- Some elements of second period `Li`, `Be`, `B` shows dissimilarities with other elements of their respective group but shows similarities with elements of third group like `Mg`, `Al`, `Si` situated diagonally to them. It is called diagonal relationship. See fig.1.

- Similarities between properties of `Li` and `Mg` are as follows :

(a) `Li` and `Mg` both reacts directly with nitrogen to form lithium nitride `(Li_3N)` and magnesium nitride `(Mg_3N_2)` whereas other alkali metals of `IA` group does not form nitride.

(b) Fluoride carbonate and phosphate of `Li` and `Mg` are insoluble in water whereas these compounds of other alkali metals are soluble.

(c) `Li` and `Mg` both are hard metals, whereas other metals of `IA` group are soft.

(d) `LiOH` and `Mg(OH)_2` both are weak base, whereas hydroxides of other elements of `IA` group are strong base.

(f) Metallic bond in `Li` and `Mg` both are strong compared to other alkali metals.

(g) Their melting and boiling points are high.

(h) By thermal disintegration of `LiNO_3` and `Mg (NO_3)_2`, `Li_2O` and `MgO` is obtained respectively.

(i) Thermal stability of `Li_2CO_3` and `Mg CO_3` is very less compared to other alkali metals and they liberates `CO_2` gas easily.


- Similarly `Be` shows similarity to `Al` of `IIIA` group compared to other elements of `IA` group which are as follows :

(a) They both elements does not provide colour to Bunsen burner.

(b) They both are comparatively stable in air.

(c) Both are insoluble in `NH_3` therefore does not form blue coloured solution.

(d) There is no tendency of making peroxide and superoxide in them.

(e) Reducing power is very less due to low value of standard electrode potential in the form of oxidation potential.

(f) `Be` and `Al` both forms halogen bridge halides.

See fig.2.