26. The World of Metals and Its Paradoxes
by L. Vlasov and D. Trifonov

 

   
   Over eighty of the elements in the Periodic System are metals. On the whole, they resemble one another more than the nonmetals. And yet there is no end of surprises in the metal kingdom.

   For instance, what colour are the different metals?

   Metallurgists divide all metals into ferrous and nonferrous. The ferrous metals include iron and its alloys. All the rest are nonferrous metals, except for the noble ones, their "Majesties" Silver, Gold and Platinum and Co.

   This is a very crude division and even the metals themselves object strongly to such lack of discrimination. Each metal actually has its own particular hue. Its dark, dull, or silvery base always has a definite tint. Scientists have become convinced of this by studying metals in the very pure state.

   Many of them when left in the air, become coated sooner or later with a very thin film of oxide which masks their true colour. But the pure metals give a very wide range of colours. The observant eye can discern metals with bluish, greenish-blue and greenish shades, with a reddish or yellowish play of colours, dark-grey like sea water on a cloudy autumn day, and shiny silvery ones which reflect solar rays like a mirror.

   The colour of a metal depends on many factors. Among others, it depends upon the method of its production. A metal obtained by sintering has a different appearance from the same metal poured into an ingot.

   If we compare metals by weight, we can distinguish light, medium and heavy ones.

   These "weight classes" have their record holders.

   Lithium, sodium, and potassium do not sink in water, because they are lighter than water. For example, the density of lithium is almost half that of water, which equals unity. Were lithium not so active an element, it would be an excellent material for a great variety of purposes. Imagine a ship or an automobile made entirely of lithium. Unfortunately, chemistry bans this attractive idea.

The "heavyweight champion" among the metals is osmium. One cubic centimetre of this noble metal weighs 22.6 grams. To balance one cube of osmium we would have to put on the other tray, say, three cubes of copper, two cubes of lead or four cubes of yttrium. The "performance" of osmium's closest neighbours, namely, platinum and iridium, is almost as high. The noble metals are also the heaviest metals.

   The hardness of metals has become proverbial. If a man is always composed and cool-headed, we say he has "iron nerves." But in the world of metals the situation is different.

   Here iron is hardly a model of hardness. The hardness champion is chromium which is just slightly inferior to diamond. By the way, paradoxical though it sounds, the hardest chemical elements are not metals at all. At the top of the conventional hardness scale stand diamond (a form of carbon) and crystalline boron. Iron should rather be classed as a soft metal; it is only half as hard as chromium. And as to the lightweights, the alkali metals, they are as soft as wax.

Liquid Metals
and a Gaseous (?) Metal

   
   All the metals are solids, harder or softer. Such is the general rule. But there are exceptions.

  Some metals are more like liquids. A chip of gallium or cesium melts on your palm, because the melting point of these metals is just below thirty degrees Celsius (86F). Francium, which has not been prepared as the pure metal so far, would melt at room temperature.

   Mercury is a classical example of a liquid metal which everybody knows. It freezes at minus 39C (-38.2F), which makes it eligible for various kinds of thermometers.

An important rival to mercury in this respect is gallium, and here is why. Mercury boils at the comparatively low temperature of about 300C (572F). This makes mercury thermometers useless for measuring high temperatures. But it takes a temperature of 2000C (3670F) to turn gallium into a vapour.

   Not a single metal can remain in the liquid state for so long, i.e., has such a large interval between its melting and boiling points, as gallium. This makes gallium an excellent material for high-temperature thermometers.

One more thing, and this is quite remarkable. Scientists have proved theoretically that if there existed a heavy analogue of mercury (an element with a very large atomic number an inhabitant of, the imaginary seventh floor - eighth period - of the Big House, unknown on Earth) its natural state at ordinary conditions would be gaseous.

   A gas possessing the chemical properties of a metal!

   Will scientists ever have such a unique element to study?

A lead wire can be melted in a match flame. Tinfoil immediately changes into a drop of liquid tin if thrown into a fire. But to liquefy tungsten, tantalum or rhenium, the temperature has to be raised above 3000C (about 5500F).

   These metals are harder to melt than any of the others. That is why the filaments of incandescent electric light bulbs are made of tungsten and rhenium.

   The boiling point of some metals are really tremendous. For instance, hafnium begins to boil at 5400 (almost 9800 F) (!), almost the temperature of the Sun's surface.