Was watching the “lead balloon” episode of mythbusters the other day and their biggest difficulty was with the fragility of the stuff. I’ve also seen (or heard of, can’t recall) it being rubbed away as easily as solid grease (i.e. the kind you’d find in a grease pencil).
Why is the densest nonradioactive metal so weak (to tension, at least)?
Hijack:
Lead 11.342
Palladium 12.020
Mercury 13.5336
Tungsten 19.25
Gold 19.282
Platinum 21.46
Osmium 22.610
Iridium 22.650
It’s a bit technical, but here goes.
The melting point of lead is about 327C, that’s 600K (Degrees Kelvin, an absolute temperature scale). Room temperature (70F or 21C) is 294K, just about half the melting point of lead. When the temperature of a metal exceeds about one third its melting point, a phenomenon known as “creep” becomes active. Creep is a deformation of the metal at stresses well below the yield strength of the material. Creep can occur very slowly, hence the name. When the deformation exceeds the ductility limit for the metal, it cracks.
As you can see, for lead, creep occurs at and below room temperature. Steel pipe is exposed to over 1000 in a modern steam electrical generating plant. This is above the creep temperature of most steels (melting point about 2700F; 2700 x 0.333 = 900F). They have specially designed steel alloys to use for these pipes that resist creep more than other alloys. Even with these alloys, a steam plant is periodically shut down and inspected. One of the things inspected for is creep damage to these high temperature pipes.
So, the simple answer is that while lead is solid at room temperature, it is close enough to its melting point to affect the properties.
An aside, I studied metallurgical engineering is college (received a BS, actually). They had a research group that performed testing on the creep phenomena. Their lab had a large sign on it, “CREEP LAB”. In general, most people just avoided the entire floor; they really weren’t interested what was behind that door.
excavating (for a mind)
in support of this, we actually used lead solder (solid core) in experiments to demonstrate creep.
it is soft and malleable which might be a better description than calling it fragile.
Why would you expect density to be correlated with hardness or strength?
And these numbers are…?
Where did you get those figures? I have long been under the impression that osmium is the densest element, and wikipedia agrees with me, giving a figure of 22.59 g/cm[sup]3[/sup]. It also explicitly says that iridium is the second densest, at 22.56 (near “room temperature”).
Of course, you are right that lead isn’t even close.
All grams per cubic centimetre, presumably.
The figures quoted agree with those from webelements, which is where I tend to go for this sort of thing. In truth, Os and Ir are so close in density, that different sources will give you different “winners”.
An article on chemistry.about.com says:
hmm, never actually thought about that before. That just seems intuitive to me. On reflection, that’s probably because most forces I’ve observed or studied are diminished by distance. Also, the more diffuse a substance, the more space for outside forces or elements to operate upon it.
Short-version college material science class: The atoms of a solid metal tend to form a crystal lattice structure, whose configuration depends on the number of unfilled positions in the outermost electron shell. These crystal lattices are short-term structures, forming grains with different orientations. The resistance of the macro structure to deformation or rupture depends on the van der Waals forces of the electron shells, the size and coherence (density of dislocations) of the grains, and the amount and relative atomic radius of other (alloying or impurity) elements that can act to restrain deformation. Pure metals are typically softer and weaker than alloyed ones, for that reason.
None of that is related to mass density.
Lead is by no means the “flimsiest” metal. The alkali metals are even softer (and have a tendency to react with water or anything moist). Indium is extremely soft, too.
Lead’s problem, if you’re making a balloon out of it, is that it’s not only soft, it’s heavy, too, and can easily collapse under its own weight. The alkali metal are extremely light.
thwe folkls at Arthur D. Little, by the way, had built one back in 1977. As the article points out, they’d previously made a Silk Purse Out Of a Sow’s Ear (which I saw at the Bicentennial exhibition in Boston in 1976):
I don’t know how this fits in, but I know that galena, lead ore, is very clearly made of some sort of cubic or crystalline structure. It has lines and faults that run parallel and perpendicular to one another in three dimensions. It makes me think that whatever forces are applied to it, it would be inclined to cleave fairly easily, and, indeed, it is easy to break apart along those lines.