INORGANIC CHEMISTRY – CLASS XI (ISC)
Properties of Group 1 elements [Alkali metals]
1. Due to high reactivity, alkali metals do not occur free in nature. Elements of group 1 (or IA) are known as alkali metals because their hydroxides are soluble in water and form strongly alkaline solutions. Alkali metals are stored under kerosene oil because they get tarnished on exposure to air. 2. The general electron configuration of alkali metals is ns1. 3. Alkali metals have largest size and lowest ionization enthalpy in their respective periods. Down the group, the size of alkali metals increases whereas ionization enthalpy decreases. 4. Alkali metals have low densities and densities increases from Li to Cs. (Exception: K is lighter than Na, due to bigger size of potassium atom). 5. Alkali metals have low melting and boiling points due to their loosely packed metallic lattices. 6.
Alkali metals, except lithium, show photoelectric effect. Due to small size and high ionization enthalpy, the electrons of lithium cannot be emitted. 7. Alkali metals do not form dipositive ions because of their very high second ionization enthalpies. 8. All alkali metals are paramagnetic but their salts are diamagnetic. 9. Alkali metals impart characteristic colors to the flame, i.e., lithium imparts crimson red, sodium imparts golden yellow, potassium imparts pale violet while rubidium and caesium impart violet color to the flame. This is due to the fact that the atoms absorb energy from the flame and the electrons jump from lower orbit to higher orbit. However, since ionization enthalpies of the lower atoms of the group are higher, the jump is small and the energy (in the form of visible light) radiated when the electrons come back to their original positions were also small (i.e. with low frequencies). Since red color is of low frequency, we see red color for the lowest element of the group, i.e. lithium. Thereafter, the frequency of the emitted light for the higher elements gradually increases. 10. Lithium burns in oxygen to form lithium monoxide while the other alkali metals form peroxides. Li does not form peroxides because the strong positive field around the lithium ion (Li+) attracts the negative charge so strongly that it does not permit monoxide (O–) ion to combine with oxygen to form peroxide ion (O22–). 11. The peroxide anions are diamagnetic and are oxidising agents. 12. Na2O2 reacts with CO2 of the air and has been used in submarines and confined places, as it removes both CO2 and produces O2.
2Na2O2 + 2CO2 → 2Na2CO3 + O2 13. Superoxides contain the ion [O2–] and are paramagnetic and colored due to the presence of an unpaired electron. LiO2 and NaO2 are yellow, KO2 is orange. RbO2 is brown and CsO2 is orange. Superoxides are stronger oxidising agents than peroxides and give H2O2 and O2 on treatment with water or acids. Potassium superoxide [KO2] is used in breathing masks, submarines and space capsules because it removes CO2 and at the same time produces O2. 4KO2 + 2CO2 → 2K2CO3 + 3O2 4KO2 + 4CO2 + 2H2O → 4KHCO3 + 3O2 14. Lithium cannot be stored in kerosene oil as it floats on the surface of oil due to low density. Therefore, it is kept wrapped in paraffin wax. All alkali metals dissolve in mercury forming amalgams with evolution of heat. Li is the exception. It is used as scavenger in the metallurgical operations to remove oxygen and nitrogen. It is the lightest metal known. It is least fusible, least dense and least soft of all alkali metals. It has highest specific heat among all the alkali metals and specific heat decreases from Li to Cs. 15. Due to large negative electrode potentials, alkali metals are strong reducing agents. The reducing character increases from Na to Cs. However, lithium is the strongest reducing agent among all the alkali metals in spite of its highest Ionization enthalpy (IE). This is...
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