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

Solution:

Physical properties of alkali metals are as follows.

(1) They are quite soft and can be cut easily. Sodium metal can be easily cut using a knife.

(2) They are light coloured and are mostly silvery white in appearance.

(3) They have low density because of the large atomic sizes. The density increases down the group from Li to Cs. The only exceptionto this isK, which has lower density than Na.

(4) The metallic bonding present in alkali metals is quite weak. Therefore, they have low melting and boiling points.

(5) Alkali metals and their salts impart a characteristic colour to flames. This is because the heat from the flame excites the electron present in the outermost orbital to a high energy level. When this excited electron reverts back to the ground state, it emits excess energy as radiation that falls in the visible region.

(6) They also display photoelectric effect. When metals such as Cs and K are irradiated with light, they lose electrons.

Chemical properties of alkali metals

Alkali metals are highly reactive due to their low ionization enthalpy. As we move down the group, the reactivity increases.

(1) They react with water to form respective oxides or hydroxides. As we move down the group, the reaction becomes more and more spontaneous.

(2) They react with water to form their respective hydroxides and dihydrogens. The general reaction for the same is given as

$2 \mathrm{M}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{M}^{+}+2 \mathrm{OH}^{\Theta}+\mathrm{H}_{2}$

(3) They react with dihydrogen to form metal hydrides. These hydrides are ionic solids and have high melting points.

$2 \mathrm{M}+\mathrm{H}_{2} \longrightarrow 2 \mathrm{M}^{+} \mathrm{H}^{-}$

(4) Almost all alkali metals, except Li, react directly with halogens to form ionic halides.

$2 \mathrm{M}+\mathrm{Cl}_{2} \longrightarrow 2 \mathrm{MCl}$

$(\mathrm{M}=\mathrm{Li}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$

Since Li+ ion is very small in size, it can easily distort the electron cloud around the negative halide ion. Therefore, lithium halides are covalent in nature.

(5) They are strong reducing agents. The reducing power of alkali metals increases on moving down the group. However, lithium is an exception. It is the strongest reducing agent among the alkali metals. It is because of its high hydration energy.

(6) They dissolve in liquid ammonia to form deep blue coloured solutions. These solutions are conducting in nature.

$\mathrm{M}+(x+y) \mathrm{NH}_{3} \longrightarrow\left[\mathrm{M}\left(\mathrm{NH}_{3}\right)_{x}\right]^{+}+\left[\mathrm{M}\left(\mathrm{NH}_{3}\right)_{y}\right]^{-}$

The ammoniated electrons cause the blue colour of the solution. These solutions are paramagnetic and if allowed to stand for some time, then they liberate hydrogen. This results in the formation of amides.

$\mathrm{M}_{(a m)}^{+}+\mathrm{e}^{-}+\mathrm{NH}_{3(l)} \longrightarrow \mathrm{MNH}_{(a m)}+\frac{1}{2} \mathrm{H}_{2(g)}$

In a highly concentrated solution, the blue colour changes to bronze and the solution becomes diamagnetic.