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What Makes an Element Reactive?

  • Teacher Resource
  • Posted 08.09.12
  • NOVA

In this video excerpt from NOVA's "Hunting the Elements," New York Times technology columnist David Pogue examines how atomic structure determines reactivity. Meet Theo Gray, chemist and author, who helps David explore how the electron configuration of an atom affects its ability to combine with other atoms. Discover why noble gases are not reactive and why halogens and alkali metals are highly reactive.

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NOVA What Makes an Element Reactive?
  • Media Type: Video
  • Running Time: 3m 24s
  • Size: 12.6 MB
  • Level: Grades 6-12


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:

  • The location of electrons in an atom can be described as concentric shells around the nucleus of the atom.
  • The number of electrons in the outermost shell of an atom determines its reactivity.
  • Noble gases have low reactivity because they have full electron shells.
  • Halogens are highly reactive because they readily gain an electron to fill their outermost shell.
  • Alkali metals are highly reactive because they readily lose the single electron in their outermost shell.

Questions for Discussion

    • Why are noble gases sometimes called inert gases?
    • Think of and describe an analogy to help you remember whether noble gases are reactive.
    • What does the host mean when he says “reactivity is a shell game”?
    • In terms of electrons, what do elements located in the same column of the periodic table have in common?


DAVID POGUE (Technology Guru): The noble gases.

Where does that term "noble gases" come from? Are they nobility? Do they rush to rescue maidens?

THEO GRAY (Chemist and Author): No, you're thinking of heroes. They are like nobility in the sense that they don't mix with the riffraff. They don't like to react with any other elements.

By and large, it's not possible to form compounds with them.

DAVID POGUE: Well, it's a shame for your collection that they are gases, because you've got big blanks here. Oh, ho, ho, ho! The noble gases, like neon and argon, pose a problem for chemists who prefer their elements to join forces and react with each other. You can run an electric current through them, excite their electrons and get pretty colors—which is how neon lights work—but the noble gases don't react.

They pretty much refuse to combine with other elements.

THEO GRAY: Being an inert gas, being unwilling to mix with the other elements, react with them, this is a very clear-cut distinction that sets apart this particular column from all the others in the periodic table.

DAVID POGUE: So why are these guys so aloof? As it turns out, protons may determine the identity of an element, but electrons rule its reactivity. And reactivity is a shell game.

Here's how the game is played.

Imagine that these balls are electrons, and the target is an atom. Electrons don't just pile on around the nucleus. As with skee-ball, where you land, relative to the center counts.

Oh come on!

The electrons take up positions in what can be thought of as concentric shells. The first shell maxes out at just two electrons, the next holds eight, then it goes up to eighteen. An atom with eight electrons in its outer shell makes one happy, satisfied atom.

And noble gases come pre-equipped with completely satisfied shells.

And is this the only column like that?

THEO GRAY: It's the only column where all the shells are completely filled.

DAVID POGUE: But what about the column just before those stable noble gases? They are called the halogens. They have an outer shell that needs just one more electron to be full. And they'll grab it any way they can. The group includes fluorine and bromine, but the most notorious is chlorine: 17 protons surrounded by 17 electrons, arranged in three shells of two, eight and seven, one short of being full.

It's that extra electron chlorine will get any way it can, sometimes with violent results. That's why chlorine gas was used as a deadly poison in World War One.

THEO GRAY: Chlorine, I mean, this is nasty stuff. This will take electrons from kittens. It'll go and steal an electron from off the water in your lungs and turn it to hydrochloric acid, because it really wants an electron.

DAVID POGUE: Yeah, maybe I'll leave that where it was.

THEO GRAY: Now, if you go the other direction, you end up with the alkali metals.

DAVID POGUE: The alkali metals are the first column. Each of them has full shells, plus one extra electron sitting in a new, outer shell.

They have familiar names like lithium, sodium and potassium. And they all want to get rid of that single, lonely electron, any way they can.

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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