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How Elements Form Compounds

  • Teacher Resource
  • Posted 08.09.12
  • NOVA

In this video excerpt from NOVA's "Hunting the Elements," New York Times technology columnist David Pogue investigates chemical reactions involving sodium. Find out how the electron configuration of sodium, an alkali metal, makes it a highly reactive element. Meet chemist and author Theo Gray, who demonstrates two sodium reactions: a spectacular explosive reaction with water; and an exciting reaction with chlorine, a highly reactive halogen. Examine how sodium and chlorine atoms combine to form the compound sodium chloride (ordinary table salt).

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NOVA How Elements Form Compounds
  • Media Type: Video
  • Running Time: 3m 22s
  • Size: 12.1 MB
  • Level: Grades 6-12

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Source: NOVA: "Hunting the Elements"

This media asset was excerpted from NOVA: "Hunting the Elements."

Teaching Tips

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

  • Sodium (Na) is an alkali metal; it is shiny and soft with a single electron in its outermost shell.
  • Alkali metals are highly reactive because they readily lose their outermost electron.
  • Sodium combines with water in an explosive reaction.
  • Chlorine (Cl) is a halogen; it is a highly reactive element that readily gains an electron to fill its outermost shell.
  • Sodium and chlorine, two highly reactive elements, combine to form the stable compound sodium chloride (ordinary table salt).

Questions for Discussion

    • Describe the electron configuration of sodium.
    • Why is sodium highly reactive?
    • How can two unstable elements, such as sodium and chlorine, combine to make a stable compound?


DAVID POGUE (Technology Guru): Sodium: symbol Na; 11 protons and 11 electrons arranged in shells as two, eight and one. Sodium is an alkali metal. Like all the elements in this group, it's desperate to get rid of that extra electron.

THEO GRAY (Chemist and Author): If you cut it quickly,…

DAVID POGUE: I should see some silvery…

THEO GRAY: …you should see a silvery surface inside.

DAVID POGUE: Indeed. Wow.

It slices like cheese, but it's actually a soft metal.

Theo's offered to put on one of his favorite sodium demonstrations. What happens when the pure element dumps its outer electron in a violent altercation with ordinary water?

He insists we wait until nightfall, when the reaction will be most spectacular.

Kids, do not try this at home!

The whole purpose of this contraption is just to dump it into the bucket of water?

THEO GRAY: Yeah. This is a, it's a sodium dumping machine.

DAVID POGUE: All right, let's give this a try. Here we go!

Nice! Oh!

What we're seeing is what happens when sodium's extra electron tears apart water molecules, releasing flammable hydrogen gas—the H in H2O—which explodes when it mixes with air.

The next day, Theo takes it up a notch. As if sodium plus water weren't violent enough, now he wants to combine the same deadly sodium with another lethal element: chlorine, one of the halogens.

The result, he claims, will be a tasty flavoring for a net full of popcorn.

Isn't chlorine deadly poison?

THEO GRAY: Absolutely.

I mean, chlorine, chlorine—they used it as a poison gas in World War I.

DAVID POGUE: It'll be perfectly safe when these two deadly ingredients combine?

THEO GRAY: I didn't say that. I said that after they're combined, the result is perfectly safe. The actual process of combining them is fraught with difficulties.

DAVID POGUE: Okay. And that's why we're dressed up like miners here.

First, a hunk of sodium in a dry metal bowl,then a jet of pure chlorine: surprisingly, no explosion.

Somehow, when these two bad boys of the periodic table come together, they calm down.

At the atomic level, sodium, an alkali metal, had an electron it didn't want, and chlorine, a halogen, wants desperately to grab an electron. Once the handoff was complete, both atoms wound up with full shells, making them stable and able to join together to form a crystal compound we can't live without: sodium chloride, table salt.

Now, I don't exactly see, like, a pile of salt anywhere.

THEO GRAY: No, the salt, most of it went up in the smoke. That is, it went in the popcorn.

DAVID POGUE: It tastes like salt, the good stuff, fresh.

THEO GRAY: Fresh salt.

DAVID POGUE: Only the freshest salt at Theo's farm.

Theo's backyard reactions have given me a crucial insight: how elements come together to form compounds is all about electrons.

Resource Produced by:

WGBH Educational Foundation

Collection Developed by:

WGBH Educational Foundation

Collection Credits

Collection Funded by:

U.S. Department of Energy

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