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Chapter 11

CHMB3 Chapter 11

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Alen Hadzovic

Chapter 11: The Group 1 Elements The Essentials • Alkali Metals: lithium, sodium, potassium, rubidium, caesium and francium; they are all metals and form simple ionic compounds which are soluble in water. • The trends in the properties of the Group 1 metals and their compounds can be explained in terms of variations in their atomic radii and ionization energies. o Their melting points decrease down the group. o Ionic and metallic radius increases down the group. o Ionization energy decreases down the group. o All the elements adopt a body-centered cubic structure, not close- packed. o The increase in atomic radius from Li to Cs leads to a decrease in the first ionization energy down the group because the valence shell is increasingly distant from the nucleus; this makes reactivity more reactive as it goes down a group. o 2 M(s) + 2 H2O(l)  2 MOH(aq) + H2(g) o Rb Cs react explosively with water because they are denser and sink below the surface, causing a sudden ignition of the hydrogen that scatters the water violently. o The enthalpies of sublimation and ionization both decrease down the group (making oxidation more favorable). • The binary compounds of the alkali metals contain the cations of the elements and exhibit predominantly ionic bonding. o Most halides have the (6,6)-coordinate rock-salt structure, but CsCl, CsBr and CsI have the (8,8)-coordinate caesium-chloride structure. o Li reacts with oxygen to give Li2O. o Sodium reacts with oxygen to give Na2O2. o Other Group 1 elements form the superoxides which contain the paramagnetic ion O2^2-. o Metals react with sulfur to form compounds with formula M2Sx, where x lies in the range 1 to 6. o Only lithium readily forms a nitride Li3N when heated in nitrogen. o Only lithium reacts with carbon to form Li2C2. o Group 1 element ions are hard Lewis acids, but their hardness decreases down the group with increasing ionic radius. • The chemical properties of Li are anomalous due to its small ionic radius and tendency to exhibit covalent bonding. o Lithium can exhibit a high degree of covalent character in its bonding; this covalent character is due to the high polarizing power of the Li+ ion associated with high charge density. o Lithium forms the normal oxide when burnt in oxygen whereas other Group 1 elements form peroxides or superoxides. o Lithium is the only alkali metal to form a nitride Li3N when heated in nitrogen and a carbide Li2C2 when heated with graphite. o Some lithium salts have very low solubilities in water; other lithium salts crystallize as hydrates or are hygroscopic. o Lithium forms many stable organometallic compounds. o Lithium nitrate decomposes directly to the oxide whereas the other alkali metals initially form nitrites, MNO2. o Lithium hydride is stable to heating to 900C whereas the other hydrides decompose above 400C. The Detail • The Group 1 elements can be extracted by electrolysis because abundance is low and should be taken from nature. • Common uses of lithium are related to its low density and are used when weight is of a premium concern; the most widely used compounds of Group 1 are NaCl and NaOH. • The hydrides of Group 1 elements are ionic and contain the H- ion.
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