Where exactly is the lead in lead crystal, and why is it not visible?
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I think the way the molecules are aligned let’s light pass between then instead of reflecting or absorbing the light. Same as with aluminum crystal that they use for defense/aerospace uses (astronaut visors, the clear dome on the head of guided missles)
A material’s properties don’t just depend on the atoms involved, but how they are bonded together. A material can have completely different appearance and properties than the atoms making it up would have on their own. It’s like how you can build different things out of the same assortment of LEGO bricks.
An example will help this make more sense: Table salt. It’s sodium chloride. That means table salt is completely made up of sodium atoms and chlorine atoms. But wait! Sodium by itself is a [shiny silver-grey metal](https://upload.wikimedia.org/wikipedia/commons/2/27/Na_%28Sodium%29.jpg) that explodes on contact with water. And chlorine by itself is a (very toxic) [greenish-yellow gas](https://groungims.groundwatergovernance.org/is_chlorine_gas_a_molefular_element.png). And yet table salt, sodium chloride, is a [transparent solid](https://www.donkom.ca/wp-content/uploads/2014/05/DKP_2131-1024×713.jpg) that dissolves in water and is safe to eat. How?? It’s because the atoms alone don’t determine the properties – the structure does! In table salt, the [sodium and chlorine are arranged like this](https://media.istockphoto.com/id/1219252433/vector/sodium-chloride-nacl-crystal-structure-over-white.jpg?s=612×612&w=0&k=20&c=PboB_LBjlVjKgg4A-YDblgI5kU9StdBzDyJ2Vi6pQIE=). That’s the key: Sodium chloride contains sodium *atoms*, but does not contain any pure metallic sodium-the-material (which is a bunch of sodium atoms all bonded only *to each other*). And sodium chloride contains chlorine *atoms*, but does not contain any chlorine-gas-the-material (which is chlorine atoms bonded to each other in pairs).
Getting back to your question, you could look at table salt ask “where is the sodium and why is it not visible?” The sodium *atoms* are there, you’re seeing them! But there is no silver sodium *metal* there to be seen, only sodium atoms connected together with chlorine atoms in the pattern that makes table salt, not the pattern that makes sodium metal.
It’s the same with lead crystal. There is no metallic lead-the-solid-grey-metal in lead crystal. The lead atoms are built in throughout the structure of the crystal in such a way that the whole thing is still transparent. The properties of lead that you’re used to (being a dark and solid metal) only arise when you have a bunch of lead atoms only bonded to other lead atoms.
You don’t see the sodium or potassium metal in glass either. It’s because they’re not free metals, but ions bound to other atoms. Ions have very different properties from their metallic counterparts, which is why sodium ignites in water, while sodium chloride (ordinary table salt) doesn’t.
Edit: fun fact, glasses that darken in sunlight contain silver ions, which temporarily turn into silver metal when subjected to light. The finely dispersed metal makes the glass look dark.
Metals act like metals because they have a very loose electron. Put a bunch of metal atoms next to each other, and the loose electrons fall off and just form a shared “sea” of electrons. That sea of electrons makes metals shiny, and it also makes them great electrical conductors and heat conductors.
When metals react with other substances, this usually changes. Oxygen is short on electrons and has a tendency to steal them, so has chlorine. So metal chemicals like sodium chloride (salt) don’t act like metals. The chlorine steals the loose electrons and the sodium and chloride now get stuck to each other to form a crystal. There is no electron sea so all the shininess and electrical properties are gone.
Glass is a mixture of various oxides of metals (silicon is the main ingredient. I’m calling it a metal here, although strictly its a semi-metal – it’s loose electrons are not quite loose enough to make it a proper metal, but the are close).
You can add various metal oxides to change the properties of glass. Pure silicon oxide melts at extremely high temperatures and is very difficult to work with. Adding fillers like sodium oxide makes it melt much more easily, so it’s much easier to make stuff with it. Adding heavy fillers like lead oxide or silver oxide makes the glass much heavier.
Finally it’s just worth pointing out that glass is not a crystal – in fact, glass is the opposite of a crystal. Crystals have all their atoms nicely aligned in precise shapes. Glasses have their atoms all over the place in a chaotic arrangement.
Lead glass is often called crystal, because its weight causes more bending of light rays. Heavy glasses are widely used in camera lenses for this reason. If you cut glass with lots of edges and facets then it will tend to sparkle. Heavy glass, because of the extra refraction tends to be more sparkly than normal light glasses.
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