托福official8听力lecture4 The Periodic Table of Elements原文解析+翻译音频

[00:00.00]Narrator: Listen to part of a lecture in a chemistry class. [00:06.01]The professor has been discussing the periodic table of elements.
[00:11.65]MALE PROFESSOR: So … are there any questions?
[00:13.79]FEMALE STUDENT: Yes, um, Professor Harrison? You were saying that the periodic table is predictive? [00:19.96]What exactly does that mean? [00:21.68]I mean, I understand how it organizes the elements, but … where’s the prediction?
[00:26.75]MALE PROFESSOR: OK, let's look at our periodic table again. [00:31.19]OK - it groups elements into categories that share certain properties, right?
[00:35.55]FEMALE STUDENT: Uh-huh …
[00:36.44]MALE PROFESSOR: And it's arranged according to increasing atomic number, which is … ? 
[00:40.73]FEMALE STUDENT: The number of protons in each atom of an element.
[00:43.96]MALE PROFESSOR: Right. [00:44.49]Well, early versions of the periodic table had gaps. Missing elements. [00:48.41]Every time you have one more proton you have another element, [00:52.06]and then — oops — there'd be an atomic number for which there was no known element. [00:56.87]And the uh prediction was that an element with that atomic number existed somewhere, but it just hadn't been found yet. [01:04.86]And its location in the table would tell you what properties it should have. [01:09.04]It was really pretty exciting for scientists at that time, to find these missing elements and confirm their predicted properties.
[01:16.67]Um actually, that reminds me of a, of a, of a very good example of all this … element 43. [01:23.46]See on the table, the symbols for elements 42 and 44? [01:29.68]Well in early versions of the table, there was no symbol for an element with 43 protons because no element had yet been discovered with 43 protons. [01:38.87]So the periodic table had a gap between elements 42 and 44.
[01:44.41]And, then, uh, in 1925 a team of chemists led by a scientist named Ida Tacke claimed that they had found element 43. [01:55.17]They had been , uh, using a relatively new technology called x-ray spectroscopy —and they were using this to examine an ore sample —  [02:07.03]and they claimed they'd found an element with 43 protons. [02:10.69]And they named it masurium.
[02:14.53]MALE STUDENT: Um, Professor Harrison? Then how come in my periodic table here element 43 is “Tc”--[02:21.74]that's technetium, right?
[02:23.57]MALE PROFESSOR: OK, let me add that …[02:26.81]Actually, uh, that's the point I’m coming to. [02:29.55]Hardly anyone believed that Tacke had discovered a new element. [02:33.05]X-ray spectroscopy was a new method at the time. [02:36.45]And they were never able to isolate enough masurium to have a weighable sample, to convince everyone of their discovery, [02:42.93]so they were discredited.[02:45.22]But then, twelve years later, in 1937, a different team became the first to synthesize an element using a cyclotron. [02:53.01]And that element had … 
[02:57.40]MALE STUDENT: 43 protons?
[02:59.07]MALE PROFESSOR: That’s right. But they named it technetium to emphasize that it was artificially created — with technology. [03:05.60]And people thought that synthesizing this element, making it artificially, was the only way to get it. [03:11.43]We still hadn’t found it occurring in nature.[03:13.67]Now, element 43, whether you call it masurium or technetium, is radioactive. [03:19.94]Why does that matter? [03:21.21]What’s true of a radioactive element?
[03:25.56]FEMALE STUDENT: It decays? It turns into other elements? [03:29.58]Oh, so does that explain why it was missing in the periodic table?
[03:34.39]MALE PROFESSOR: Exactly. Because of its radioactive decay, element 43 doesn't last very long … [03:40.45]and therefore… if it ever had been present on Earth it would have decayed ages ago … [03:45.52]So … the masurium people were obviously wrong and the technetium people were right … [03:50.66]right?Well, that was then.
[03:53.04]Now we know that element 43 does occur naturally — [03:56.64]it can be naturally generated from uranium atoms that have spontaneously split. [04:01.35]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. [04:10.23]So Tacke’s team might very well have found small amounts of masurium in their ore sample. [04:14.77]It’s just that once it was generated from split uranium, it decayed very quickly.
[04:19.63]And you know, here’s an incredible irony. [04:22.14]Ida Tacke — the chemist who led the masurium team — well, 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!
[04:34.68]MALE STUDENT: So is my version of the periodic table wrong? [04:37.94]Should element 43 really be called masurium?
[04:40.97]MALE PROFESSOR: Maybe. But you know it's hard to tell for sure after all this time if Ida Tacke's group did discover element 43. [04:48.50]They didn’t um publish enough detail on their methods or instruments for us to know for sure.
[04:54.71]But I like to think element 43 was discovered twice.[04:58.21]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 a laboratory. [05:09.13]And, of course, it was an element the periodic table led us to expect existed … before anyone had found it — or made it.