Obviously I am speaking here of elemental metals. Alloys and whatnot can be any color.
Metals are, with two exceptions, ALL grey/silver. There’s some minor variations - platinum is a bit lighter than iron, for instance, and I know from personal experience that very pure lead has a sort of blue/purple sheen in certain light - but they’re all basically grey. EXCEPT copper, which is sort of orange, and gold, which is yellow.
Why is that?
Sorry, there are three. Cesium metal (caesium for those across the pond) looks like gold. Of course, it explodes on contact with water and ignites spontaneously in water.
From Wikipedia: "Whereas most metals are gray or silvery white, gold is slightly reddish-yellow.[21] This color is determined by the frequency of plasma oscillations among the metal’s valence electrons, in the ultraviolet range for most metals but in the visible range for gold due to relativistic effects affecting the orbitals around gold atoms.[22][23] Similar effects impart a golden hue to metallic caesium. "
For copper, “Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells corresponds to orange light.”
Why do you get all those rainbow hues on bismuth? (I could look at Wikipedia for an explanation. I have a strong feeling I would not understand it).
It has to do with the thickness of the oxide layer on the surface of the bismuth. Kinda sorta like the film that makes soap bubbles, the thickness determines which wavelengths of light are absorbed or reflected.
Well-explained. Thanks!
Just to clarify, of course when I say almost all metals are silver, I am assuming BEFORE oxidization. Once they oxidize, they aren’t pure anymore. They can be lots of different colors. Copper turns green, of course.
Gold is unique in its malleability (ability to form thin sheets). You can make a gold sheet so thin, that light will pass through it like glass or paper. The transmitted light is blue-green.
Gold is a noble metal and it can be powdered to very small particle size without oxidation. Since 100s of years back, people have used powdered/colloidal gold to get many different colors depending on the concentration.
oxides , sulfides, chlorides of bismuth have all sorts of colours.
I dont see how thickness matters, those substances aren’t translucent.
That’s not where the bismuth iridescence comes from, though. It’s thin-film interference.
In a thin enough film, damn near anything is translucent.
An interesting fact about aluminum- I used to think it didn’t oxidize, just like gold. But aluminum does oxidize- the aluminum oxide forms a ‘clear coat’ that protects the inner aluminum. Aluminum oxide is the main chemical that sapphires are made of.
https://www.aluminummanufacturers.org/aluminum-sulfate/sapphire/
The oxide ‘clear coat’ prevents Aluminum from reacting with water, the way sodium does.
But you can dissolve Aluminum (like foil) in Gallium (available on amazon …) and it will readily react with water making a lot of Hydrogen.
(please understand and take necessary safety precautions )
Well you can’t just drop something that cool-sounding in here and not comment on it. What’s thin film interference? Is that like oxidation?
No, it’s when light passing through a very thin surface - like oxide in this case - is basically double-bounced - some by the outer surface, some by the inner interface between the oxide layer and the metal. Those two streams will interfere with each other, The amount of interference is dependent on the wavelength of the light and the thickness of the film relative to it. So some colours are reinforced, others diminished. Net result: colours dependent on the layer thickness (but also the angle the light is hitting the film).
It’s the same effect as soap bubbles and oil slicks, except the oxide layer isn’t liquid and so you don’t get the swirling effects you see in those.
I wonder if this is why lead has green and purple streaks in it. (I have a client who pours lead for various applications; the ingots are dark gray but have odd colors streaking through. It’s one of the ways you can visually tell the difference in purity - the purest lead is colorful.)
Yes.
It’s lead oxide (the “dross”) on the surface of the molten lead.
<birds and insects reading this thread> : What are you talking about all metals look the same?!
Aluminum oxidizes fast. When bare aluminum is exposed to oxygen it takes less than 1 nanosecond to oxidize. Bare aluminum unfiltered by a layer of oxide has to be kept in an inert environment to be seen.
Interesting note – the metals aren’t uniformly “silver” across all wavelengths. Or even opaque, for that matter. Silver has a transmission “hole” near 300 nm, and all of the alkali metals become transmissive in the ultraviolet (although you’re going to need very thin samples to “see” through. Gold Leaf, by the way, transmits green light.
I was going to object to the second poster’s calling cesium “gold”. I’ve worked with cesium, and I wouldn’t call it gold - it’s more like silver. If you look at the picture in the Wikipedia article you can see irs true color, which Wikipedia describes as “silvery gold”.
Looks golden to me:
https://upload.wikimedia.org/wikipedia/commons/thumb/0/03/Rb%26Cs_crystals.jpg
Sorry, that was a messed-up link, this is what I’m talking about (Cæsium being the lower vial)
