Why carbon sulphide a simple molecular structure?
It depends on the relative propensity of the atoms to form double bonds. Double bonds are formed by the ‘sideways-on’ overlap of p-orbitals and can only really occur with any effectiveness when: a) the atoms are small, and b) the atoms are of a similar size In CO2, the atoms are both small and of a similar size so double bond formation is favoured. This leads to a molecular structure (O=C=O) in which the carbon atom is double-bonded to two oxygens. In SiO2, the size difference between Si and O is too great to form double bonds of sufficient strength to allow the formation of molecular SiO2. Consequently, Si forms single bonds to four oxygens and oxygens form single bonds to two silicons resulting in a giant covalent lattice structure similar to that of diamond. In CS2, there is a significant size difference between C and S but not as big as between Si and O (see figures in reference). This allows the formation of double bonds which are just strong enough to stabilise the molecular form
