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1 .<-NARRATOR:->Listen to part of a lecture in a chemistry class. The professor has been discussing the periodic table of elements.

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1 .<-MALE PROFESSOR:->So … are there any questions?

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1 .<-FEMALE STUDENT:->Yes, em, professor Harrison, you were saying that the periodic table is predictive [hesitant upspeak]? What exactly does that mean?

2 .I mean I understand how it organizes the elements, but … where is the prediction?

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1 .<-MALE PROFESSOR:->OK, let's look at periodic table again.

2 .OK, it groups elements into categories that share certain properties, right?

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1 .<-FEMALE STUDENT:->Um-huh~

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1 .<-MALE PROFESSOR:->And it's arranged according to increasing atomic number, which is … ? [prompting for answer]

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1 .<-FEMALE STUDENT:->The number of protons in each atom of an element.

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1 .<-MALE PROFESSOR:->Right, well, early versions of the periodic table had gaps, missing elements.

2 .Every time you have one more proton you have another element, and then — oops — there’d be an atomic number for which there was no known element. And the uh prediction was that an element with that atomic number existed somewhere, but it just hadn’t been found yet.

3 .And its location in the table would tell you what properties it should have.

4 .It was really pretty exciting for scientists at that time to find these missing elements and confirm their predictive properties.

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1 .Um actually, that reminds me of a, of a, of a very good example of all this … element 43.

2 .See on the table, the symbols for element 42 and 44.

3 .Well, in early versions of the table, there was no symbol for element 43 protons because no element with 43 protons had been discovered yet.

4 .So the periodic table had a gap between elements 42 and 44. And, then, uh, in 1925 a team of chemists led by a scientist named Ida Tacke claimed that they had found element 43.

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1 .They had been, uh, using a relatively new technology called x-ray spectroscopy —and they were using this to examine an ore sample — and they claimed they’d found an element with 43 protons. And they named it masurium.

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1 .<-MALE STUDENT:->Um, professor Harrison, then, how come in my periodic table, here, element 43 is “Tc”, that's technetium, right?

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1 .<-MALE PROFESSOR:->OK, let me add that …[writing “technetium” on blackboard]

2 .Actually, um, that's the point I'm coming to.

3 .Hardly anyone believed that Tack has discovered the new element.

4 .X-ray spectroscopy was a new method at that time.

5 .And they were never able to isolate enough masurium to have a weighable sample, to convince everyone of their discovery, so they were discredited.

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1 .But then, twelve years later, in 1937, a different team became the first to synthesize an element using a cyclotron. And that element had … [dramatic pause, leading the students to answer]

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1 .<-MALE STUDENT:->43 Protons?

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1 .<-MALE PROFESSOR:->That's right, but they named it Technetium to emphasize that it was artificially created with technology.

2 .And people thought that synthesizing this element, making it artificially was the only way to get it.

3 .We still haven't found it occurring in nature.

4 .Now, element 43 what they called Masurium or Technetium is radioactive.

43号元素无论称为Masurium还是Technetium，是具有放射性的。

5 .Why is that matter? What's true of radioactive element?

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1 .<-FEMALE STUDENT:->It decays? [adding to her answer] It turns into other elements? Oh [light dawning], so does that explain why it was missing in the periodic table?

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1 .<-MALE PROFESSOR:->Exactly. Because of its radioactive decay, element 43 doesn’t last very long … and therefore… if it ever had been present on Earth it would have decayed ages ago …

2 .[Reporting on what people believed then, not what he himself thinks] So the Masurium people were obviously wrong, and the Technetium people were right, right?

3 .Well, that was then. Now we know that element 43 does occur naturally—it can be naturally generated from uranium atoms that have spontaneously split.

4 .And guess what ... the ore sample the masurium group was working with had plenty of uranium in it—enough to split into measurable amounts of masurium.

5 .So Tacke’s team might very well have found small amounts of masurium in their ore sample. It’s just that once it was generated from split uranium, it decayed very quickly.

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1 .And you know, here is an incredible irony.

2 .Ida Tack, the chemist led the Masurium team, when she was the first to suggest that Uranium could break up into smaller pieces but she didn't know that that was the defense of her own discovery of element 43.

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1 .<-MALE STUDENT:->So is my version of the periodic table wrong? Should element 43 really be called Masurium?

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1 .<-MALE PROFESSOR:->Maybe, but you know it‘s hard to tell for sure after all this time, if Ida Tack’s group did discover element 43.

2 .They didn't, um, publish enough detail on their methods or instruments for us to know for sure.

3 .But I'd like to think that element 43 was discovered twice, as Masurium, it was the first element discovered that occurs in nature only from spontaneous fission, and as Technetium, it was the first element discovered in laboratory.

4 .And of course, it was an element the periodic table let us to expect existed before anyone had found it or made it.