Thorium is named after Thor, obviously. I wanted to give it a symbol for electricity, lightningh, or thunder, but I found none. However, I found some symbols for electrum.
Electrum is a naturally occurring alloy of gold and silver, with trace amounts of copper and other metals. The word for 'electricity' is actually derived from 'electrum'.
And the symbol I picked looks kind of like a hammer, if you squint your eyes.