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 [Music]
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 Welcome to Everything Matters - Tales From the
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 Periodic Table I'm your host, Ron Hipschman and today
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 we're going to talk about the element Cadmium.  Here I have a sample of Cadmium
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 for you to see, kind of a silvery element, let's get back to our
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 presentation. Here we see the beautiful periodic table
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 produced by Theodore Gray. Incidentally Theo has written one of my
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 favorite books called "The Elements". You can pick it up from the
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 online Exploratorium store if you'd like. Check out his fantastic site
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 periodictable.com. Cadmium is the 48th element in the
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 periodic table its atomic number is 48 because that's
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 how many protons are in its nucleus. The number of protons in a nucleus
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 determines which element we're talking about.
[00:01:01.22]

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 Cadmium was discovered simultaneously in 1817
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 by Frederick Stromeyer and Carl Samuel Leberecht Hermann, both in Germany.
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 They discovered it as an impurity in Zinc carbonate.
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 That Zinc carbonate is found in nature as the mineral
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 Smithsonite, historically called Calamine, an old name for Zinc ores. Incidentally
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 calamine lotion is a mixture of Zinc oxide,
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 not Zinc carbonate with a little bit of Iron oxide and is really
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 unrelated to this Zinc ore. Cadmium is obtained almost
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 exclusively as a byproduct of Zinc smelting,
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 and to a lesser extent of Lead and Copper smelting.
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 Smaller quantities are also produced by the recycling of
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 iron and steel. Cadmium is from the latin "cadmia" (or greek "cadmia"),
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 meaning "calamine", that was named after the Greek
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 character Cadmus, the mythological founder,
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 and first king of Thebes, who we see here fighting the dragon in a painting
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 by Hendrick Goltzius. From this rich history, we get the name of the
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 element Cadmium. Cadmium belongs to a large group of
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 elements in the middle of the periodic table called the "transition
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 metals". These elements are defined as ones with an incomplete
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 "d shell" of electrons. This is also known as the
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 "d block" of the periodic table. Cadmium is the last element in its row
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 that's a member of the d block, though there is some debate among chemists as
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 to whether Cadmium really belongs in that d block.
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 The universe produces some Cadmium in
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 merging neutron stars (pretty exotic) with about equal amounts
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 also produced in dying low mass stars. Here on earth, the main suppliers of
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 Cadmium are, not surprisingly, also the main suppliers
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 of Zinc. China, with almost one-sixth of the
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 world's Cadmium production, is closely followed by South Korea and Japan
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 as the major suppliers. Use of Cadmium has grown steadily since nineteen
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 hundred. The world now produces about twenty five
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 thousand metric tons per year and this figure does not
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 include U.S. production whose production numbers are
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 withheld for some odd reason. Some Zinc ores contain up to 1.4 percent
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 Cadmium. In the 1970's, the output of Cadmium was
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 6.5 pounds of Cadmium per ton of zinc.
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 The American Chemical Society's endangered element list
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 places Cadmium as "limited availability -
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 future risk to supply", so we need to keep our eye on this and be
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 sure to recycle our Cadmium carefully, not only because it's relatively rare,
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 but also it's highly toxic. How common is Cadmium?
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 Not very. Let's see. It's the 58th most common element in the Universe,
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 the 42nd most common element in the Sun, the 48th most common element in
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 meteorites, the 66th most common in the Earth's
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 crust, a little more common in the oceans at
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 45th most common element, that actually accounts for about 8
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 million tons of Cadmium - about the same as the amount
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 of gold in the ocean, 8 million tons of gold in the ocean as well.
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 It's the 22nd most common element in humans -
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 more on that later. Cadmium, despite its limited abundance, is
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 actually pretty cheap. A little less than three
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 dollars per kilogram in large quantities. If you had invested in Cadmium about
