NCERT Class XI Chemistry The s-Block Elements Solutions

Question : 1
Total: 32
What are the common physical and chemical features of alkali metals ?
General characteristics of alkali metals :
(i) Electronic configuration : General electronic configuration of the valence shell is ‘ns1’ where ‘n’ gives the number of the outermost shells.
(ii) Atomic and ionic sizes : They have the largest size in their respective periods. The atomic size regularly increases upon descending the group.
(iii) Ionisation enthalpy : It decreases down the group due to decreasing effective nuclear charge. 2nd ionisation enthalpy is very high as now an electron has to be removed from stable noble gas configuration.
(iv) Melting and boiling points : Due to weak metallic bond strength and large atomic sizes, all alkali metals are soft and have low melting and boiling points.
(v) Metallic character : Alkali metals readily lose their valence electron and form M+ ions showing +1 oxidation states. Metallic character increases as we descend.
(vi) Flame colouration : They impart characteristic colours to flame. (vii) Photoelectric effect : Due to very low value of ionisation enthalpies, alkali metals exhibit photoelectric effect.
(viii) Nature of compounds : Due to highly electropositive character, compounds of alkali metals are ionic in nature.
(ix) Reactivity towards air : Their lustrous surface gets tarnished when exposed to air. They burn violently in air or oxygen forming oxides. Li forms monoxide (Li2O), Na forms peroxide (Na2O2) while K, Rb, Cs form superoxides (KO2,RbO2,CsO2).
(x) Reactivity towards water : Alkali metals form hydroxides when they come in contact with water and liberate hydrogen gas.
(xi) Reactivity towards hydrogen : They form ionic hydride (M+X). Ionic character of hydrides increases on descending the group. They act as strong reducing agents.
(xii) Reactivity towards halogen : They react with halogens to form ionic halides (M+X).The ionic character of metal halides increases down the group.
(xiii) Reducing character : An element which acts as a reducing agent, must have low ionisation energy. Alkali metals act as strong reducing agents as their ionisation energy values are low. Since ionisation energy decreases on moving down from Li to Cs, the reducing property increases in the same order. Thus, Li is weakest reducing agent while Cs is the strongest reducing agent amongst alkali metals in free state.
(xiv) Solution in liquid ammonia : They dissolve in liq. NH3 forming blue coloured solutions, which are good reducing agents and good conductors of electricity.
M + (x + y) NH3

The blue colour of the solution is due to ammoniated electron.
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