s-Block Element (Alkali and Alkaline Earth Metals)

SYNOPSIS

Hydration enthalpy:
Hydration enthalpy of alkali metal ions decreases with increase in ionic size.

Physical properties:

• All alkali metals are silvery white, soft and light metals. Because of their large size, these elements have low density.
• Melting and boiling points of alkali metals are low, indicating weak metallic bonding.
• Alkali metals and their salts impart a characteristic colour to the oxidising flame.
  
  

  Metal	Li	Na	K	Rb	Cs
  Colour	Crimson red	Yellow	Pale violet	Red violet	Blue

Chemical properties:
Alkali metals are highly reactive due to their large size and low ionisation enthalpy.

• Reactivity towards air: They burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide, and the other metals form superoxide.
• Reducing nature: They are strong reducing agents; lithium showing the most reducing nature and sodium showing the least reducing nature.

• Solution in liquid ammonia: They dissolve in liquid ammonia to produce a deep blue solution which is conducting in nature.
  The blue colour of the solution is due to the ammoniated electron, and the solution is paramagnetic.
  
  In concentrated solution, the blue colour changes to bronze colour and becomes diamagnetic.

ANOMALOUS PROPERTIES OF LITHIUM
Although the lithium atom and ion are exceptionally small, it has high polarising power (i.e. charge–radius ratio).

GROUP 2 ELEMENTS: ALKALINE EARTH METALS

Hydration enthalpies:

• Order of hydration enthalpies of alkaline earth metal ions: Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺
• The hydration enthalpies of alkaline earth metal ions are higher than those of alkali metal ions. Thus, compounds of alkaline earth metals are more extensively hydrated compared to alkali metals, e.g. MgCl₂ and CaCl₂ exist as MgCl₂.6H₂O and CaCl₂.6H₂O, while NaCl and KCl do not form such hydrates.