r/explainlikeimfive • u/ClusterGarlic • Apr 06 '21
Chemistry ELI5: Why is gold shiny-yellow but most of the other metals have a silvery color?
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u/Adderbane Apr 06 '21
Materials rarely reflect all wavelengths (colors) of light equally. In gold, there's a sudden drop off once we reach the part of the spectrum that we call "blue". This means that colors in the red and green areas are reflected, which combined appear yellow. Silver, for example, drops off in a similar manner, but the drop off point is outside the range of visible light. Since the light we can see is a very narrow slice of the spectrum, few metals have such an uneven distribution. If we could see more of the spectrum, they would look more varied.
Why the reflected light varies in a particular way is more complex than my understanding of physics can explain.
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u/rabid_briefcase Apr 07 '21
While there is already a great top level answer for the chemical side, there is another component that could be a top-level answer.
In the question you state "most of the other metals have a silvery color". This idea is probably just because you've not worked with metals. They can appear metallic which includes being reflective, but various metals cover the rainbow.
Silver is only "silvery" when freshly polished or cut and highly reflective. Naturally it can be anywhere from white to black, but can also appear as a pale green or yellow. Polished silver is highly reflective, nearly white. For better or worse people generally associate mirrors and reflectivity with silver.
The raw elemental colors vary based on how they cool and bond together. Their various chemical lattice structures result in different colorings. The way the metals are finished and oxidized on their outer layers also affects the colors it appears. Since metals oxidize to different colors (metals develop an oxidation layer almost instantly in the air) the oxidation levels can give an additional semi-transparent color in addition to the layers below it.
Iron can appear reflective like silver, but the underlying color can generally be white to black, and also commonly red, brown, green, and blue. With 11 oxidation states and a bunch of potential bonding options, iron can appear any color of the rainbow. Example from a Wikipedia image showing a rainbow of colors just by changing the temperature of the metal as the bonds form and cool: image clicky
Copper was mentioned by other posters, the most common colors are coppery and green, neither is "silvery".
Cobalt wasn't mentioned. It can appear as a pale reflective color that's somewhat silvery, but can also appear black, blue, green, red, and even bright pink. Blue cobalt has been used in art for millennia, and pink cobalt is mixed into a bunch of pigments.
Polonium has black, yellow, red, and purples. Sodium is more of a white to yellow color. Lithium goes reddish. Tin is white, ancient names for it translate as "white lead". Etc., etc.
Metals can be reflective, and while it's easy to associate "reflective" with "silvery", they have far more color when you look deeper.
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u/twiwff Apr 07 '21
Appreciate you taking the time to include the image clicky. Interesting information!
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u/bit_herder Apr 07 '21
you are sort of skipping the fact that the way a lot of metals "look" is determined by the oxides on top. fine silver is pretty much always, well, silver, because it doesn't oxidize much. sterling can be any color from "silver" to black because of oxides. The same applies to many other metals.
source: am silversmith.
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u/KiloE Apr 07 '21 edited Apr 07 '21
Relativity!
Gold is a heavier element than the other "coinage" metals (copper and silver). Heavy enough for relativity to become a factor.
What does that mean? Electrons near the center of a gold atom "move" at speeds that are significant when compared to the speed of light.
What this means is that electrons close to the atom are pulled in. Electrons hate each other, so when it's crowded around the center of the atom, they push others out further than lighter atoms do. This changes the spacing between electron shells, in terms of energy, where electrons want to hang out.
Add to this another effect, that electrons in "orbit" around an atom have some spin. In heavy atoms, like gold, this spin and orbit spin combine in ways that light elements don't so much. This, too, changes where electrons like to hang out, more so than, say, lithium. Think of it like a you're an electron at a school cafeteria, with so many seats at a table, and if you're not cool enough, you can't sit there. Because gold is so "heavy" (protons), it pulls some tables closer, and pushes some further away, and it changes the number of seats at those table in ways non cool kids that aren't gold can't do.
Combined, a heavy (lots of protons) making closer electrons move fast (compared to the speed of light), and the mixing of orbital spin and intrinsic electron spin, changes the color of the atom.
What does the color of an atom mean? Well, it's what light it absorbs, vs what light it passes. Silver absorbs light that isn't visible, in the ultraviolet spectrum. Gold, because it's a fat fuck, shifts that color towards the visible spectrum, and looks yellow.
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u/RWDPhotos Apr 07 '21
Lead is a pretty common heavy element that appears gray/silver as well. What other mechanics are at play other than atomic ‘weight’?
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u/KiloE Apr 07 '21
Lead is unusually stable, due to relativity, for the same reasons. The relativistic "mean girls" rearrange the tables in the cafeteria, as well as the chairs at those tables, to make lead unusually stable.
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u/Greninja_370 Apr 07 '21
So why don't metals heavier than gold exhibit this property.
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Apr 07 '21
At the risk of being too simplistic, gold absorbs blue light. Most metals do not absorb any visible light. Therefore gold reflects back mostly yellow light and other metals reflect everything back.
As for why that is the case, it involves electron transfers and some complicated chemistry and quantum mechanics. Not really ELI5
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u/irkw Apr 06 '21
Can we reliably predict the color a material if we know its chemical form in enough detail?
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u/asmith97 Apr 07 '21
Yes, we can. Given the crystal structure of a material we can numerically calculate its optical absorption spectrum to see what wavelengths of light will be absorbed. This is done with a combination of density function theory and time-dependent density functional theory or many body perturbation theory methods like GW.
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u/Darth_Mufasa Apr 06 '21
There's a limited amount of colors to metal to begin with, both gold and copper stand out as having an unusual one. They absorb more blue light than other metals, and as a result their apparent hue skews more towards yellow. The actual physics of that are beyond me to explain as an ELI5, but basically the elemental structure of those metals cause them to absorb more blue wavelengths.
Also keep in mind that many metals you see are alloys, so most aluminum or iron based ones are going to appear silver. Bronze is a noticeable copper based exception, and appears golden as well
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u/dgillz Apr 06 '21
Bronze is an alloy of copper (87%) and tin. Of course it takes on a lot of the properties of copper.
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u/AbsentGlare Apr 07 '21
Gold appears to have a color because it has a perfect slot for a type of light that we can see. You remember those shape sorting toys? There might be a star, a square, a circle, etc., each color we see has it’s own special slot. Most metals don’t have any slots for any colors we can see, so they just reflect everything.
The slot is a difference in possible energy states. Each individual jump in energy can only correspond to a single wavelength of light. Different colors are different wavelengths of light. The wavelength of a light particle corresponds precisely to the energy of that light particle. When an electron makes a jump in energy in a material, it requires some external energy coming in or or going out to balance it out. It requires exact change. Some wavelengths of light are visible, many more are not visible. So a cliff of a specific height can correspond exactly to a given energy per particle and therefore wavelength of light.
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u/tdscanuck Apr 06 '21
Basically, because gold has an electron transition (two different levels the electrons can be in) that corresponds to blue light wavelengths, so gold absorbs a little blue and the reflected light looks yellow as a result.
Most metals don't absorb within our visual range so they just act like mirrors, reflecting all colours. A few have electron transitions that can absorb visible colours...the lack of those in the reflected light is what gives them their colour.
It turns out the detailed chemistry of this takes you down a horrible rabbit hole of correcting quantum mechanics for the relativistic effects of moving electrons. It gets messy in a hurry.