All the diatomic molecules such as HF, HCL etc are linear region regardless of the number of active electron pairs surrounding the central atom. It is due to the reason that there are only two atoms in an diatomic molecules.
Molecules of more than two atoms can all have linear structure e.g. BeCL2, CO2, CQHQ, CS2, etc.
Structure of BeCL2
BeCl2 is a diatomic linear molecule with bond angle 180°.
Cl – Be – Cl
i. In this case, there are two active sots of electrons surrounding the Be atom. In terms of electron repulsion theory, these pairs must be arranged as for as possible therefore, two CL atoms arranged themselves in a linear structure.
ii. In terms of hybrid orbital theory, Be used its Sp1 hybrid orbitals for bonding.
Be (ground) —-> 1s2, 2s2
Be (excited) —->1s2, 2s2, 2p1x (hybrid to form sp1 orbital)
iii. As Sp1 hybrid orbitals are at the angle of 180° arranged linearly therefore, when these orbitals overlaps with 2px1 orbital of CL atoms forming sigma bond, the structure is straight.