Gold
Gold is a difficult system. ~ Indian chemist Sourav Pal
Chemistry is mostly concerned with the reactivity of elements, which owes to the number of electrons in the outer shell; the fewer electrons there, the more reactive.
Cesium and gold both have a single electron in their outer shell (the 6th such shell for them). Cesium is the most alkaline of natural elements, and highly reactive: it explodes if dropped in water, and even reacts to ice.
In contrast, gold is stalwart to a fault. Gold does not react to oxygen at any temperature, nor with ozone. Gold is unaffected by most acids and most bases. Hence gold does not tarnish. Special relativity accounts for gold’s stability and its color.
Negatively charged electrons whirl about their atomic nucleus with a speed and tightness corresponding to the intensity of the positively charged protons within, along with the nuclear core’s mass. With 79 protons in gold’s nucleus versus the 55 protons in cesium, and half again as many neutrons, gold’s tightly bound nuclear core has much more pull on its orbiting electrons.
The electrostatic attraction of gold’s nucleus relativistically speeds up, and tucks in, gold’s electrons, making it less reactive, and increasing its light absorption. (The subatomic attraction of gold’s positively charged protons to the negatively charged electrons in orbit both increases the electrons’ speed and increases their mass, causing a relativistic contraction in their orbits because, as an electron’s mass increases, the radius of its orbit with constant angular momentum shrinks proportionately.) Thus, gold soaks up blue light; reflecting the reds and greens which combine into the golden hue we see.
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Gold is not the only element under the influence of special relativity. Mercury is another heavy atom, with electrons held close to the nucleus. But the bonds between mercury atoms are weak; hence mercury has a low melting point and is liquid at ambient temperature.