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 five years ago, you would have made a tidy profit. It's
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 almost doubled in price during those five years.
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 That's about 14 percent per year increase on the average. That's
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 a pretty decent return on investment. If we compare the size of the Cadmium
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 atom to that of Hydrogen, we'd see something like this.
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 Most of the transition elements in that d-block have similar sized atoms.
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 Here, our atom size is sorted from largest
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 (cesium, at the left) to helium (the smallest, on the right).
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 You can see that Cadmium is pretty close to the middle of the pack.
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 Each element has many different forms. For each specific element
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 the number of protons in the nucleus is the same,
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 48 protons for Cadmium like we mentioned, but there can be different numbers of
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 neutrons in the nucleus. All these different forms are called
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 "isotopes" and they're chemically identical but
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 with slightly different weights. The number you see next to the chemical
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 symbol is the total number of protons and neutrons in the nucleus. There are 38
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 isotopes of Cadmium, and of these there are six stable,
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 non-radioactive isotopes. Cadmium 106, 108, 110, 111, 112,
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 and 114. They occur in various abundances in nature.
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 But if you add up the six stable isotopes,
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 you don't get one hundred percent, you get close to eighty percent.
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 The radioactive isotopes Cadmium 113 and Cadmium 116
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 account for the rest. We'll see why in the next slide.
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 By the way the word isotope comes from the greek "isos",
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 meaning "same", and "topos" meaning "place", since they all occupy the same
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 place in the periodic table. Of the radioactive isotopes of Cadmium,
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 these are the longest lived - the ones with
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 half-lives all over one hour. If you wait
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 for one half-life, half of the isotope you have
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 will decay. If you wait for one more half-life,
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 half of that half decays, leaving you with one
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 quarter, and so on. As you can see, the longest half-lives here are for
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 those two isotopes making up about 20 percent
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 of the existing Cadmium. Cadmium 113 has a half-life of 8 times 10 to the
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 15th years. That's 583 times the age of the
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 universe! While Cadmium 116 has a half-life of 2.8
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 times 10 to the 19th years, or over 2
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 billion times the age of the universe! So you see that not much of either of
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 those isotopes has had enough time to decay.
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 significantly. Cadmium is moderately dense at 8.65
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 grams per cubic centimeter. Remember that water has a density of one
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 gram per cubic centimeter. I've put up a
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 couple more densities for comparison. Cadmium is slightly denser than iron at
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 7.8 grams per cubic centimeter. Here is a graph of the elements from
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 highest density on the left, to lowest density on the
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 right. When we do this talk at the Exploratorium,
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 we have a set of blocks, so you can feel the effects of density yourself,
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 but we'll have to wait to do this until we're back in the museum.
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 Our set of blocks have a wide range of densities
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 with the heaviest at Tungsten, to Lead, to Copper, to Iron
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 to Titanium, to Aluminum, to Magnesium. We also have a plastic and
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 wood blocks, but those are not technically elements. Again Cadmium's density is 8.65
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 grams per cubic centimeter, just above Iron
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 and below Copper making it the 36th densest element. Cadmium
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 has the 73rd highest melting point on this chart,
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 with a melting point of 321 degrees celsius. Pretty low. There are only 16
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 other solid elements that melt at lower temperatures.
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 Cadmium also has the 73rd highest boiling point
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 at 767 degrees Celsius. Its boiling point is only 446 above its
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 melting point. There are only four other metal elements
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 known to have a smaller difference between the melting and
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 boiling points. Those would be magnesium, thulium, mercury, and ytterbium.
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[00:10:05.22]
 Cadmium is slightly exceptional because it has the seventh highest
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 thermal expansion rate - three one hundred thousandths
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[00:10:14.12]
 per degree Celsius. That means that if you had a
[00:10:18.00]

