Atomic Structure of an Alloy

Here are some of the main ideas students should take away from this video:

• An alloy is a combination of elements that has properties different from its components.
• Bronze is an alloy made primarily of copper (Cu); tin (Sn) is typically its main alloying ingredient.
• Bronze was the first manmade alloy.
• Bell makers still use bronze because it has desirable properties, such as hardness and sound quality.
• The atomic structure of a pure metal is orderly and allows electrons to flow freely through the material.
• In bronze, the addition of tin to copper restricts the movement of the copper atoms.

DAVID POGUE (Technology Guru): Tin; symbol Sn; atomic number 50—50 protons and 50 electrons.

Tin added in small amounts to copper makes bronze, the first manmade metal alloy. Bronze helped to spur global trade, and, once forged into tools and weapons, it played a defining role in the empires of antiquity. Bronze named an entire age of human civilization. And even today, it's still hanging around.

This is The Verdin Company, a 170-year-old family-run business in Cincinnati, Ohio.

I'm here because they're about to cast several bells. Even with all the other modern materials available, they still choose bronze. I want to know why. Hasn't something better come along, after all these years?

Ralph Jung offers to make the case for bronze.

RALPH JUNG (Bell Maker, The Verdin Company): This is our pattern that we're going to use to actually make the form in the sand.

DAVID POGUE: So this looks like a finished bell. This isn't a bell?

RALPH JUNG: Yes, it does. This is just the pattern. Yeah. It's made out of aluminum, so it's real easy to handle.

DAVID POGUE: Well, what's wrong with that? Aluminum's good: aluminum doesn't rust; aluminum's light.

RALPH JUNG: You're right. It doesn't.

DAVID POGUE: Why don't you make the bells out of this?

RALPH JUNG: Well, the sound. It doesn't have that lasting ring. And it just…

DAVID POGUE: You don't like how that sounds?

RALPH JUNG: Not really. It sounds kind of tinny, also.

DAVID POGUE: Hey, thanks a lot, buddy.

RALPH JUNG: Well, you know.

DAVID POGUE: I practiced.

RALPH JUNG: We'll show you what a real bell sounds like.

DAVID POGUE: The quality of the sound depends on the atomic structure of the material. In pure metals, the atoms are arranged in orderly rows and columns. Each atom gives up some of its electrons to create a kind of sea of these randomly moving charged particles.

It's these free-flowing electrons that make metals conductive. When placed in a circuit, the negatively charged particles line up and flow as an electric current.

The sea of electrons also creates flexible, metallic bonds among the atoms. In copper, they can slide past each other easily, which makes it relatively soft and easy to dent, not right for a bell. That's why Verdin uses stiffer stuff.

RALPH JUNG: So, we'll put this down into here.

DAVID POGUE: Ralph places the form into a circular steel sleeve, then fills the space around it with a mixture of sand and epoxy, to withstand the searing heat of the hot metal.

When this company started, they used a mixture of horsehair, manure and just about anything else that would hold a shape without burning, but the goal was the same: to create a hollow shape that follows the inner and outer perimeter of the bell.

Once he removes the aluminum and joins the two halves, a bell-shaped space remains on the inside, ready to accept the molten bronze.

RALPH JUNG: And what we have here, David, is the bronze ingots that we use to put in the furnace. As you can see, they're, they've got a little bit of heft to them.

DAVID POGUE: Yeah, it's like…

RALPH JUNG: They average about 20 pounds. That's a, that's a mixture, actually, of 80 percent copper and 20 percent tin. And what we have here is the tin in a raw form. This is how it comes out of the ground. This is from Malaysia.

DAVID POGUE: Okay.

RALPH JUNG: And we have a chunk of copper the way it comes out of the ground. And that's from South Africa.

DAVID POGUE: So, that's the recipe for bronze?

RALPH JUNG: Exactly.

DAVID POGUE: So, you've got copper plus tin equals bronze.

RALPH JUNG: Equals bronze, yeah.

DAVID POGUE: Why couldn't you use one of those metals by themselves? Why don't you make bells out of just copper?

RALPH JUNG: If it was all copper, it would, first of all, be too soft, and we wouldn't get that sound that we want from a bell. Tin with copper gives us that hardness.

DAVID POGUE: Adding tin to copper during melting changes the properties of the metal. The larger tin atoms restrict the movement of the copper atoms, making the material harder.

A blow causes the atoms to vibrate, but the tin prevents them from moving too far out of position. Tin is good for a bell, but only in the right proportion.

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