Topic: Group 1 Elements – Alkali Metals
Q.1. Alkali metals are paramagnetic but their salts are diamagnetic. Explain.
Ans. Alkali metals contain one unpaired electron (ns1) and hence are paramagnetic. However, during salt formation, this unpaired electron is transferred to the non-metallic atom forming its anion. As a result, salt has paired electrons both in the cation as well as in the anion and hence alkali metal salts are diamagnetic in nature.
Q.2. Which out of sodium or potassium has higher melting point?
Ans. On going from Na to K, the size of the atom increases and hence the metallic bonding weakens. In other words, sodium has higher melting point than potassium because of stronger metallic bonding.
Q.3. Among alkali metals in aqueous solution, Li+ ion has the lowest mobility. Why?
Ans. Because of smallest size among alkali metals, Li+ ions are most highly hydrated in aqueous solution. As a result, among alkali metals, mass of hydrated lithium ion is the highest and thus lowest ionic mobility.
Q.4. Why alkali metals impart color to the flame?
Ans. Alkali metals have low ionization enthalpies. Their valence electrons easily absorb energy from the flame and are excited to higher energy levels. When these excited electrons return to the ground state, the absorbed energy is emitted in form of light.
Q.5. Why can caesium be used in photoelectric cell while lithium cannot?
Ans. Caesium has the lowest while Lithium has the highest ionization enthalpy. Hence, Cs can lose electrons very easily whilie Li cannot.
Q.6. Lithium has highest ionization enthalpy in group 1 elements, yet it is the strongest reducing agent, Why?
Ans. In aqueous solutions, the tendency of an element to lose electrons does not entirely depend upon its ionization enthalpy. It also depend upon its enthalpy of Sublimation and the enthalpy of Hydration of the ion left after the loss of an electron. The combined effect of these factors is measured in terms of electrode potential. Since lithium has the most negative (-3.04 V) electrode potential of group 1 elements, therefore, lithium is the strongest reducing agent.
Q.7. Sodium fire in the laboratory should not be extinguished by pouring water. Why?
Ans.7. The reaction of sodium with H2O is so exothermic that the H2 produced catches fire.
2 Na + 2 H2O ===> 2 NaOH + H2
As a result, the fire spreads rather than being extinguished. Therfore, H2O should not be used for extinguishing sodium fire. Instead pyrene (CCl4) should be used. When CCl4 is sprinked over fire, it being volatile (b.p. 350K) immediately forms vapors. These vapours being non-inflammable and heavy, surround the fire. As a result, supply of air is cut off and the fire gets extinguished.
Q.8.Why is it that on being heated in excess supply of air, K, Rb and Cs form superoxides in preference to oxides and peroxides?
Ans.8. K+, Rb+, and Cs+ are large cations and superoxide ion (O2)- is larger than oxide (O2-) and peroxide (O2)2- ions. Since due to higher lattice energies, a large cation stabilizes a large anion, therefore, these metals form superoxides in preference to oxides and peroxides.
Q.9. Why superoxides of alkali metals are paramagnetic while normal oxides are diamagnetic?
Ans.9. Superoxieds contain the ion (O2)- which has the structure
containing a three electron bond, i.e., it has one unpaired electron and hence is paramagnetic. Normal oxides containing the ion 
do not have any unpaired electron and hence are diamagnetic.
Q.10. Why alkali metals are normally kept in kerosene oil?
Ans. This is because in the air they are very easily oxidized to oxides which may dissolve in the moisture of the air to form hydroxides or they also combine directly with water vapors present in moisture to form hydroxides. The hydroxides, in turn, may combine with CO2 of the atmosphere to form carbonates.
4 M + O2 ====> 2 M2O
M2O + H2O ====> 2 MOH
2 MOH + CO2 ====> M2CO3 + H2O
Therefore, to protect them from air and moisture, they are kept under kerosene oil.
Q.11. What makes lithium to show properties uncommon to the rest of the alkali metals?
Ans.11. The unusual properties of lithium as compared to other alkali metals is due to (i) the exceptionally small size of its atom and ion, (ii) high polarizing power (charge/size ratio), (iii) high ionization enthalpy and (iv) absence of d-orbitals.
Q.12. When is cation highly polarizing ? Which alkali metal cation has the highest polarizing power ?
Ans. A cation is highly polarizing if its charge/size ratio is very high. Because of its small size, Li+ has the highest polarizing power among the alkali metal ions.
Q.13. Why does table salt get wet in rainy season?
Ans.13. Pure NaCl is not hygroscopic but table salt is impure NaCl containing impurities of Mg2SO4, CaSO2, MgCl2 and CaCl2. All of these being hygroscopic absorb moisture from air in rainy season. As a result, table salt get wet.