[00:10:18.00]
 one meter bar of Cadmium, it would get longer by three
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 one hundred thousandths of a meter (or three hundredths of a millimeter)
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 when you raised its temperature by one degree Celsius.
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 I know that doesn't sound like much, but it does add up when you change the
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 temperature significantly, or you have a long bar of Cadmium.
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 Cadmium is a very soft solid with a hardness of only
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 two on mohs scale of hardness. You might be able to scratch this metal with your
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 fingernail it's so soft! In this chart of hardness sorted from
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 hardest on the left (Boron), to softest on the right (Cesium),
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 Cadmium is the 43rd hardest (and 24th softest)
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 element. Cadmium is the 15th best conductor of electricity.
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 No slouch, but it wouldn't make very good wire either. With its higher resistance,
[00:11:16.21]

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 you'd waste most of your electricity in heat it would generate,
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 plus, remember, that it melts at a very low temperature.
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 Electricians frown on melty wires! Cadmium is the 19th best conductor of
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 heat. Again, not bad but not really good enough
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 to use for this purpose either. From our periodic table of the spectra,
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 we see that Cadmium displays a variety of emission lines
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 across the spectrum. What's not obvious from this picture, is
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 that most of the light comes from the green
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 and blue end of the spectrum. The bright lines given off by Cadmium
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 are seen all combined in this special Cadmium lamp. When the
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 lamp is first turned on, you see the purplish glow of argon gas.
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 This gas vaporizes the solid Cadmium in the glass tube,
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 and you then see its beautiful bluish green glow.
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 The biggest use of Cadmium is (or was) in the making of rechargeable batteries.
[00:12:30.13]

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 Here we see a variety of Nickel Cadmium
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 batteries. The disadvantages of these is that they
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 have a limited number of charge- discharge cycles before they would no
[00:12:41.12]

[00:12:41.12]
 longer hold a charge, and the fact that the toxic nature of
[00:12:46.04]

[00:12:46.04]
 Cadmium meant we had to be very careful to recycle them properly.
[00:12:51.07]

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 They also have a fully charged voltage of only 1.2 volts, making them
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 unsuited in some voltage sensitive applications
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 as direct replacements for the more common 1.5 volt
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 carbon zinc batteries of the same sizes.
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 Electroplated Cadmium, banned in most circumstances because of the metal's
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 toxicity, functions as a sacrificial coating,
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 corroding before the substrate material. Cadmium plating's main use is in
[00:13:25.01]

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 aeronautical applications.
[00:13:28.17]

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 Compounds containing Cadmium were used as a phosphor
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 in black & white television picture tubes to create the white light
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[00:13:36.19]
 forming the picture we watched. Different Cadmium compounds were used in
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[00:13:41.22]
 color televisions to create the green and blue glowing phosphors.
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[00:13:47.01]
 Of course, now that we've moved beyond picture tubes,
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 that use is only academic. Here's how it worked.
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 Electrons, boiled off the heated cathode in the neck of the tube,
[00:13:59.07]

[00:13:59.07]
 are accelerated, focused, and bent to strike the Cadmium oxide phosphor
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[00:14:04.23]
 screen to create the white glow we looked at.
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[00:14:10.06]
 In 1860, Barnabas Wood announced the discovery of an alloy of Bismuth,
[00:14:16.15]

[00:14:16.15]
 Lead, Tin, and Cadmium in proportions such that it had a very low melting
[00:14:23.03]

[00:14:23.03]
 point. Wood's metal melts at a low low
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[00:14:27.14]
 70 degrees Celsius (or 158 degrees Fahrenheit).
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[00:14:32.15]
 A nice cup of tea would melt a spoon made of this,
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[00:14:36.10]
 but i wouldn't drink that tea, Not only does Wood's metal contain
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[00:14:41.01]
 toxic Cadmium, but also poisonous lead. Here you see a small ingot
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 of Wood's metal melting in a hot beaker of water.
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[00:14:54.04]
 In a nuclear fission reactor, rods of Cadmium are used to absorb
[00:14:59.01]

