Discuss the general characteristics and gradation in properties of alkaline earth metals.

Explanation

The general characterstics and gradation in alkaline earth metals follows:
i) Oxides and hydroxides:
Alkaline earth metals forms oxides of type Mo.
These are formed by burning in oxygen.
BeO is amphoteric and covalent in nature where as other oxides are ionic and basic in nature.
Other oxides i.e, except BeO forms hydroxides with water.
Eg: `MgO+ H_2O to Mg(OH)_2`
The solubility thermal stability and the basic character of these hydroxides increase with increase of atomic no. from `Mg(OH)_2` to `Ba(OH)_2`
These hydroxides are less basic, less stable than alkali metal hydroxide
`Be(OH)_2` is amphoteric in nature. This can be evidented by the following reactions.
`Be(OH)_2` reacts with both acids and alkalis.
`Be(OH)_2 +2OH^(-) to [Be(OH)_4]^(2-)`
Beryllation
`Be(OH)_2 +2Hcl+2H_2O to [Be(OH)_4]Cl_2`
Hence `Be(OH)_2` is amphoteric in nature.
ii) Halides:
These forms `MX_2` type halides.
Excepe Be- halides, all other halides of these metal are ionic.
Be-halides are covalent and soluble in organic solvents.
In vapour phase `BeCl_2` forms a bridged dimer which disociates into monomer at high temperatures (around 1200k)
`Cl-Be -Cl`
In solid state `BeCl_2` has a chain structure.

THe tendency to form halide hydrates gradually decreases- down the group.
Eg: `MgCl_2, 8H_2O, CaCl_2, 6H_2O, BaCl_2, 2H_2O`
Ca, Sr and Ba halides, dehydration can be done by heating.
Fluorides are less soluble than the chlorides due to their high lattice energies.
i) carbonates:
Alkaline earth metals forms `MCO_3` type carbonates
These carbonates are insoluble in water
The solubility of these carbonates in water decreases as the atomic no. of the element increases.
The thermal stability increases with increasing cationic size.
These carbonates decompose on heating to give `CO_2`
`CaCO_3 overset(Delta) to Cao+CO_2`
`BeCO_3` is unstable and kept only in at atmosphere of `CO_2`
ii) Sulphates:
Alkaline earth metals forms `MSO_4` type sulphates
These are white solids and are stable to heat
`BeSO_4 and MgSO_4` are readily soluble in water due to high hydration enthalpies of `Be^(2+), Mg^(2+)`
The solubility decrease from `CaSO_4` to `BaSO_4`
iii) Nitrates:
Alkaline earth metals forms `M(NO_3)_2` type nitrates.
These are formed by the reaction of carbonates in `dil. HNO_3`
`Mg(NO_3)_2` crystallises with six water molecules and `Ba(NO_3)_2` is an hydrous.
All of these decompose on heating to give the respective index.
`2M(NO_3)_2 to 2MO +4NO_2 +O_2`
`M=Be, Mg, Ca, Sr, Ba`