Explanation Of S – Block (Group II A Elements)

Group IIA Elements:

Beryllium (Atomic number – 4)

Magnesium (Atomic number – 12)

Calcium (Atomic number – 20)

Strontium (Atomic number – 38)

Barium (Atomic number – 56)

Radium (Atomic number – 88)

  •  Have 2 electrons in the outermost orbit.
  •  By loosing two electrons they form stable + 2 ions.
  •  They are divalent and show + 2 as the stable oxidation state.

General trends in Periodic Table:

  • As we move from top to bottom, atomic radius increases.
  • Order of atomic radius: Be < Mg < Ca < Sr < Ba < Ra
  • Ionization potential decreases down the group.
  • Density increases with increase in atomic mass.
  • Electronegativity decreases from top to bottom.
  • Stability of thermal decomposition of metal carbonates increases down the group.
  • Solubility of carbonates in water decreases down the group.
  • Hydration enthalpy decreases down the group.
  • Reactivity towards Halogens

All the alkaline earth metals combine with halogen forming their halides.

M + X₂ → M X₂ (X = F, Cl, Br, I)

Flame Test: Be and Mg give negative flame test as the electrons are strongly bound.

Hydration Enthalpy: Like alkali metals ions, the hydration enthalpies of alkaline earth metals ion decrease with increase in ionic size down the group.

General characteristics of compounds of alkaline earth metals:-

1. Oxides and Hydroxides: The alkaline earth metals burn in oxygen to form monoxide of the form MO. BeO is Amphoteric and rest are ionic in nature.

MO + H₂O → M (OH)₂ (hydroxide)

The solubility, thermal stability and the basic character of hydroxides increases with increase in atomic number.

2. Halides: Except for Beryllium halides, all halides of alkaline earth metals are ionic in nature.

3. Salts of Oxoacids:

(i) Carbonates:

  • Carbonates of alkaline earth metals are insoluble in water.
  • Metal carbonates on heating give metal oxide and carbon dioxide gas on heating.
  • Beryllium carbonate in highly unstable hence are kept in CO₂ atmosphere.
    MCO₃ (s) → MO (s) + CO₂ (g).

 (ii) Nitrates:

  • On heating give metal oxides and Nitrogen dioxide.
  • Tendency of formation of hydration decreases with increase in size.
    M (NO₃)₂ (s) → 2MO (s) + 4NO₂ (g) + O₂ (g).