Similarity in properties between first element of a group and second element of the next group.
the two elements differ in
Anomalous behavious beryllium -
1) Due to small atomic and ionic size
2) High ionisation energy
3) No vacant d-orbitals
4) High hydration energy
Due to diagonal relationship Be and Al exhibit many similar properties but covalency of Be is 4 because it has an absence of d-orbital whereas, covalency of Al is up to 6 due to the presence of vacant d-orbital.