Atomic structure of a lithium atom, illustration. In the centre of the atom is the nucleus, made up of a tightly-packed cluster of protons (red) and neutrons. Three electrons (blue) are orbiting around the nucleus. The electrons each have a negative charge, which is balanced by positively-charged protons in the nucleus. This classical model is known as the Bohr Model, after the Danish physicist Neils Bohr. However, in modern quantum physics it has proved impossible to precisely locate the position of atomic electrons. Instead they are represented by fuzzy clouds, large areas within which there is a high probability of an electron being found.