Topic E1) and are also known in solid compounds. The trends inBrønsted acidity of hydrides and oxoacids in water are described in Topic E2. pKa values of oxoacids may changemarkedly down a group as the structure changes (e.g. HNO3 is a strong acid, H3PO4 a weak acid; the elements Sb, Teand I in period 5 form octahedral species such as [Sb(OH)6]−, which are much weaker acids).
Brønsted basicity ofcompounds with lone pairs follows the ‘hard’ sequence discussed above (e.g. NH3>H2O>HF, and NH3≫ PH3> AsH3).Redox chemistryThe elements O, F, Cl and Br are good oxidizing agents. Compounds in high oxidation states (e.g. oxides and halides)are potentially oxidizing, those in low oxidation states (e.g. hydrides) reducing.
Oxidizing power increases with groupnumber, and reducing power correspondingly declines. The trends down each group are dominated by bond strengthchanges (see Topic C8). Between periods 2 and 3 bonds to hydrogen become weaker (and so hydrides become morereducing and the elements less oxidizing) whereas bonds to oxygen and halogens become stronger (and so oxides andhalides become less oxidizing). Compounds of AsV, SeVI and BrVII in period 4 are more strongly oxidizing thancorresponding ones in periods 3 or 5. This alternation effect can be related to irregular trends in ionization energies,associated with the way that electron shells are filled in the periodic table (see Topics A4 and A5).Section F—Chemistry of nonmetalsF2HYDROGENKey NotesThe elementHydrides of nonmetalsHydrides of metalsThe hydrogen bondDeuterium and tritiumRelated topicsHydrogen occurs on Earth principally in water, and is a constituent oflife.
The dihydrogen molecule has a strong covalent bond, which limitsits reactivity. It is an important industrial chemical.Nonmetallic elements form molecular hydrides. Bond strengths andstabilities decline down each group. Some have Brønsted acidic andbasic properties.Solid hydrides with some ionic character are formed by many metals,although those of d- and f-block elements are often nonstoichiometricand metallic in character. Hydride can form complexes such as AlH4−and many examples with transition metals.Hydrogen bound to a very electronegative element can interact with asimilar element to form a hydrogen bond. Hydrogen bonding isimportant in biology, and influences the physical properties of somesimple hydrides.Deuterium is a stable isotope occurring naturally; tritium isradioactive. These isotopes are used in research and in thermonuclearweapons.Chemical periodicity (B2)Industrialchemistry:Brønsted acids and bases (E2)catalysts (J5)The elementHydrogen is the commonest element in the Universe and is a major constituent of stars.
It is relatively much lesscommon on Earth but nevertheless forms nearly 1% by mass of the crust and oceans, principally as water and inhydrates and hydroxide minerals of the crust. It is ubiquitous in biology (see Topics J1–J3).The dihydrogen molecule H2 is the stable form of the element under normal conditions, although atomichydrogen can be made in the gas phase at high temperatures, and hydrogen may become a metallic solid or liquid atextremely high pressures. At 1 bar pressure, dihydrogen condenses to a liquid at 20 K and solidifies at 14 K, these beingthe lowest boiling and melting points for any substance except helium.
The H-H bond has a length of 74 pm and adissociation enthalpy of 436 kJ mol−1. This is the shortest bond known, and one of the strongest single covalent bonds.Although it is thermodynamically capable of reacting with many elements and compounds, these reactions often have alarge kinetic barrier and require elevated temperatures and/or the use of catalysts (see Topic J5).F2—HYDROGEN153Dihydrogen is an important industrial chemical, mostly made from the steam re-forming of hydrocarbons frompetroleum and natural gas. The simplest of these reactions,is endothermic, and temperatures around 1400 K are needed to shift the equilibrium to the right.
Major uses ofhydrogen are in the synthesis of ammonia, the hydrogenation of vegetable fats to make margarine, and the production oforganic chemicals and hydrogen chloride (see Topic J4).Hydrides of nonmetalsHydrogen forms molecular compounds with nonmetallic elements. Table 1 shows a selection. With the exception of theboranes (see Topic F3) hydrogen always forms a single covalent bond. Complexities of formula or structure arise fromthe possibility of catenation, direct element-element bonds as in hydrogen peroxide, H-O-O-H, and in many organiccompounds. The International Union of Pure and Applied Chemistry (IUPAC) has suggested systematic names ending in-ane, but for many hydrides ‘trivial’ names are still generally used (see Topic B5).
In addition to binary compounds,there are many others with several elements present. These include nearly all organic compounds, and inorganicexamples such as hydroxylamine, H2NOH. The substitutive system of naming inorganic compounds derived fromhydrides is similar to the nomenclature used in organic chemistry (e.g. chlorosilane, SiH3Cl; see Topic B5).Table 1 shows the bond strengths and the standard free energies of formation of hydrides. Bond strengths andthermodynamic stabilities decrease down each group.
Compounds such as boranes and silanes are strong reducingagents and may inflame spontaneously in air. Reactivity generally increases with catenation.Table 1. A selection of nonmetal hydrides (E indicates nonmetal)aIUPACrecommended systematic names that are rarely used.values for compounds decomposing before boiling at atmospheric pressure.bExtrapolated154SECTION F—CHEMISTRY OF NONMETALSGeneral routes to the preparation of hydrides include:(i) direct combination of elements:(ii) reaction of a metal compound of the element with a protonic acid such as water:(iii) reduction of a halide or oxide with LiAlH4 or NaBH4:Route (ii) or (iii) is required when direct combination is thermodynamically unfavorable (see Topic B6). Catenatedhydrides can often be formed by controlled pyrolysis of the mononuclear compound.Brønsted acidity arises from the possibility of transferring a proton to a base, which may sometimes be the samecompound (see Topic E2 for discussion of trends).
Basicity is possible when nonbonding electron pairs are present (seeTopics C1 and C9). Basicity towards protons decreases towards the right and down each group in the periodic table, sothat ammonia is the strongest base among simple hydrides.Hydrides of metalsNot all metallic elements form hydrides. Those that do may be classified as follows.• Highly electropositive metals have solid hydrides often regarded as containing the H− ion. They have structuressimilar to halides, although the ionic character of hydrides is undoubtedly much lower.
Examples include LiH(rocksalt structure) and MgH2 (rutile structure; see Topic D3).• Some d- and f-block elements form hydrides that are often metallic in nature, and of variable (nonstoichiometric)composition. Examples include TiH2 and CeH2+x.• Some heavier p-block metals form molecular hydrides similar to those of nonmetals in the same group, examplesbeing digallane (Ga2H6) and stannane (SnH4), both of very low stability.Hydrides of more electropositive elements can be made by direct reaction between elements.
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