An atomic nucleus isn't always round. It can stretch out like a melon or flatten like a pumpkin. And its size depends on this shape — just as the size of a pie depends on the shape of the dough.

Scientists calculated the shapes and sizes of nuclei of berkelium isotopes — a heavy element produced in reactors. It turned out that flattened nuclei are larger than elongated ones at the same degree of deformation. This is unexpected: previously, it was thought that size depends only on how strongly the nucleus deviates from a sphere, not on the direction of that deviation.

This hollow center is what increases the radius. The nucleus kind of puffs up from within, even though it contains the same number of particles. This connection between shape, level filling, and size helps us better understand how the heaviest nuclei are structured — and why some are more stable than others.