Alkali metals and Alkaline Earth metal

 The elements in periodic table in which last electron enters in 'S'  orbital of valance shell  are called as 'S'  block elements

As this block accommodate only 2 electrons, there are total 2 groups, group first and second belong to the  S  block of periodic table. 

Group first elements are called as alkali metals while group second elements are called as alkaline        earth metal except beryllium because their  oxides and  hydroxides are alkaline in nature and they found in earth crust 

                                    The Electronic configuration of group 1 elements 

                                 The Electronic configuration of group 2  elements

PERIODIC TREND FOR ALKALI METALS :

Atomic and Ionic radii :

alkali metals have largest size in particular period of the periodic  table as we move down the group due to increase in the number of shells  size increases from Li to Cs

Ionization enthalpy : 

Ionization enthalpies of alkali metals are very low and decrease down the group from Li to Cs because as the size increases effective nuclear charge decreases

Hydration Enthalpies :

hydration enthalpy is defined as the amount of energy release on dilution of one mole of substance in large amount of water if hydration enthalpy is high it means the hydrated iron is very stable the hydration enthalpies of alkali metals decreases with increase in size

Li has maximum degree of hydration 

Physical properties of alkali metals :

All the alkali metals are silvery white soft and light because of large size in period These elements have low density which increases down the group 

The melting and boiling point of alkali metals are low, indicating weak metallic bonding due to presence of single valence electron

Alkaline metals and salts imparts characteristic color to an oxidizing flame due to the excitation of the outermost electron to a higher energy level and then return to their ground state by emitting radiation.

Chemical Properties : 

i) Reactivity towards air

Alkali metals get tarnished in the presence of dry air due to the formation of their respective oxides.

The metallic oxide so formed further reacts with moisture to form hydroxides.

The alkali metals burn vigorously in oxygen and form their respective oxides.

4Li + O2 --> 2Li2O (Oxide)

 (ii) Reactivity towards water:

The alkali metals react with water and forms hydroxide and dihydrogen.

Lithium due to its small size and very high hydration energy does not react with water vigorously even though it has most negative E0 value.

But on the other hand sodium do reacts with water vigorously even though it has least negative E0 value.

(iii) Reactivity towards dihydrogen

The alkali metals react with dihydrogen at about 673K (lithium at 1073K) and form hydrides.

2M + H2 --> 2M+H-

(iv) Reactivity towards halogens

The alkali metals react vigorously with halogens and form ionic halides, M+X–

(v) Reducing nature

The alkali metals are strong reducing agents.

Lithium is the most powerful and sodium is the least powerful.

(vi) Solutions in liquid ammonia

The alkali metals dissolve in liquid ammonia and impart a deep blue color to the solutions. The color in the solution nature is due to the ammoniated electron which absorbs the energy in the visible region of light and imparts the blue color to the solution.

The solutions are paramagnetic and liberate hydrogen on standing which in turn results in the formation of amide.

The blue colour in concentrated solution changes to bronze colour and becomes diamagnetic.