What were Democritus ideas about atoms?

Democritus' only belief about atoms was that they were one of the possibilities when debating the structure of matter.

The Ancient Greeks loved to debate things, with a very strong liking for the abstract. One of the essentials for a debate is that one or more people should support one side, and one or more should support the other. If everyone agrees on something no debate is possible, which the Greeks would have regarded as no fun at all.

The original question was "Can matter be divided indefinitely, or will you eventually find a small unit which can not be further divided ?". Democritus argued that there was a limit, after which things become indivisible. The Greek Word for indivisible was atomos from which we got the Word atom. Many people feel that the Word atom and the very concept of indivisibility,were the only two things of interest that came out of this great debate.

In fact, there was one more thing, that was conspicuous only by its absence. The Greeks argued, and argued, and argued; they discussed and debated and argued, and they talked and they talked and they talked. Not once did the Greeks look for evidence. Did they do lots of experiments ? Oh my goodness ! No! None at all. Not one. An experiment would have meant doing rather than talking; doing was work and work was done by slaves (of which they had many). No Greek would demean himself by working.

It is difficult for us, at this remove, to pass any judgment on the Greeks viewpoint. What we can say, with 20/20 hindsight, is that the Greeks contributed remarkably little to engineering and even less to physics or chemistry. However, their contribution to politics, philosophy and mathematics is a stunning legacy. They did much more than their share for the advancement of human knowledge.