Introduction: For divers, a lot of oxygen can rush into the body, sometimes more than the body can handle. Nausea underwater can often result from this increased amount of oxygen in the body. However, helium is often mixed with oxygen in the body, because it does not tend to react with other elements. The lack of reactivity of helium with oxygen is the consequence of a property known as ionization energy. Ionization energy refers to the amount of energy needed to remove an electron from the outermost shell of an atom of an element.

Based on the locations of elements on the Periodic Table, properties like ionization energy will change going down a group or across a period. The trends in periodic properties are partly a result of the number of protons in the nucleus of atoms and the number of energy levels (or electron shells) in an atom of an element. Generally, going down a group, the number of energy levels will increase with an increase in the number of protons, so there is less attraction, a greater size for the atom, and less energy necessary to remove electrons from the outermost shell of the atom. On the other hand, going across a period, the number of energy levels remains the same with an increase in the number of protons, so there is greater attraction for electrons caused by the protons, a smaller size for the atom, and more energy necessary to remove electrons from the outermost shell of the atom.