Alkali Metals

3 Li

11 Na

19 K

37 Rb

55 Cs

87 Fr

Alkali Metal is the name given to the first group of elements in the Periodic Table. They have 1 valence electron and thus, valency +1. All of the alkali merals are metals as they lose electron to form cations. They form basic oxides. Francium is the most metallic element both in group and the periodic table.

Among the alkali metals sodium and potassium are abundant and lithium, rubidium and cesium have much lower abundances. Francium is highly radioactive; its longest-lived isotope ²²³Fr has a half-life of only 21 minutes. The general electronic configuration of Alkali metals is [Noble Gas]ns¹.

Physical Properties

Atomic Size and Ionization Enthalpy

On moving from top to bottom in a group, principal quantum number (n) increases. Therefore, the shielding effect of the electrons in the inner orbital outweighs the effective nuclear charge(Z_eff). Thus atomic size increases and Ionization enthalpy decreases. Elements Potassium and Cesium are often used as photo-voltaic cell due to this reason.

Hydration Enthalpy

The hydration enthalpy is the most in case of Lithium due to high charge to radius ratio. As we go down the group, the Hydration enthalpy decreases.

Colour to Flame

All the group 1 elements give colour to the flame because they have a general electronic configuration of ns¹ due to which they readily lose the outermost electron. The electron jumps from higher energy level to lower energy level in the oxidising flame. When the electron returns it emits radiation in the visible region of spectrum.

Chemical Properties

With air(N₂ and O₂)

1. lithium:

• $Li + N_2 \longrightarrow 2Li_3N$

lithium reacts with nitrogen directly to give lithium nitride.

• $Li + O_2 \longrightarrow Li_2O$

1. sodium

$Na + O_2 \longrightarrow Na_2O_2$

sodium reacts with Oxygen(in excess) to form sodium peroxide.

1. potassium

$K + O_2 \longrightarrow KO_2$

potassium reacts with Oxygen(in excess) to form potassium superoxide.

With Water

$2Na + 2H_2O \longrightarrow 2NaOH + H_2$

All alkali metals react with water to form hydroxide with the release of hydrogen gas. It is a highly exothermic reaction.

With Halogen

$2Na + Cl_2 \longrightarrow 2NaCl$

All alkali metals react with halogens to give halides.

With Ammonia

Let M be any metal belonging to alkali metals.

$M + (x+y)NH_3 \longrightarrow [M(NH_3)_x]^+ + [e(NH_3)_y]^-$

Due to the presence of ammoniated electron the resultant product is:

1. Paramagnetic
2. Blue in colour
3. Electrically conducting

On standing for sometime the product's properties will change:

1. Diamagnetic
2. now bronze in colour
3. electrically non-conducting