Q.1. Alkali metals are paramagnetic but their salts are diamagnetic. Explain.
Ans. Alkali metals contain one unpaired electron (ns1) and hence are paramagnetic. However, during salt formation, this unpaired electron is transferred to the non-metallic atom forming its anion. As a result, salt has paired electrons both in the cation as well as in the anion and hence alkali metal salts are diamagnetic in nature.
Q.2. Which out of sodium or potassium has higher melting point?
Ans. On going from Na to K, the size of the atom increases and hence the metallic bonding weakens. In other words, sodium has higher melting point than potassium because of stronger metallic bonding.
Q.3. Among alkali metals in aqueous solution, Li+ ion has the lowest mobility. Why?
Ans. Because of smallest size among alkali metals, Li+ ions are most highly hydrated in aqueous solution. As a result, among alkali metals, mass of hydrated lithium ion is the highest and thus lowest ionic mobility.
Q.4. Why alkali metals impart color to the flame?
Ans. Alkali metals have low ionization enthalpies. Their valence electrons easily absorb energy from the flame and are excited to higher energy levels. When these excited electrons return to the ground state, the absorbed energy is emitted in form of light.
Q.5. Why can caesium be used in photoelectric cell while lithium cannot?
Ans. Caesium has the lowest while Lithium has the highest ionization enthalpy. Hence, Cs can lose electrons very easily whilie Li cannot.
Q.6. Lithium has highest ionization enthalpy in group 1 elements, yet it is the strongest reducing agent, Why?
Ans. In aqueous solutions, the tendency of an element to lose electrons does not entirely depend upon its ionization enthalpy. It also depend upon its enthalpy of Sublimation and the enthalpy of Hydration of the ion left after the loss of an electron. The combined effect of these factors is measured in terms of electrode potential. Since lithium has the most negative (-3.04 V) electrode potential of group 1 elements, therefore, lithium is the strongest reducing agent.
Q.7. Sodium fire in the laboratory should not be extinguished by pouring water. Why?
Ans.7. The reaction of sodium with H2O is so exothermic that the H2 produced catches fire.
2 Na + 2 H2O ===> 2 NaOH + H2
As a result, the fire spreads rather than being extinguished. Therfore, H2O should not be used for extinguishing sodium fire. Instead pyrene (CCl4) should be used. When CCl4 is sprinked over fire, it being volatile (b.p. 350K) immediately forms vapors. These vapours being non-inflammable and heavy, surround the fire. As a result, supply of air is cut off and the fire gets extinguished.
Q.8.Why is it that on being heated in excess supply of air, K, Rb and Cs form superoxides in preference to oxides and peroxides?
Ans.8. K+, Rb+, and Cs+ are large cations and superoxide ion (O2)- is larger than oxide (O2-) and peroxide (O2)2- ions. Since due to higher lattice energies, a large cation stabilizes a large anion, therefore, these metals form superoxides in preference to oxides and peroxides.
Q.9. Why superoxides of alkali metals are paramagnetic while normal oxides are diamagnetic?
Ans.9. Superoxieds contain the ion (O2)- which has the structure
containing a three electron bond, i.e., it has one unpaired electron and hence is paramagnetic. Normal oxides containing the ion 
do not have any unpaired electron and hence are diamagnetic.Q.10. Why alkali metals are normally kept in kerosene oil?
Ans. This is because in the air they are very easily oxidized to oxides which may dissolve in the moisture of the air to form hydroxides or they also combine directly with water vapors present in moisture to form hydroxides. The hydroxides, in turn, may combine with CO2 of the atmosphere to form carbonates.
4 M + O2 ====> 2 M2O
M2O + H2O ====> 2 MOH
2 MOH + CO2 ====> M2CO3 + H2O
Therefore, to protect them from air and moisture, they are kept under kerosene oil.
Q.11. What makes lithium to show properties uncommon to the rest of the alkali metals?
Ans.11. The unusual properties of lithium as compared to other alkali metals is due to (i) the exceptionally small size of its atom and ion, (ii) high polarizing power (charge/size ratio), (iii) high ionization enthalpy and (iv) absence of d-orbitals.
Q.12. When is cation highly polarizing ? Which alkali metal cation has the highest polarizing power ?
Ans. A cation is highly polarizing if its charge/size ratio is very high. Because of its small size, Li+ has the highest polarizing power among the alkali metal ions.
Q.13. Why does table salt get wet in rainy season?
Ans.13. Pure NaCl is not hygroscopic but table salt is impure NaCl containing impurities of Mg2SO4, CaSO2, MgCl2 and CaCl2. All of these being hygroscopic absorb moisture from air in rainy season. As a result, table salt get wet.
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