What kind of chain should I buy if my gears are internal in a drum? I have seven gears but should I buy a one speed chain? What would happen if I would use a seven speed chain?
The chain you need is 1/8 inch chain, usually sold as singlespeed chain.
A derailleur chain, which are sold as multi-speed chains, is narrower and the cogs may be too thick to fit.
EDIT: As said in the other answers, some singlespeed and internal gear hubs have narrower cogs and can use the 3/32 inch chain. An 1/8 inch chain may be slightly noisier with these cogs, but works.
To answer your question, if you used a 7 speed chain it will do one of two things
1) It will either work perfectly (if the bike has "normal" thinner teeth on the chainring and cog) then its 3/32" chain.
2) or it won't sit on the teeth properly and will ride high, plus it might wedge onto the teeth and not come off when its supposed to. This would be "Chain Suck" and would make for very bad riding. In this case you require wider 1/8" chain
Note: 1/8" is "wider" than 3/32". 1/8 is the same as 4/32 and is therefore 25% more, or 33% additional space between the inner plates.
So you need the right chain. Consider checking with a caliper, or take the old chain to your LBS and ask for one of those please.
You need a chain that is specifically a single-speed chain, not a multi gear derailleur chain.
For an internal gear hub (IGH) bike you probably need a 1⁄8 inch (3.2 mm) single-speed chain.
However some single speed or IGH bikes use a narrower 3⁄32 inch (2.4 mm)
single-speed chain, which is the same as derailleur chains.
To determine what you have currently, measure the distance between the inner plates of your current chain.