Draw all the plausible resonance structures of thiophene,

Here's what I find.

Explanation:

I get the five resonance contributors shown below.

Except for the bottom structure, the S atom always has a + charge and the C atoms have a negative charge.

The resonance hybrid has six π electrons, so thiophene is aromatic.

Note that only one of the lone pairs on S is involved in the resonance system.

The other lone pair is in an sp² orbital, which is in the plane of the ring and therefore cannot participate in resonance.