Normally, atoms are electrically neutral. They have the same number of electrons (charge -1) as protons (charge +1). Ions are atoms with either more or less electrons than protons.
Negative ions are relatively rare in the Universe, because they require adding electrons to atoms, which want to get rid of them. Positive ions, on the other hand, exist whenever electrons can be removed from atoms. There are many positive ions in stars, nebulae, and other astronomical objects.
The most positive ion possible for a given element consists of the bare nucleus with no electrons around it. For hydrogen, the requires removing only one electron, which requires an energy of only about 13.6 electron volts (eV). For uranium, 92 electrons must be removed, and the energy required to remove the last electron is about 131,820 eV, almost 10,000 times as much as is required to ionize hydrogen.
We refer to the ionization state of the ion by how many electrons it is missing, or by the number of electrons it still retains. For example, U82+ has 82 of the normal 92 electrons removed, leaving 10 electrons, just like neutral neon. Therefore, we often refer to it as neonlike urnaium.
We can show the energy levels of ions by means of an energy level diagram, such as these:
The primary orbital levels are shown. In a neutral atom, all the orbitals are filled, but in an ion, the electrons are missing, so there are orbital vacancies. Ions try to capture electrons into the orbital vacancies by a variety of recombination processes.