Chemistry DIAGONAL RELATIONSHIPS

Diagonal Relationship of `Li` with `Mg` :

Due to its small size lithium differs from other alkali metals but resembles with `Mg` as its size is closer to `Mg`. Its resemblance with `Mg` is known as diagonal relationship. Generally the periodic properties show either increasing or decreasing trend along the group and vice versa along the period which brought the diagonally situated elements to closer values. Following are the characteristic to be noted.

(i) Both `Li` and `Mg` are harder and higher m.pt than the other metals of their groups.

(ii) Due to covalent nature, chlorides of both `Li` and `Mg` are deliquescent and soluble in alcohol and pyridine while chlorides of other alkali metals are not so.

(iii) Fluorides, phosphates of `Li` and `Mg` are sparingly soluble in water whereas those of other alkali metals are soluble in water.

(iv) Carbonates of `Li` and `Mg` decompose on heating and liberate `CO_2`. Carbonates of other alkali metals are stable towards heat and decomposed only on fusion.

`Li_2CO_3-> Li_2O+CO_2; Mg CO_3-> MgO+CO_2`

(v) Hydroxides and nitrates of both `Li` and `Mg` decompose on heating to give oxide. Hydroxides of both `Li` and `Mg` are weak alkali.

`4 LiNO_3 -> 2Li_2O + 4NO_2 + O_2 ; 2Mg(NO_3)_2 -> 2MgO + 4NO_2 + O2`

`2LiOH -> Li_2O + H_2O; Mg(OH)_2 -> MgO + H_2O`

Hydroxides of other alkali metals are stable towards heat while their nitrates give `O_2` and nitrite.

`2KNO_3 -> 2KNO_2 + O_2`

(vi) Both `Li` and `Mg` combine directly with `N_2` to give nitrides `Li_3 N` and `Mg_3 N_2`. Other alkali metals combine at high temperature.

`6Li + N_2 -> 2Li_3N`; `3Mg + N_2 -> Mg_3N_2.`

Both the nitrides are decomposed by water to give `NH_3`.

`Li_3N + 3H_2O -> 3LiOH + NH_3`; `Mg_3N_2 + 6H_2O ->3Mg(OH)_2+ 2NH_3`

(vii) Bicarbonates of `Li` and `Mg` are more soluble in water than carbonates whereas carbonates of alkali metals are more soluble.

(viii) Both `Li` and `Mg` combine with carbon on heating.

`2Li + 2C -> Li_2C_2; Mg + 2C -> Mg C_2`

(ix) The periodic properties of `Li` and `Mg` are quite comparable

`text( Li Mg)`

Electronegativity `1.0` `1.2`

Atomic radii `quad quad1.23` `1.36`

Ionic radii `0.60(Li^+ )` `0.65(Mg^(+ 2))`

Atomic volume `12.97 c.c` `13.97 c.c`

(x) Both have high polarizing power. Polarizing Power = `text(Ionic charge)/text(ionic rardius)^2`

(xi) Lithium and `Mg` form only monooxide on heating in oxygen.

`4Li ^+ O_2 -> 2 Li_2O`; `2Mg + O_2 -> 2 MgO`

(xii) `Li_2SO_4` like `MgSO_4` does not form alums.

(xiii) The bicarbonates of `Li` and `Mg` do not exist in solid state, they exist in solution only.

(xiv) Alkyls of `Li` and `Mg` (`R.Li` and `R. MgX`) are soluble in organic solvent.

(xv) Lithium chloride and `MgCl_2` both are deliquescent and separate out from their aqueous solutions as hydrated crystals, `LiCl*2H_2O ` and `MgCl_2* 2H_2O`.

Diagonal Relationship of `Be` with `Al` :

Due to its small size `Be` differs from other earth alkaline earth metals but resembles in many of its properties with `Al` on account of diagonal relationship.

(i) `Be^(2+)` and `Al^(3+)` have almost same and smaller size and thus favour for covalent bonding.

(ii) Both these form covalent compounds having low m. pt. and soluble in organic solvent.

(iii) Both have same value of electronegativity (i.e.`1.5`).

(iv) The standard O.P of these elements are quite close to each other;

`Be^(2+)=1.69` volts and `Al^(3+)= 1.70` volts.

(v) Both become passive on treating with conc. `HNO_3` in cold.

(vi) Both form many stable complexes e.g. `(BeF_4)^(-)`, `(AlH_4)^(-)`.

(vii) Like `BeO`, `Al_2O_3` is amphoteric in nature. Also both are high m. pt. solids.

`Al_2O_3 + 2NaOH -> 2NaAlO_2 + H_2O`; `Al_2O_3 + 6HCl -> 2AlCl_3 + 3H_2O`

(viii) `Be` and `Al` both react with `NaOH` to liberate `H_2` forming beryllates and alluminates.

`Be + 2NaOH -> Na_2BeO_2 + H_2`; `2Al + 6NaOH -> 2Na_3AlO_3 + 3H_2`

(ix) `Be_2C` and `Al_4C_3` both give `CH_4` on treating with water.

`Be_2C + 2H_2O -> CH_4 + 2BeO`; `Al_4C_3 + 6H_2O -> 3CH_4 + 2Al_2O_3`

(x) Both occur together in nature in beryl ore, `3BeO*Al_2O_3*6SiO_2`.

(xi) Unlike other alkaline earths but like aluminium, beryllium is not easily attacked by air (Also `Mg` is not attacked by air).

(xii) Both `Be` and `Al` react very slowly with dil. `HCl` to liberate `H_2`.

(xiii) Both `Be` and `Al` form polymeric covalent hydrides while hydrides of other alkaline earth are ionic.

(xiv) Both `BeCl_2` and `AlCl_3` are prepared in similar way.

`BeO+ C + Cl_2 -> BeCl_2 + CO`; `Al_2O_3 + 3C + 3Cl_2 -> 2AlCl_3 + 3CO`

(xv) Both `BeCl_2` and `AlCl_3` are soluble in organic solvents and act as catalyst in Friedel-Crafts reaction.

(xvi) Both `Be (OH)_2` and `Al (OH)_3` are amphoteric whereas hydroxides of other alkaline earths are strong alkali.

(xvii) The salts of `Be` and `Al` are extensively hydrated.

(xviii) `BeCl_2` and `AlCl_3` both have a bridged polymeric Structure.

(xix) `Be` and `Al` both form fluoro complex ions `[BeF_4]^(2-)` and `[AlF_6]^(3-)` in solution state whereas other members of `2^(nd)` group do not form such complexes.

Difference between Alkali Metals and Alkaline Earth Metals :

See Table.