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

Explanation

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 exception to this isk, which has lower density than Na.
(4) The matallic 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 excites electron reverts back to the ground state, it emits excees 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 enthalphy 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 spon taneous
They react with water to form their respective hydroxides and dihydroges The general reaction for the same is given as
`2M+2H_(2)Orarr 2M^(+)+2OH^(Theta)+H_(2)`
(3) They react with dihydrogen to form metal hydrides. These hydrides are ionic solids and have high melting points.
`2M+H_(2)rarr 2M^(+)H^(-)`
(4) Almost all alkali metals, except Li, react directly with halogens to form ionic halides
`2M+Cl_(2)rarr 2MCl`
`(M=Li, K, Rb, 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 There solutions are conducting in nature
`M+(a+y)NH_(3)rarr[M(NH_(3))_(x)]^(+)+[M(NH_(3))_(y)]^(-)`
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
`M_((cm))^(+)+e^(-)+NH_(3(l))rarrMNH_((cm))+1/2H_(2(g))`
n a highly concentrated solution, the blue colour changes to bronze and the solution becomes diamagnetic.