Trends in Chemical Reactivity with Halogens

All the alkali and alkaline earth metals are highly reactive.They react with air, water,hydrogen,halogens  and a number of other non metals. The alkali metals have one electron in their valence shell and possess large atomic sizes due to which they are highly reactive.

 Reactivity of alkali metals with halogens

All the alkali metals react with the halogens directly  and form the corresponding halides with the general formula MX.

The general reaction of alkali metals with halogen can be represented as follows:

               2M+X2→2MX

Where M is an alkali metal(Li,Na,K,Rb,Cs) and X is a halogen(F2,Cl2,Br2, I2)

For example,sodium reacts with chlorine and forms sodium chloride

                       2Na+Cl2→2NaCl

From lithium to caesium the reactivity of alkali metals with a particular halogen increases in the following order.

Li < Na < K < Rb < Cs

The reactivity of halogens with a particular alkali metal decreases from fluorine to iodine as follows.

F2 > Cl2 > Br2 > I2

Except for some lithium halides all the alkali metal halides are ionic in nature.

All the alkali metal halides are crystalline solids and have high melting and boiling points except lithium halides.The melting points of lithium halides are low due to its covalent nature.

All the alkali metal halides except lithium halides, in their fused state and in solutions are good conductors of electricity and they are used for the preparation of the corresponding alkali metals.

All the alkali metal halides are freely soluble in water except lithium fluoride.Lithium halides are sparingly soluble in water.

Lithium chloride is deliquescent and is soluble in alcohol and pyridine.Lithium chloride separates  out from solution as hydrated crystals LiCl.2H2O

The halogen compounds of alkali metals are less hydrated.

Sodium chloride, potassium chloride and rubidium chloride form non-hydrated chlorides.

Reactivity of alkaline earth metals with halogens

All the alkaline earth metals react with halogens directly  when heated and form the corresponding halides with the general formula MX2.

The general reaction of alkaline earth  metals with halogens can be represented as follows:

               M+X2→MX2

Where M is an alkaline earth metal(Mg,Ca,Sr,Ba) and X is a halogen(F2,Cl2,Br2, I2)

For example, magnesium reacts with chlorine and forms magnesium chloride

                            Mg+Cl2→MgCl2

By treating halogen acids with metals, metal hydroxides, metal oxides and metal carbonates the alkaline earth metal halides can be prepared. M+2HX→MX2+H2

MO+2HX→MX2+H2O

M(OH)2+2HX→MX2+2H2O

MCO3+2HX→MX2+H2O+CO2

By passing a current of chlorine gas on a mixture containing beryllium oxide and carbon at 870 to 1070K beryllium chloride can be prepared.

BeO+C+Cl2→BeCl2+CO

Beryllium halides are covalent in nature. This is due to small size and high

charge on  the bivalent beryllium ion (Be+2 ion) .It has high polarising power.

 Beryllium chloride does not conduct electricity in fused state as it is a covalent compound.It is soluble in organic solvents like benzene and ether.

Beryllium chloride acts as a Lewis acid.Due to the presence of vacant orbital in beryllium, it can accept a lone pair of electrons.

The salts of Beryllium will not give any colour during flame test.

The fluorides,chlorides,bromides and iodides of the alkaline earth metals except beryllium are ionic solids.

The solubility of these halides decreases from beryllium to barium as hydration energy decreases with the increase in size of the metal ion.