| MadSci Network: Physics |
Greetings:
Your question is related to a number of factors concerning how atoms interact. In the first chapter "Atoms in Motion" in the "Feynman Lectures on Physics" Professor Feynman a Nobel Laureate writes:
If, in some cataclysm, all of scientific knowledge were to be destroyed and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or what ever you wish to call it) that all things are made of atoms-little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see that there is an enormous amount of information about the world, if just a little imagination and thinking are applied.
Professor Feynman also goes on to say that atoms are very, very small. If we could magnify an apple up to the size of the earth the atoms would be about the size of an apple!
Professor Feynman starts his discussion of melting and boiling with water molecules which are composed of two small hydrogen atoms and one large oxygen atom. At very low temperatures water is a solid (ice). The hydrogen atoms of the water molecules are all stuck (bonded) to each other by attraction and although they are all vibrating as if they are attached by springs, the atoms are all trapped in a fixed location in the solid ice crystal. As the temperature is increased the vibrations become more violent until near zero degrees Celsius (32 F) the molecules break loose from each other (melt) and liquid forms. The hydrogen's still attract each other but the molecules are free to move about sharing hydrogen bonds with each other as they move (flow). If we raise the temperature to 100 degrees Celsius (212 F) the liquid begins to boil and the molecules are vibrating so violently that they break loose from from each other and expand in rapid motion and form a gas (steam).
If you measure the melting points and boiling points of the 100 plus elements you will find a huge range of temperatures. For example helium (He) boils at 0.9 degrees above absolute zero (-273.15 C) and tungsten (Wf) melts at 3422 C and boils at 5555 C (10031F) !
Mercury (Hg) and gallium (Ga) can exist at one atmosphere pressure in three different states; sold, liquid and gas (some scientists would add a fourth state, the highly excited plasma state). The state in which a group of atoms exhist is dependent on a number of factors including the temperature and the pressure. Solids are further complicated because they form in seven different crystal systems. These crystal structures define the optical, electrical, thermal, magnetic and elastic properties of the atoms in the crystal. Many molecules can exhist in several crystaline states called phases, each with different physical properties.
The point of this is that the melting point of a group of atoms is determined by the strength of the bonds between the atoms and that the crystal structure plays a major role in the bond strength. One interesting fact that Hg and Ga have in common is that they prefer the orthorhombic crystal form ( a tilted eight corner box with an atom in each corner) while all the other metals form more complex crystals such as body centered cubes and face centered cubes. Oklahoma State University has a super Web site on crystal structures with animated models showing the cubic forms. You can find this at:
http://bubba.ucc.okstate.edu/jgelder/solstate.html
Another interesting fact is that Hg melts at -38.84 C and boils at 356.73 C while Ga also melts near room temperature at 30 C but boils at 2205 C, a temperature eight times higher than Hg.. Thus while the two metals have solid state bonds that break near room temperature, Ga has much stronger bonds in the liquid state.
Regards - Adrian Popa