[00:14:59.01]
 neutrons. Those neutrons are responsible for the chain reaction
[00:15:03.20]

[00:15:03.20]
 causing the nuclear material in the reactor core,
[00:15:07.05]

[00:15:07.05]
 usually uranium, to break into smaller atoms,
[00:15:10.21]

[00:15:10.21]
 releasing tremendous amounts of energy. Pulling the rods out of the core allows
[00:15:16.12]

[00:15:16.12]
 more neutron reactions to take place, heating up the core. Pushing the rods in,
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[00:15:22.12]
 slows the reaction, cooling the reactor. Hence, "control rods".
[00:15:29.12]

[00:15:29.18]
 In 1907 Cadmium became the standard by which
[00:15:33.16]

[00:15:33.16]
 length was measured. In that year the International Union for the Cooperation
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[00:15:39.05]
 in Solar Research (which later became the International
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[00:15:42.19]
 Astronomical Union), defined the international angstrom by
[00:15:47.03]

[00:15:47.03]
 declaring the wavelength of this red line of Cadmium equal
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[00:15:52.02]
 to 6438.4696 international angstroms. This definition
[00:16:00.04]

[00:16:00.04]
 was endorsed by the International Bureau of Weights and Measures in 1927.
[00:16:05.22]

[00:16:05.22]
 By the way, the angstrom unit is named after the Swedish physicist,
[00:16:10.15]

[00:16:10.15]
 and one of the founders of the science of spectroscopy,
[00:16:14.04]

[00:16:14.04]
 Anders Jonas Angstrom. The angstrom unit is 10 to the minus 10th (or one ten
[00:16:21.03]

[00:16:21.03]
 billionth) of a meter. Pretty small. However,
[00:16:24.10]

[00:16:24.10]
 in 1960 the definition of both the meter and the angstrom were changed to use the
[00:16:31.03]

[00:16:31.03]
 element Krypton instead. Poor Cadmium.
[00:16:37.18]

[00:16:37.18]
 Cadmium sulfide is used as a yellow pigment
[00:16:41.12]

[00:16:41.12]
 called "Cadmium yellow" and Cadmium selenide,
[00:16:46.02]

[00:16:46.02]
 or in this case Cadmium selenosulfide, is used as a red pigment often called
[00:16:52.19]

[00:16:52.19]
 "Cadmium red"
[00:16:55.18]

[00:16:56.06]
 This is a photocell or a photoresistor. The electrical resistance of a
[00:17:02.12]

[00:17:02.12]
 photoresistor decreases with brighter and brighter light
[00:17:06.15]

[00:17:06.15]
 intensity, allowing more current to flow when it's brightly
[00:17:10.06]

[00:17:10.06]
 illuminated. It can change its resistance by a factor
[00:17:14.04]

[00:17:14.04]
 of up to 10,000 going from darkness to light.
[00:17:19.03]

[00:17:19.03]
 This is the electronic symbol for the photoresistor, for the next time you find
[00:17:23.07]

[00:17:23.07]
 yourself reading a schematic. The photoresistor gets its light
[00:17:27.11]

[00:17:27.11]
 sensitivity from Cadmium sulfide  - the same ingredient
[00:17:32.04]

[00:17:32.04]
 in the pigment Cadmium yellow.
[00:17:36.02]

[00:17:36.04]
 Cadmium selenide quantum dots (or QDs), are nano particles that fluoresce in a
[00:17:42.06]

[00:17:42.06]
 variety of colors determined by the size of the particle.
[00:17:46.13]

[00:17:46.13]
 The same exact material, Cadmium selenide, is in each vial of liquid here.
[00:17:53.16]

[00:17:53.16]
 Only the size of the particles is different,
[00:17:56.17]

[00:17:56.17]
 and is used to tune the color. Those sizes
[00:18:00.08]

[00:18:00.08]
 are labeled above the vials in nanometers, or
[00:18:03.09]

