A Conversation About Electron Affinity
Gold: Have you seen this club before? It seems very interesting!
Platinum: I feel inert about it.
Gold: You feel inert about anything, donít you?!
Chlorine: Hey everybody, look whoís here.
Oxygen: What? Metals at our club?!, grab their electrons and throw them out of club!
Fluorine: My favorite hobby, consider it done!†
Carbon: Wait, we donít want to make a salt/assault here, hey you, gold and platinum, look here is a private club exclusively for nonmetals, no place for electron losers! but for strong electron gainers.
Gold: Strong electron gainers, huh?† Is that over there records of your electron affinities?
Carbon: Yeah, we appreciate our electron-gaining capabilities and whatever measuring that.
Gold: So did you know with the exception of halogens, platinum and I have higher electron affinities in comparison to other nonmetals?
Oxygen: Wait? What?
Carbon: Alright! This doesnít make you a nonmetal but demonstrates how loose some conclusions are, for example the one that assumes nonmetals have higher electron affinities than metals. Anyway, letís cheer up!
Yes, thatís true!, as the above conversation also points out, the electron affinities of gold and platinum metals are higher than those of nonmetals except halogens. For comparison, some electron affinities in electronvolts (eV) are as follows: F (3.40), Cl (3.61), Br (3.36), I (3.06), At (2.8), Au (2.31), Pt (2.13), S (2.08), Se (2.02), Te (1.97), Po (1.9) and O (1.46). This seems to be in contradiction with the common behavior of metals and nonmetals (Seventeen elements are generally considered as nonmetals which are hydrogen, helium, carbon, nitrogen, oxygen, fluorine, neon, phosphorus, sulfur, chlorine, argon, selenium, bromine, krypton, iodine, xenon and radon). Nonmetals located towards top and right of the common periodic table, generally have higher ionization energies, electron affinities and electronegativities than metals since in general nonmetals have higher effective nuclear charges and smaller atomic radii in comparison to metals. However, in some cases, such common behavior doesnít hold true. For example, in some atomic properties like electron affinity (and also electronegativity), gold and platinum metals seem to not fit well with such expectations. Relativistic effects play a major role in high electron affinities (and also electronegativities) of gold and platinum.
But what are relativistic effects? IUPAC (in its famous gold book) considers relativistic effects as corrections to exact non-relativistic energy from the fact that inner shell electrons in heavy atoms move with velocities comparable in order of magnitude to the velocity of light. The fact is that the SchrŲdinger equation generally used to analyze quantum systems like atoms doesnít consider the theory of relativity developed by Albert Einstein. Most of time, this isnít problematic since in simple words, there is no need to consider theory of relativity at speeds much lower than the speed of light which is the case for many quantum systems. In other words, we get a same analysis with or without consideration of theory of relativity in the cases with speeds much lower than the speed of light. However, analyses with and without consideration of theory of relativity (relativistic and non-relativistic analyses) deviate from each other as the speeds of particles present in quantum systems increase and get closer to the speed of light. Such deviation increases by increase of speed and for velocities comparable in order of magnitude to the velocity of light, deviation isnít negligible and one should consider the theory of relativity to obtain the true analysis of examined system. For small but yet not negligible deviations, it is usually possible to correct non-relativistic analysis by a term or correction to get it close enough to the relativistic analysis where such corrections are generally mentioned as relativistic effects. In the case of great deviations, it is usually necessary to consider the theory of relativity from the beginning as non-relativistic analyses may completely get wrong and simple corrections maynít compensate.
In the case of atoms, inner shell electrons in heavy atoms move with velocities comparable in order of magnitude to the velocity of light and therefore we need to consider relativistic effects for them. In such cases, beside inner shell electrons, relativistic effects can affect outer shell electrons too since changes in states of inner shell electrons can result in changes in states of outer shell electrons beside the fact that outer shell electrons can also act as inner electrons partially as a result of their penetration to the space close to the atomic nucleus. Relativistic effects generally get stronger as atom gets heavier, or in fact as atomic number increases. Relativistic effects can change energy levels of atomic orbitals and they may increase or decrease stability of electrons. For example, there exists an extra stability for the newly added electron in electron affinity reactions of gold and platinum metals where that extra stability is attributed to relativistic effects. It may be interesting to know that color of gold metal is also attributed to relativistic effects. Most metals are silvery or gray however gold is yellow. Relativistic effects cause energy levels of 6s and 5d orbitals of gold get closer to each other in the way that their difference moves from the ultraviolet region of electromagnetic spectrum to the visible region. This increases absorption of blue light by gold and gold appears yellow, the complementary color to blue.