What is the centre of an atom called?

The nucleus, holding the protons and neutrons, and it is where essentially all the mass is - more than 99.9 per cent of it. The electrons contribute almost nothing to the weight and everything to the size.

The scale is hard to hold in the head. If an atom were a sports stadium, the nucleus would be a pea on the centre spot and the electrons would be specks somewhere in the stands. Matter is overwhelmingly empty space, and the reason your hand does not pass through a table is not that the atoms are touching but that their electrons repel each other.

Ernest Rutherford found it by accident in 1909, by firing alpha particles at gold foil and expecting them all to pass through. A tiny fraction bounced straight back. He said it was as though you had fired a fifteen-inch shell at a sheet of tissue paper and it had come back and hit you - and the only explanation was that the atom's mass and charge were concentrated in something very small and very dense at the centre.