BeCl2 in gaseous state is linear and sp hybridised , herein, each Be atom it bonded to two chlorides and one empty valence orbital remains non-bonding but in solid state, it polymerizes, where Be gets bonded to four chlorides, so according to the concepts of hybridisation, for forming four covalent bonds, there needs four orbitals that participate in hybridisation, so four valence orbitals of Be atom (one s and three p ) undergoes hybridisation to give sp3 hybridised orbitals and forms polymeric BeCl2 molecule!
