What is meant by the diagonal relationship of elements? Discuss the diagonal relationship of beryllium with aluminium.
Beryllium exhibits some similarities with aluminium. Point out three such properties.
When an element shows similarities to another element in the next higher group, and in the next period, these are said to exhibit diagonal relationship. For example, Li resembles Mg, Be resembles Al and B resembles Si.
Diagonal relationship of beryllium with aluminium:
Since beryllium (Be2+) and aluminium (Al3+) have similar charge/radius ratio, they exhibit diagonal relationship. They resemble as follows:
(i) Both these elements dissolve in strong alkalies to liberate hydrogen and forming beryllate's and aluminates.
(ii) Both have strong tendency to form covalent compounds. Both BeCl2 and AlCl3 are covalent.
(iii) Both form non-volatile, hard oxides (BeO and Al2O3) having very high melting points.
(iv) Both Be and Al form fluoro-complex anions
(v) Carbides of both the metals react with water liberating methane gas.