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Why are these denser than gold?
Old 04-07-2012, 09:24 AM   #1
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Why are these denser than gold?

I watched Nova on PBS this week. The show was called Hunting the Elements and very interesting.

Initially they discussed gold. It's atomic weight is 197 (protons + neutrons) it's atomic number is 79 (protons). They said only 4 elements are denser than gold - rhenium 186 and 75, osmium 190 and 76, iridium 192 and 77 and finally platinum 195 and 78.

How can these 4 be denser than gold when their atomic weight is less than gold's?

There are many elements with atomic weights above gold's, aren't they denser if a solid vs a gas?

I wonder if this comment was a mistake? Nova is a pretty good show and I would be surprised if they made a blunder like that. Anyone here know?

Here's and excellent periodic table - Dynamic Periodic Table
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Old 04-07-2012, 09:34 AM   #2
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Google-fu:

The atomic mass or weight of Gold is 197 and its density under normal conditions is 19.3 g/cm3. The atomic mass of Uranium is 238 and its density is 19 g/cm3, slightly lower than Gold, even though its atomic mass is larger. How is that possible?

The reason is that the Gold atoms are packed in closer under normal conditions than the Uranium atoms. Most of the time, the heavier atom will result having greater density.
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Old 04-07-2012, 01:25 PM   #3
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The was a great show; that Pogue guy is pretty funny. I agree with Soup, there are other things that affect density.
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Old 04-07-2012, 03:21 PM   #4
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As stated above, there is more to density than atomic weight. The density of water behaves in such a way to sustain life as we know it. As water cools the density increases, in line with what happens to almost every other liquid. However, once you get to about 3.9 C, as the water cools more it becomes LESS dense and floats to the top.

When a normal liquid freezes it freezes from the bottom up but water freezes from the top down (the cooler liquid is at the top). This means you get an insulating cover of ice keeping the water below liquid at extremely cold surface temperatures.
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Old 04-07-2012, 03:45 PM   #5
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Thanks, that's the answer. I assumed (!) the atomic weight was directly proportional to the density - more protons and neutrons the more dense the atom but then read below! Seems reasonable but incorrect.

The amount of empty space in the nucleolus of an atom is hard to imagine, it's mostly empty! I love this stuff but it is mind blowing.
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