$\mathrm{Be}$ belongs to the alkaline earth metals and its general electronic configuration is ${}_4\mathrm{Be}\to 1{\mathrm{s}}^2 2{\mathrm{s}}^2$. It is able to form the dispositive ion. Due to small size and high charge, it has more polarising power and it shows most covalent nature in its respective family members. Due to this slight covalent nature it forms the amphoteric oxides similar to the $\mathrm{Al}$.

Bur $\mathrm{Al}$ belongs to the p-block element and it has ability to show its covalency $6$ and form polymeric covalent compounds. But $\mathrm{Be}$ is very small in size, it can expand its covalency only up to $4.$