Action with air or oxygen-formation of oxides
These metals react with air or oxygen slowly upon heating (being less electropositive than alkali metals) to form oxides (MO), except Ba and Ra, which form peroxides.
Due to increasing electropositive character of the elements down the group, the reactivity with oxygen increases.
Monoxides are prepared by decomposition of their carbonates:
While all oxides are basic in nature, BeO is amphoteric reacting with acids as well as alkalies.
The basic character of oxides decreases from Be to Ba because of decrease in polarizing power with increase in ionic size.
BeO MgO CaO, SrO, BaO
Amphoteric Weakly basic Basic
The oxides of Be and Mg (BeO, MgO) are almost insoluble in water (due to their large lattice energies) while the oxides of rest of the metals dissolve in water to form hydroxides.
Combination with hydrogen - Formation of hydrides
All alkaline Earth metals combine with hydrogen upon heating to form ionic hydrides, MH2, excepting beryllium.
BeH2 is produced by the reduction of BeCl2 with LiAlH4.
Some basic characteristics of hydrides are:
- MgH2 is covalent in nature while all other hydrides i.e. CaH2, SrH2, BaH2, are ionic. Calcium hydride is also known as hydrolith.
- All hydrides have high reactivity with water and form hydroxides and liberate hydrogen gas.
Action with water - Formation of hydroxides
Alkaline Earth metals have lesser tendency to react with water to form hydroxides as compared to alkali metals.
The hydroxides are basic in nature and basic strength increases down the group. This is due to the low ionization energies of these metals where the M-O bond in MOH is weak and it can cleave to give OH- ions in solution. As the ionization energy decreases down the group, the basic strength of the hydroxides increases. For example, Be(OH)2 is amphoteric, Mg(OH)2 is mildly basic while others are strong bases.
The hydroxides of alkaline Earth metals are less basic than alkali metals of the corresponding periods due to:- High ionization energies
- Small ionic size and
- Dipositive charge on the ions.
The hydroxides of alkaline Earth metals are less soluble than alkali metal hydroxides. Their solubility in water increases with increase in size of the metal: Be(OH)2 and Mg(OH)2 are almost insoluble, Ca(OH)2 is sparingly soluble while Sr(OH)2 and Ba(OH)2 are fairly soluble. This is due to increase in size of the cation and decrease in their lattice energies on moving down the group. Hydration energies become more due to decrease in lattice energies and so the solubility of hydroxide in water increases. This explains the increasing solubility of alkaline Earth metal hydroxides on moving down the group. Lattice energy decreases with increase in size of the cation and therefore, solubility increases from Be to Ba.
Formation of carbonates
The carbonates of alkaline Earth metals are prepared by passing carbon dioxide in limited supply, through the solution of their hydroxides.
The carbonates decompose on heating to give carbon dioxide and metal oxide.
Stability of these carbonates increase down the group. For example, BeCO3 is least stable while BaCO3 is the most stable. The solubilities of the metal carbonates in water decreases down the group. For example, BeCO3 is slightly soluble, while BaCO3 is completely insoluble.
Formation of sulphates
The action of dilute H2SO4 on metals, metal oxides, metal hydroxides and metal carbonates gives sulphates.
Formation of halides
Alkaline Earth metals combine with halogens at higher temperatures to form halides.
They can also be obtained by the action of halogen acids on metals, their oxides, carbonates and hydroxides.
Properties
- Beryllium halides are covalent while all other halides are ionic and are readily soluble in water.
- BeCl2 is relatively low melting solid and volatile while others have high melting points. BaCl2 has very high melting point.
- BeCl2 is soluble in organic solvents due to its covalency.
- Their anhydrous halides are hygroscopic (absorb water molecules to form hydrates) e.g. BeCl2.4H2O, MgCl2.6H2O, CaCl2.6H2O. Because of this property, anhydrous calcium chloride is used as a dehydrating agent. BeCl2 gets hydrolysed by moisture and gives fumes of hydrochloric acid.
- All the chloride of the other members impart characteristic flame colors except for BeCl2 and MgCl2.
CaCl2 SrCl2 BaCl2
Brickred Crimson Grassy green
Be and Mg chlorides do not give flame colorations because of their high ionization energies (energy of bunsen flame not sufficient to excite the outermost electron).
Beryllium chloride
It is prepared by heating beryllium oxide with chlorine vapors in the presence of carbon.
Beryllium chloride has a polymeric chain structure in the solid state due to its electron deficient nature. It has only four electrons in the valence shell and can accept two electron pairs (from neighboring Cl atom forming coordinate bonds) to complete its octet.
Polymeric structure of BeCl2
Each Be atom is terahedrally surrounded by four C atoms. Two of the chlorine atoms are bonded by covalent bonds while the other two by coordination bonds.BeCl2, exists as dimer in the vapor phase, which dissociates into the linear monomer at 1200 K.
Complex compounds
Due to their small size and higher charge density these elements form stable complexes. Of these, beryllium and magnesium have maximum tendency to form complexes. For example, beryllium form complexes of the type [BF4]2-
Chlorophyll is an important complex of magnesium.
