Bite-size science: The emptiness within ...

The dimensions within an atom are difficult to visualize using drawings or visual techniques due to the great differences in size and distance of the various parts. Most of us have seen representations of atoms showing the nucleus in the center and the electrons depicted around the outside in some manner. If we put this scale into perspective by using powers of ten, we would find that the nucleus of an atom would be 10,000 times as large as an electron. But the distance of the electrons from the nucleus would make the size of the atom approximately 10,000 times the size of the nucleus. 

To get some idea of this comparison, imagine a pea placed in the center of a large football stadium. If the pea represents the nucleus of an atom, the closest electrons in the atom would be spinning around the outside of the stadium's walls. With that in mind it becomes clear that 99.999999999% of the volume of an atom is just empty space! If we were to get rid of all this empty space within the atoms that we are made of, squeezing electrons and nuclei close together, the entire human population would fit into a volume the size of a sugar cube. 

If you find this fact unworldly and difficult to digest you'll be surprised to learn that the universe is scattered with stars which have such unimaginable densities. These are called neutron stars and are the result of the gravitational collapse of a massive star during a supernova event. The neutron star's density varies from below 1×109 kg/cbm in the crust, increasing with depth to above 6×1017 or 8×1017 kg/cbm deeper inside. This density is approximately equivalent to the mass of the entire human population compressed to (surprise!) a volume the size of a sugar cube.