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given element。） there was still a great deal thatwasn’t known or understood。 hydrogen is the most mon element in the universe； and yetno one would guess as much for another thirty years。 helium； the second most abundantelement； had only been found the year before—its existence hadn’t even been suspectedbefore that—and then not on earth but in the sun； where it was found with a spectroscopeduring a solar eclipse； which is why it honors the greek sun god helios。 it wouldn’t beisolated until 1895。 even so； thanks to mendeleyev’s invention； chemistry was now on a firmfooting。
for most of us； the periodic table is a thing of beauty in the abstract； but for chemists itestablished an immediate orderliness and clarity that can hardly be overstated。 “without adoubt； the periodic table of the chemical elements is the most elegant organizational chartever devised；” wrote robert e。 krebs in the history and use of our earth’s chemicalelements； and you can find similar sentiments in virtually every history of chemistry in print。
today we have “120 or so” known elements—ninety…two naturally occurring ones plus acouple of dozen that have been created in labs。 the actual number is slightly contentiousbecause the heavy； synthesized elements exist for only millionths of seconds and chemistssometimes argue over whether they have really been detected or not。 in mendeleyev’s dayjust sixty…three elements were known； but part of his cleverness was to realize that theelements as then known didn’t make a plete picture； that many pieces were missing。 histable predicted； with pleasing accuracy； where new elements would slot in when they werefound。
no one knows； incidentally； how high the number of elements might go； though anythingbeyond 168 as an atomic weight is considered “purely speculative；” but what is certain is thatanything that is found will fit neatly into mendeleyev’s great scheme。
the nineteenth century held one last great surprise for chemists。 it began in 1896 whenhenri becquerel in paris carelessly left a packet of uranium salts on a wrapped photographicplate in a drawer。 when he took the plate out some time later； he was surprised to discoverthat the salts had burned an impression in it； just as if the plate had been exposed to light。 thesalts were emitting rays of some sort。
considering the importance of what he had found； becquerel did a very strange thing： heturned the matter over to a graduate student for investigation。 fortunately the student was arecent émigré from poland named marie curie。 working with her new husband； pierre； curiefound that certain kinds of rocks poured out constant and extraordinary amounts of energy；yet without diminishing in size or changing in any detectable way。 what she and her husbandcouldn’t know—what no one could know until einstein explained things the followingdecade—was that the rocks were converting mass into energy in an exceedingly efficient way。
marie curie dubbed the effect “radioactivity。” in the process of their work； the curies alsofound two new elements—polonium； which they named after her native country； and radium。
in 1903 the curies and becquerel were jointly awarded the nobel prize in physics。 （mariecurie would win a second prize； in chemistry； in 1911； the only person to win in bothchemistry and physics。）at mcgill university in montreal the young new zealand–born ernest rutherford becameinterested in the new radioactive materials。 with a colleague named frederick soddy hediscovered that immense reserves of energy were bound up in these small amounts of matter；and that the radioactive decay of these reserves could account for most of the earth’s warmth。
they also discovered that radioactive elements decayed into other elements—that one dayyou had an atom of uranium； say； and the next you had an atom of lead。 this was trulyextraordinary。 it was alchemy； pure and simple； no one had ever imagined that such a thingcould happen naturally and spontaneously。
ever the pragmatist； rutherford was the first to see that there could be a valuable practicalapplication in this。 he noticed that in any sample of radioactive material； it always took the same amount of time for half the sample to decay—the celebrated half…life—and that thissteady； reliable rate of decay could be used as a kind of clock。 by calculating backwards fromhow much radiation a material had now and how swiftly it was decaying； you could work outits age。 he tested a piece of pitchblende； the principal ore of uranium； and found it to be 700million years old—very much older than the age most people were prepared to grant theearth。
in the spring of 1904； rutherford traveled to london to give a lecture at the royalinstitution—the august organization founded by count von rumford only 105 years before；though that powdery and periwigged age now seemed a distant eon pared with the roll…your…sleeves…up robustness of the late victorians。 rutherford was there to talk about his newdisintegration theory of radioactivity； as part of which he brought out his piece of pitchblende。
tactfully—for the aging kelvin was present； if not always fully awake—rutherford notedthat kelvin himself had suggested that the discovery of some other source of heat wouldthrow his calculations out。 rutherford had found that other source。 thanks to radioactivity theearth could be—and self…evidently was—much older than the twenty…four million yearskelvin’s calculations allowed。
kelvin beamed at rutherford’s respectful presentation； but was in fact unmoved。 he neveraccepted the revised figures and to his dying day believed his work on the age of the earth hismost astute and important contribution to science—far greater than his work onthermodynamics。
as with most scientific revolutions； rutherford’s new findings were not universallyaccepted。 john joly of dublin strenuously insisted well into the 1930s that the earth was nomore than eighty…nine million years old； and was stopped only then by his own death。 othersbegan to worry that rutherford had now given them too much time。 but even withradiometric dating； as decay measurements became known； it would be decades before we gotwithin a billi