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1. Atomic and Ionic Radii: Alkali metals have largest atomic and ionic radii in their respective periods and on moving down the group, atomic and ionic radii increases

     Li < Na < K < Rb < Cs

2. Ionisation enthalpy:

  • IE1 values of alkali metals are lowest in each period
  • IE2 values of alkali metals are very high and there is large difference between IE1 and IE2.
  • On moving down in the group IE1, value decreases from Li and Cs.

Li > Na > K > Rb > Cs

3. Hydration Enthalpy: Hydration enthalpy of alkali metal ions decrease with increase in ionic size.

Li+ > Na+ > K+ > Rb+ >Cs+

Li+has maximum degree of hydration and for this reason Lithium salts are mostly hydrated.

Example. LiCl. 2H2O.

4. Electropositive and Metallic character: Increases down a group

Li < Na < K < Rb < Cs

5. Melting Point and Boiling Point:

Li > Na > K > Rb > Cs Melting Point and Boiling Point order

  • All these metals possess low M.P and B.P.

Reason: In alkali metals, the atoms are bulb together by metallic bonds and stronger is the metallic bond, higher is the M.P and B.P. As the size of atom increases the metallic bonds becomes weak, this results in low as P and B.P. On moving from Li to Cs, size of atom increases so inter atomic interaction becomes weak resulting in low M.P and B.P

6. Heat of Atomisation:

Li > Na > K > Rb > Cs because metallic bonding becomes weaker from Li to Sc

7. Density:

 Li > K > Na > Rb > Cs

Density of K is lower than Na due to abnormal increase in size of K

8. Standard Oxidation Potential and Reducing Properties: All alkali metals can easily lose electrons and thus they have high values of oxidation potential. So that these are good Reducing agent. More is oxidation potential, more is the tendency to get oxidised and thus more powerful is reducing nature in aqueous medium.

9. Flame Colouration: All alkali metals and their salts impart colour to an oxidising (or Bunsen) flame. This is because the heat from the flame excites the outermost to a higher energy level. When the excited comes back to the ground state, there is emission of radiation in the visible region.