Beryllium atom has electronic configuration $1s^2 2s^2$. So, in $\mathrm{Be}_2$ there are a total of eight electrons which must be filled in four molecular orbitals. M.O. configuration of $\mathrm{Be}_2$ → $\sigma 1s^2 \sigma^* 1s^2 \sigma 2s^2 \sigma^* 2s^2$ B.O. = 1/2(4 – 4) = 0. Since the bond order is zero it shows that molecular beryllium will not exist.