Hi,
The melody and words are the bits that can be copyrighted. That is the song, in essence.
Chords, instrumentation etc fall into the category of "arrangement".
For example, another band come along with a completely new arrangement of the song, using the same essential melody and words but everything else is different.
Chords are the difficult part. Strictly speaking they could be a part of a copyright, but copyright itself is intended to cover what is original in the work. Words and melody can easily be deemed original enough to warrant this, but very rarely is a chord progression original enough, so generally they are not part of the copyright claim,.
Bands tend to claim the arrangement, while the songwriter claims the song itself.