Question

What are the common physical and chemical features of alkali metals ?

Answer 1

Common physical features of alkali metals:
(1) They are soft and can be easily cut.
(2) They have silvery white appearance. They are light coloured.
(3) Due to large atomic size, they have low density. On moving down the group the density increases with exception of K which has lower density than Na.
(4) Due to weak metallic bonding, alkali metals have low melting and boiling points.
(5) They provide characterisitc colour to flames.
(6) They show photoelectric effect. On irradiation with lightCs and K lose electrons.
Common chemical features of alkali metals:
Due to low ionization enthalpy, akali metals have high reactivity.
On moving down the group, the reactivity increases.
(1) On reaction with water, oxides and hydroxides are formed.
On moving down the group, the reaction becomes more and more vigorous.
(2) On reaction with water, hydroxide and hydrogen are formed.
$2M+2H_{2}O\to 2MOH+H_2$
(3) On reaction with hydrogen, hydrides are formed.
$2M+H_2\to MH$
(4) They directly combine with halogens to form ionic halides. Li is an exception.
$2M+C{l}_2\to 2MCl,(M=Li,K,Rb,Cs)$
(5) They are strong reducing agents. On moving down the group, the reducing power increases with exception of Li. Due to high hydration energy, Li is the strongest reducing agent among alkali metals.
(6) They dissolve in liquid ammonia. The solution is deep blue in colour due to presence of ammoniated electrons.