the arrangement of the atoms are in the order O-Si-S
with Si as the central atom
Number of valence electrons around each of the atoms
Si - 4
S - 6
O - 6
total number of valence electrons - 16
the number of lone pairs - 16/2 = 8
1 pair is shared between O and Si and 1 pair shared between Si and S
8-2 = 6 pairs are remaining
if we add 3 pairs each to O and Si as lone pairs their octets are complete however Si has an incomplete octet
then we convert the single bond between to O and Si to a double bond and bond between Si and S also to a double bond. By doing so the number of lone pairs around S and O are reduced to 2 lone pairs
. . . .
: O = Si = S :
