The Noble Gases don't form ions. Gold and platinum are also pretty nonreactive.

1) "The ionization potential, ionization energy or EI of an atom or molecule is the energy required to remove one mole of electrons from one mole of isolated gaseous atoms or ions. More generally, the nth ionization energy is the energy required to strip it of an nth mole of electrons after the first n − 1 mole of electrons have already been removed. It is considered in physical chemistry as a measure of the "reluctance" of an atom or ion to surrender an electron, or the "strength" by which the electron is bound; the greater the ionization energy, the more difficult it is to remove an electron. The ionization potential is an indicator of the reactivity of an element. Elements with a low ionization energy tend to be reducing agents and to form salts." Source and further information: http://en.wikipedia.org/wiki/Ionization_potential Further information: http://en.wikipedia.org/wiki/Ionization_energies_of_the_elements http://en.wikipedia.org/wiki/Ionization_energies_of_the_elements_%28data_page%29 2) "The electron affinity, Eea, of an atom or molecule is the energy required to detach an electron from a singly charged negative ion, i.e., the energy change for the process X- → X + e− An equivalent definition is the energy released (Einitial − Efinal) when an electron is attached to a neutral atom or molecule. It should be noted that the sign convention for Eea is the opposite to most thermodynamic quantities: a positive electron affinity indicates that energy is released on going from atom to anion. All elements whose EA have been measured using modern methods have a positive electron affinity, but older texts mistakenly report that some elements such as alkaline earth metals have negative Eea, meaning they would repel electrons. This is not recognized by modern chemists. The electron affinity of the noble gases have not been conclusively measured, so they may or may not have slightly negative EAs. Atoms whose anions are relatively more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. Eea of noble gases are close to 0. Although Eea vary in a chaotic manner across the table, some patterns emerge. Generally, nonmetals have more positive Eea than metals." Source and further information: http://en.wikipedia.org/wiki/Electron_affinity 3) "Metals usually have low ionization potential whereas non-metals have high ionization potential. Metalloids have intermediate ionization potential. The inert gases have very high ionization potential, due to the stability of the outer shell. Helium has the highest ionization potential. Within a group, the ionization potential generally decreases with increasing atomic number. Increasing atomic number results in increasing atomic radii. Thus, the electrons of the outer shell are further away than those of the previous element. The effective nuclear charge decreases as atomic size increases. Thus it is easier to pull one electron from the outermost shell of the atom. Ionization potential does not necessarily vary uniformly from one element to another. But it is a periodic property. It increases from group 1 to group 18. But the increase is not very regular. Ionization potential increases across the period because of increase in nuclear charge due to which the atomic size decreases. Thus, more energy is required to pull away the electron from the outermost shell of the atom of smaller size." "Electron affinity increases from left to right across the period because of increase in nuclear charge and decrease in atomic size. This causes the incoming electron to experience a greater pull of the nucleus thus giving a higher electron affinity. Electron affinity decreases down the group because the number of shells increases i.e., the atomic size increases and the effective nuclear charge decreases. This causes the incoming electron not to experience much attraction of the nucleus thus giving a lower electron affinity. The electron affinity of completely filled atoms is almost zero. An atom does not accept an electron in its outermost shell if it already has a stable configuration i.e. a duplet or octet, as in the case of inert gases. " Source and further information: http://www.tutorvista.com/content/science/science-ii/periodic-classification-elements/trends.php