The Alkali Metals

The H ion is also a strong reducing agent, as in this example  

Covalent (Molecular) Hydrides Hydrogen reacts with nonmetals to form many covalent hydrides. In most of them, hydrogen has an oxidation number of +1 because the other nonmetal has a higher electronegativity. 

Conditions for preparing covalent hydrides depend on the reactivity of the other nonmetal. For example, with stable, triple-bonded N2, the reaction needs high temperatures ( 400°C), high pressures ( 250 atm), and a catalyst  

Industrial facilities throughout the world use this reaction to produce millions of tons of ammonia each year for fertilizers, explosives, and synthetic fibers. On the other hand, hydrogen combines rapidly with reactive, single-bonded F2, even at extremely low temperatures (—196°C)  

The intense nucleophilic reactivity of these compormds is ascribed to the presence of nearly a full unit of negative charge on one of the carbon atoms. n-Butyl-lithium, on the other hand, is a colourless liquid which readily dissolves in saturated hydrocarbons, as a hexamer (Bu°Li) , believed to have an electron-deficient covalent constitution though the lithium-carbon bonds are certainly highly polar. The more covalent character of lithium alkyls is likely to be due mainly to the smaller radius and higher polarizing power of lithium relative to sodium. 

The structure of methylpotassium is different from those of methyl-lithium and -sodium, and is consistent with an ionic constitution, K Me , each methyl group being surrounded by six potassium atoms in a trigonal prismatic arrangement. 

Whereas the negative charge on a phenyl anion would be mainly confined to one doubly occupied orbital of the a-framework, 

Addition compounds such as sodium- or lithium-naphthalene, Li [CjoHg ], are to be distinguished from substitution compounds such as 1- or 2-naphthyl-lithium, CxoH7Li. The formation of hydrocarbon anions is a typical property of alkali metals only on account of their relatively low ionization potentials. Aromatic hydrocarbons whose electron afSnities are high also form anions on reaction with some of the more electropositive Group II metals (e.g. Mg, Ca). The subject has been very clearly reviewed (de Boer, see p 119). 

The alkali metals occupy the first column of the periodic table. Their electronic configurations are the following  

What we see is that each alkali metal has a single outermost (valence) electron in an s subshell. That commonality gives each element similar properties. For example, they are all extremely reactive metals. Each alkah metal reacts readily, sometimes even explosively, with water. Therefore, none of them is found in nature in pure form. All of these elements form +1 ions—Li, Na, K, etc. The reason for that is that by losing one electron, alkali metals become isoelectronic (having the same number of electrons) as the noble gas elements in the preceding rows. Therefore, like the noble gases, alkali metals are extremely stable entitles. 

The only compounds alkali metals form are ionic compounds such as NaCl, NaNOj, KI, and K COj. These compounds all tend to be soluble in water. They tend to be fairly soft metals with a low melting point (for metals). Like metals in general, they conduct heat and electricity. 

In Sections 23.5 through 23.7 we will study the chemistry of selected metals from Group lA (the alkali metals). Group 2A (the alkaline earth metals), and Group 3A (aluminum). 

As a group, the alkali metals (the Group lA elements) are the most electropositive (or the least electronegative) elements known. They exhibit many similar properties, some of which are listed in Table 23.4. Based on their electron configurations, we expect the oxidation number of these elements in their compounds to be +1 because the cations would be isoelectronic with the preceding noble gases. This is indeed the case. 

Periodic Table—properties of the alkali and alkaline earth metals. 

Metallic potassium cannot be easily prepared by the electrolysis of molten KCl because it is too soluble in the molten KQ to float to the top of the cell for collection. Moreover, it vaporizes readily at the operating temperatures, creating hazardous conditions. Instead, it is usually obtained by the distillation of molten KQ in the presence of sodium vapor at 892°C. The reaction that takes place at this temperature is 

Sodium and potassium are both extremely reactive, but potassium is the more reactive of the two. Both react with water to form the corresponding hydroxides. In a limited supply of oxygen, sodium bums to form sodium oxide (Na20). In the presence of excess oxygen, however, sodium 

Standard reduction potential (V)t -3.05 Refers to the cation M , where M denotes an alkali metal atom. -2.71 2.93 2.93 -2.92 

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An emulsifying agent generally produces such an emulsion that the liquid in which it is most soluble forms the external phase. Thus the alkali metal soaps and hydrophilic colloids produce O/W emulsions, oil-soluble resins the W/O type (see emulsion). 

It is also possible to explain, from hydration models, the differences between equally-charged cations, such as the alkali metals = 73,5, = 50,1 land 38.68, all in units of mor cm ). From atomic... 

Of the quantities shown in figure A2.4.8 is measurable, as is Sp, but the remainder are not and must be calculated. Values of 1-2 V have been obtained for although smaller values are found for the alkali metals. 

