Ionic compounds alkali metals

As pure substances, Group 1A or alkali metals are soft metallic solids with low densities and low melting points. They easily form 1+ cations. They are highly reactive, reacting with most nonmetals to form ionic compounds. Alkali metals react with hydrogen to form hydrides such as NaH. Alkali metals react exothermically with water to produce the metal hydroxide and hydrogen gas. In nature, alkali metals exist only in compounds. 

The elements show increasing metallic character down the group (Table 14.6). Carbon has definite nonmetallic properties it forms covalent compounds with nonmetals and ionic compounds with metals. The oxides of carbon and silicon are acidic. Germanium is a typical metalloid in that it exhibits metallic or nonmetallic properties according to the other element present in the compound. Tin and, even more so, lead have definite metallic properties. However, even though tin is classified as a metal, it is not far from the metalloids in the periodic table, and it does have some amphoteric properties. For example, tin reacts with both hot concentrated hydrochloric acid and hot alkali ... 

The group 1 and 2 metals form compounds M3P and M3P2 respectively, which are hydrolysed by water and can be considered to be ionic. The alkali metals also form phosphides which contain groups of P atoms forming chains or cages, the cages being either [Pv] (15.18) or [Pn] (15.19). Lithium phosphide of stoichiometry LiP consists of helical chains and is better formulated as Li [P ], the [P ]" ... 

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. 

The small size of lithium frequently confers special properties on its compounds and for this reason the element is sometimes termed anomalous . For example, it is miscible with Na only above 380° and is immiscible with molten K, Rb and Cs, whereas all other pairs of alkali metals are miscible with each other in all proportions. (The ternary alloy containing 12% Na, 47% K and 41% Cs has the lowest known mp, —78°C, of any metallic system.) Li shows many similarities to Mg. This so-called diagonal relationship stems from the similarity in ionic size of the two elements / (Li ) 76pm, / (Mg ) 72pm, compared with / (Na ) 102pm. Thus, as first noted by Arfvedson in establishing lithium as a new element, LiOH and LiiCOs are much less soluble than the corresponding... 

The compounds which most nearly fit the clas-sicial conception of ionic bonding are the alkali metal halides. However, even here, one must ask to what extent it is reasonable to maintain that positively charged cations M+ with favourably... 

Analysis of weight loss isotherms displayed in Fig. 8 shows that the first step in the interaction between Nb02F and carbonates of other alkali metals is similar to the interaction described by Equation (11). However, compounds of the M(Nb04F form, where M = Na, K, Rb, Cs, were not found [85]. The instability of such compounds is related to the ionic radii of the alkali metals, which are greater than that of Nb5+, thus the ions are too large to occupy the octahedral cavities formed by the oxygen and fluorine ions. 

MiNbC F compounds have a NaC 1-type structure, and are stable only in the case of lithium due to the steric similarity between the lithium ion and Nb3. In the case of other alkali metal cations with larger ionic diameters, the M4Nb04F compounds decompose yielding orthoniobates and simple fluorides of alkali metals, as follows ... 

It is important to note that the number of different compounds found in such systems increases significantly when moving along the sequence of alkali metals, from lithium to cesium, as does their thermal stability. This phenomenon is related to the systematic increase of both ionic radii and polarity of alkali metals ions when moving from lithium to cesium. 

The principal product of the reaction of the alkali metals with oxygen varies systematically down the group (Fig. 14.15). Ionic compounds formed from cations and anions of similar radius are commonly found to he more stable than those formed from ions with markedly different radii. Such is the case here. Lithium forms mainly the oxide, Li20. Sodium, which has a larger cation, forms predominantly the very pale yellow sodium peroxide, Na202. Potassium, with an even bigger cation, forms mainly the superoxide, K02, which contains the superoxide ion, O,. ... 

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