Question

What is the Lewis structure of `OCS`?

Answer 1

If you think about it, `"S"` is right below `"O"`, so the number of valence electrons it contributes is the same.

Therefore, `"OCS"` is isoelectronic with `"CO"_2`, having the same number of valence electrons but not identical atoms.

So, what you have in comparison is:

`color(blue)( :stackrel(..)"O"="C"=stackrel(..)"S": )` `=>` `"OCS"`

`:stackrel(..)"O"="C"=stackrel(..)"O":` `=>` `"CO"_2`

You can do the electron-counting method and get the same result.

`"O": 6`
`"C": 4`
`"S": 6`

`6+4+6 = 16` valence electrons

Distribute `8` of them in two double bonds (`2` per bond line), and you have `8` remaining.

`"O"="C"="S"`

So, `4` as lone pairs on `"O"` and `4` as lone pairs on `"S"`. This also gives you the above result.

`:stackrel(..)"O"="C"=stackrel(..)"S":`