Snapping a picture of the inside of an atom - the electrons, the protons, the neutrons - is no easy task. Quantum mechanics makes it virtually impossible to pin down these subatomic particles.
Instead of having the ability to describe where a particle is, quantum theory provides a description of its whereabouts called a wave function.
Wave functions work like sound waves, except that whereas the mathematical description of a sound wave defines the motion of molecules in air at a particular place, a wave function describes the probability of finding the particle.
Physicists can theoretically predict what a wave function is like, but measuring a wave function is very hard because they are exquisitely fragile.
In another bit of quantum weirdness, most attempts to directly observe wave functions actually destroy them in a process called collapse.