So let me do a little background here. Today the pedestrian understanding of Atoms isn't much better than Leucippus. It's usually seen as an the smallest unit of matter. It's seen as stable, and foundational. This is of course destroyed by any understanding of subatomic particles... but even these basic atomic building blocks are in flux. Electrons for example can jump between energy levels. It can jump up a level by absorbing light. Then by emitting light if can jump down a level. There are a total of 5 levels each which has a known energy state. More here and here.