Why can sulfur have an expanded octet but not oxygen?
sulfur has 2 electrons in its 3s subshell , and 4 in 3p subshell. On absorbing energy the paired electrons get excited to to the next higher energy level which is the 3d subshell. So after exciting it's electrons six unpaired electrons are present which can take part in bonding via overlapping of orbitals. Other compounds can also be explained similarly. Also the d- orbitals contribute to only two bonds out of six bonds( which would have been meant in the paper by Reed and Weinhold (1986) ). Oxygen has no empty orbitals with it . It only possess orbitals upto 2p which contains 4 electrons. So it cannot expand its octet by exiting it's electrons.