An individual iron atom as well as one terbium atom were X-rayed, which revealed their electron arrangements.

But the smallest amount to date that can be X-rayed is an attogram, or roughly 10,000 atoms. That's because the X-ray emission of a single atom is just too weak to be detected—until now.

Single atom X-ray mechanism When X-rays illuminate an atom (red ball at the centre of the molecule), core level electrons are excited. X-ray-excited electrons then tunnel to the detector tip via ...