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

Physical and chemical features of alkali metals-

Physical features-

• All the alkali metals are silvery white, soft and light metals. Because of the large size, these elements have a low density which increases down the group from Li to Cs. However, potassium is lighter than sodium.
• The melting and boiling points of the alkali metals are low indicating weak metallic bonding due to the presence of only a single valence electron in them.
• The alkali metals and their salts impart characteristic colour to an oxidizing flame.

Chemical features-

• Due to low ionisation enthalpy, they are highly reactive. As we move down the group reactive nature increases.
•  The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture to form hydroxides.  They burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide, the other metals form superoxides. The superoxide $O^{2-}$  ion is stable only in the presence of large cations such as K, Rb, Cs.
•  The alkali metals react with water to form hydroxide and dihydrogen.

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