One current limitation of orbital-free DFT is that since only the total density is calculated, there is no way to identify contributions from electronic states of a certain angular momentum character /. This identification is exploited in non-local pseudopotentials so that electrons of different / character see different potentials, considerably improving the quality of these pseudopotentials. The orbital-free metliods thus are limited to local pseudopotentials, connecting the quality of their results to the quality of tlie available local potentials. Good local pseudopotentials are available for the alkali metals, the alkaline earth metals and aluminium [100. 101] and methods exist for obtaining them for other atoms (see section VI.2 of [97]). 

The full quantum mechanical study of nuclear dynamics in molecules has received considerable attention in recent years. An important example of such developments is the work carried out on the prototypical systems H3 [1-5] and its isotopic variant HD2 [5-8], Li3 [9-12], Na3 [13,14], and HO2 [15-18], In particular, for the alkali metal trimers, the possibility of a conical intersection between the two lowest doublet potential energy surfaces introduces a complication that makes their theoretical study fairly challenging. Thus, alkali metal trimers have recently emerged as ideal systems to study molecular vibronic dynamics, especially the so-called geometric phase (GP) effect [13,19,20] (often referred to as the molecular Aharonov-Bohm effect [19] or Berry s phase effect [21]) for further discussion on this topic see [22-25], and references cited therein. The same features also turn out to be present in the case of HO2, and their exact treatment assumes even further complexity [18],... 

H3 (and its isotopomers) and the alkali metal triiners (denoted generally for the homonuclears by X3, where X is an atom) are typical Jahn-Teller systems where the two lowest adiabatic potential energy surfaces conically intersect. Since such manifolds of electronic states have recently been discussed [60] in some detail, we review in this section only the diabatic representation of such surfaces and their major topographical details. The relevant 2x2 diabatic potential matrix W assumes the fomi... 

The alkali metals of Group I are found chiefly as the chlorides (in the earth s crust and in sea water), and also as sulphates and carbonates. Lithium occurs as the aluminatesilicate minerals, spodimene and lepidolite. Of the Group II metals (beryllium to barium) beryllium, the rarest, occurs as the aluminatesilicate, beryl-magnesium is found as the carbonate and (with calcium) as the double carbonate dolomite-, calcium, strontium and barium all occur as carbonates, calcium carbonate being very plentiful as limestone. 

The alkali metals have the interesting property of dissolving in some non-aqueous solvents, notably liquid ammonia, to give clear coloured solutions which are excellent reducing agents and are often used as such in organic chemistry. Sodium (for example) forms an intensely blue solution in liquid ammonia and here the outer (3s) electron of each sodium atom is believed to become associated with the solvent ammonia in some way, i.e. the system is Na (solvent) + e" (sohem). 

As with the hydroxides, we find that whilst the carbonates of most metals are insoluble, those of alkali metals are soluble, so that they provide a good source of the carbonate ion COf in solution the alkali metal carbonates, except that of lithium, are stable to heat. Group II carbonates are generally insoluble in water and less stable to heat, losing carbon dioxide reversibly at high temperatures. 

The properties of lithium resemble those of the alkaline earth metals rather than those of the alkali metals. Discuss this statement. 

The alkali metal tetrahydridoborates are salts those of sodium and potassium are stable in aqueous solution, but yield hydrogen in the presence of a catalyst. They are excellent reducing agents, reducing for example ion(III) to iron(II). and silver ions to the metal their reducing power is used in organic chemistry, for example to reduce aldehydes to alcohols. They can undergo metathetic reactions to produce other borohydrides, for example... 

Chemically, carbon dioxide is not very reactive, and it is often used as an inactive gas to replace air when the latter might interact with a substance, for example in the preparation of chromium II) salts (p. 383). Very reactive metals, for example the alkali metals and magnesium can, however, continue to bum in carbon dioxide if heated sufficiently, for example... 

Liquid ammonia. This can be prepared by compressing ammonia gas. It has a boiling point of 240 K and is an excellent solvent for many inorganic and organic substances as well as for the alkali metals. Liquid ammonia is slightly ionised. ... 

Nitrates are prepared by the action of nitric acid on a metal or its oxide, hydroxide or carbonate. All nitrates are soluble in water. On heating, the nitrates of the alkali metals yield only oxygen and the nitrite ... 

These all contain the ion NO2. They are much more stable than nitrous acid, and those of the alkali metals can be fused without... 

These are ionic solids and can exist as the anhydrous salts (prepared by heating together sulphur with excess of the alkali metal) or as hydrates, for example Na2S.9HjO. Since hydrogen sulphide is a weak acid these salts are hydrolysed in water,... 

These are similar to those of the alkali metals but are rather less soluble in water. However, calcium sulphide, for example, is not precipitated by addition of sulphide ions to a solution of a calcium salt, since in acid solution the equilibrium position... 

These closely resemble the corresponding sulphides. The alkali metal selenides and tellurides are colourless solids, and are powerful reducing agents in aqueous solution, being oxidised by air to the elements selenium and tellurium respeetively (cf. the reducing power of the hydrides). 

The hydrogensulphates (or bisulphates) containing the ion HSO4, are only known in the solid state for the alkali metals and ammonium. Sodium hydrogensulphate is formed when sodium chloride is treated with cold concentrated sulphuric acid ... 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

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