Simple Nature - Light and Matter

the muddle. The row-and-column scheme he came up with is essentiallyour modern periodic table. The columns of the modern versionrepresent groups of elements with similar chemical properties, andeach row is more massive than the one above it. Going across eachrow, this almost always resulted in placing the atoms in sequenceby weight as well. What made the system significant was its predictivevalue. There were three places where Mendeleev had to leavegaps in his checkerboard to keep chemically similar elements in thesame column. He predicted that elements would exist to fill thesegaps, and extrapolated or interpolated from other elements in thesame column to predict their numerical properties, such as masses,boiling points, and densities. Mendeleev’s professional stock skyrocketedwhen his three elements (later named gallium, scandiumand germanium) were discovered and found to have very nearly theproperties he had predicted.One thing that Mendeleev’s table made clear was that mass wasnot the basic property that distinguished atoms of different elements.To make his table work, he had to deviate from orderingthe elements strictly by mass. For instance, iodine atoms are lighterthan tellurium, but Mendeleev had to put iodine after tellurium sothat it would lie in a column with chemically similar elements.Direct proof that atoms existedThe success of the kinetic theory of heat was taken as strong evidencethat, in addition to the motion of any object as a whole, thereis an invisible type of motion all around us: the random motion ofatoms within each object. But many conservatives were not convincedthat atoms really existed. Nobody had ever seen one, afterall. It wasn’t until generations after the kinetic theory of heat wasdeveloped that it was demonstrated conclusively that atoms reallyexisted and that they participated in continuous motion that neverdied out.The smoking gun to prove atoms were more than mathematicalabstractions came when some old, obscure observations were reexaminedby an unknown Swiss patent clerk named Albert Einstein.A botanist named Brown, using a microscope that was state of theart in 1827, observed tiny grains of pollen in a drop of water on amicroscope slide, and found that they jumped around randomly forno apparent reason. Wondering at first if the pollen he’d assumed tobe dead was actually alive, he tried looking at particles of soot, andfound that the soot particles also moved around. The same resultswould occur with any small grain or particle suspended in a liquid.The phenomenon came to be referred to as Brownian motion, andits existence was filed away as a quaint and thoroughly unimportantfact, really just a nuisance for the microscopist.It wasn’t until 1906 that Einstein found the correct interpreta-464 Chapter 8 Atoms and Electromagnetism