介绍:
TPO8 Lecture 4 Chemistry
Pro: So, are there any questions?
Stu: Yes, um, Professor Harrison, you were saying that the periodic table is predictive. What exactly does that mean? I mean I understand how it organizes the elements but where’s the prediction?
Pro: Ok, let’s look at our periodic table again. Ok, it groups elements into categories that share certain properties, right?
Stu: Uh-huh.
Pro: And it is arranged according to increasing atomic number, which is…
Stu: The number of protons in each atom of an element.
Pro: Right, well, early versions of the periodic table had gaps, missing elements. Every time you had one more proton, you had another element. And then, oops, there’d be an atomic number, for which there was no known element. And the prediction was that an element with that atomic number existed somewhere, but it just hadn’t been found yet. And its location in the table would tell you what properties it should have. It was really pretty exciting for scientists at that time to find these missing elements and confirm their predictive properties.
Um, actually, that reminds of a ... of a very good example of all these, element 43. See on the table, the symbols for elements 42 and 44. Well, in early versions of the table, there was no symbol for an element 43 protons because no element with 43 protons had been discovered yet. So the periodic table had a gap between elements 42 and 44.
And then in 1925, a team of chemists led by a scientist named Ida Tacke claimed that they had found element 43. They had been using a relatively new technology called X-ray spectroscopy, and they were using this to examine an ore sample. And they claimed that they’d found an element with 43 protons. And they named it Masurium.
Stu: Um, Professor Harrison, then, how come in my periodic table here, element 43 is Tc, that’s Technetium, right?
Pro: Ok, let me add that. Actually, um, that’s the point I’m coming to. Hardly anyone believed that Tacke’d discovered a new element. X-ray spectroscopy was a new method at that time. And they were never able to isolate enough Masurium to have a weighable sample to convince everyone of the discovery. So they were discredited.
But then, 12 years later in 1937, a different team became the first to synthesize an element using a cyclotron. And that element had…
Stu: 43 protons?
Pro: That’s right, but they named it Technetium to emphasize that it was artificially created with technology. And people thought that synthesizing this element, making it artificially was the only way to get it. We still hadn’t found it occuring in nature.
Now element 43, whether you call it Masurium or Technetium, is radioactive. Why does that matter? What is true of a radioactive element?
Stu: It decays, it turns into other elements. Oh, so does that explain why it was missing in the periodic table?
Pro: Exactly, because of its radioactive decay, element 43 doesn’t last very long. And therefore, if that ever had been present on Earth, it would have decayed ages ago. So the Masurium people were obviously wrong, and the Technetium people were right. Right? 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. 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. So Tacke’s team might very well have found small amounts of Masurium in the ore sample. It’s just that once was generated from split Uranium, it decayed very quickly.
And you know here’s an incredible irony, Ida Tacke, the chemist 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.
Stu: So is my version of the periodic table wrong? Should element 43 really be called Masurium?
Pro: Maybe, but you know it’s hard to tell for sure after all this time, if Ida Tacke’s group did discover element 43. They didn’t, um, publish enough detail on their methods or instruments for us to know for sure. But I’d like to think 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 a laboratory. And of course, it was an element the periodic table let us to expect existed before anyone had found it or made it.
