G. F. Grinyer

Imagine a multi-story building where protons and neutrons live on each floor. The floors are energy levels, and usually there's a stairwell of a certain height between the sixth floor and the one above — the so-called gap. In the oxygen-20 nucleus, this gap acts as a barrier that particles find hard to cross.

A pear lies on the table, and if it had an electric charge, that charge would gather at one end. Many nuclei are built this way: the displacement of protons and neutrons creates an intrinsic dipole moment. But radium-224, which is indeed pear-shaped, has a dipole moment that is almost zero.