[00:18:03.09]
 billionths of a meter. These vials are being illuminated
[00:18:06.23]

[00:18:06.23]
 with an ultraviolet light to make them fluoresce.
[00:18:10.00]

[00:18:10.00]
 This has some interesting applications. For instance, in making LCD panels,
[00:18:16.19]

[00:18:16.19]
 one can make the colors from the tv brighter
[00:18:20.04]

[00:18:20.04]
 and more saturated with the use of a quantum dot panel
[00:18:23.18]

[00:18:23.18]
 that emits light at just the right colors
[00:18:26.19]

[00:18:26.19]
 to more efficiently pass through the red, green, and blue filters in front of the
[00:18:32.15]

[00:18:32.15]
 liquid crystals.
[00:18:35.14]

[00:18:36.02]
 Relatively low in cost, Cadmium and Tellurium can be compounded into
[00:18:42.06]

[00:18:42.06]
 Cadmium telluride thin film photovoltaic modules.
[00:18:46.23]

[00:18:46.23]
 Unlike silicon solar cells that use actual
[00:18:50.10]

[00:18:50.10]
 bulk slices of silicon, these only use a thin vapor deposited film,
[00:18:57.11]

[00:18:57.11]
 so they use much less material and are hence
[00:19:00.13]

[00:19:00.13]
 cheaper to manufacture. The thinness means that you can deposit the film on
[00:19:05.18]

[00:19:05.18]
 flexible materials like you see here. This also adds some unique manufacturing
[00:19:11.03]

[00:19:11.03]
 possibilities over the rigid and brittle silicon cells.
[00:19:17.05]

[00:19:17.11]
 One of the early lasers developed was the Helium
[00:19:20.21]

[00:19:20.21]
 Cadmium laser. It put out a very deep violet beam of light very different
[00:19:27.16]

[00:19:27.16]
 than the much more common red of the helium neon
[00:19:31.01]

[00:19:31.01]
 (or HeNe) laser. Cadmium plays no natural biological role,
[00:19:37.16]

[00:19:37.16]
 and should not normally be found in the human body.
[00:19:41.22]

[00:19:41.22]
 Cadmium and its compounds are highly toxic
[00:19:45.05]

[00:19:45.05]
 and exposure to this metal is known to cause cancer.
[00:19:48.21]

[00:19:48.21]
 Cadmium targets the body's cardiovascular,
[00:19:52.04]

[00:19:52.04]
 renal, gastrointestinal, neurological, reproductive, and respiratory systems.
[00:19:58.12]

[00:19:58.12]
 Is there a system it doesn't attack? Of course,
[00:20:02.10]

[00:20:02.10]
 Cadmium is found in the human body because of environmental pollution,
[00:20:06.13]

[00:20:06.13]
 but thankfully in small quantities. We'll end today's talk with Mary
[00:20:12.23]

[00:20:12.23]
 Soon Lee's elemental haiku about Cadmium. Defined the angstrom,
[00:20:19.05]

[00:20:19.05]
 until krypton ousted you, now looking for work, Thank you for watching
[00:20:26.13]

[00:20:26.13]
 Everything Matters -  Tales Fom the Periodic Table. The next
[00:20:30.13]

[00:20:30.13]
 program in this series will examine another very interesting
[00:20:34.12]

[00:20:34.12]
 element -  Indium. We hope you'll join us.
[00:20:39.07]

[00:20:39.07]
 This program, like all Exploratorium programs, is only possible
[00:20:43.11]

[00:20:43.11]
 because of donors like you. We know that this time is challenging, but if you can,
[00:20:49.16]

[00:20:49.16]
 help us keep educational content like this free and accessible to all
[00:20:54.10]

[00:20:54.10]
 by donating today at
[00:20:58.06]

[00:20:58.07]
 www.exploratorium.edu/connect Thank you
[00:21:05.00]

[00:21:05.02]
 [MUSIC]
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