Physical properties:

(a) Large atomic radii: The atomic radii of alkali metals are the largest in their respective periods. These increase as we travel down the group. 

(b) Large ionic radii: The ionic radii increase as we move down the group due to the addition of a new energy shell with each succeeding element. 

(c) Low ionization enthalpy: The ionization enthalpies decrease as we move down the group.The ionization enthalpies of the alkali metals are the lowest due to loosely held s- electron. 

(d) Hydration enthalpy: It decreases with the increase in ionic radii.The hydration enthalpy of Li ion is the maximum and the hydration enthalpy of Cs ion is the minimum. 

(e) Oxidation state: The alkali metals exhibit oxidation state of +1 in their compounds and are strongly electropositive in character. The electropositive character increases from Li to Cs. 

(f) Metallic character: The metallic character increases down the group. 

(g) Melting point and boiling point: The m.p and b.p of alkali metals are very low and decrease with increase in atomic number. 

(h) Nature of bonds formed: These metals form ionic bonds. The ionic character increases as we down the group. 

(i) Flame colouration: All the alkali metals impart a charactersistic colour to the flame. 

(j) Photoelectric effect: Alkali metals (except Li) exhibits photoelectric effect.