Diagonal Relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table. These pairs (Li & Mg, Be & Al, B & Si etc.) exhibit similar properties; for example, Boron and Silicon are both semiconductors, form halides that are hydrolyzed in water and have acidic oxides. Such a relationship occurs because crossing and descending the periodic table have opposing effects. On crossing a period of the periodic table, the size of the atoms decreases, and on descending a group the size of the atoms increases. Similarly, on moving along the period the elements become progressively more covalent, less reducing and more electronegative, whereas on descending the group the elements become more ionic, more basic and less electronegative. Thus, on both descending a group and crossing by one element the changes cancel each other out, and elements with similar properties which have similar chemistry are often found - the atomic size, electronegativity, properties of compounds (and so forth) of the diagonal members are similar.

The similarity between Lithium and Magnesium is particularly striking and arises because of their similar sizes: atomic radii, Li = 152 pm, Mg = 160 pm: ionic radii: Li+ = 76 pm, Mg2+ = 72 pm. The main points of

similarity are:

(i) Both lithium and magnesium are harder lighter than other elements in the respective groups.

(ii) Lithium and magnesium react slowly with water. Their oxides and hydroxides are much less soluble and their hydroxides decompose on heating.

(iii) The oxides, Li2O and MgO do not combine with excess oxygen to give any superoxide.

(iv) The carbonates of lithium and magnetism decompose easily on heating to form the oxides and CO2 .

(v) Both LiCl and MgCl2 are soluble in ethanol.

(vi) Both LiCl and MgCl2 are deliquescent and crystallize from aqueous solution as hydrates, LiCl.2H2O and MgCl2.8H2O.