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perceptive。 because the properties repeated themselves periodically； the inventionbecame known as the periodic table。
mendeleyev was said to have been inspired by the card game known as solitaire in northamerica and patience elsewhere； wherein cards are arranged by suit horizontally and bynumber vertically。 using a broadly similar concept； he arranged the elements in horizontalrows called periods and vertical columns called groups。 this instantly showed one set ofrelationships when read up and down and another when read from side to side。 specifically；the vertical columns put together chemicals that have similar properties。 thus copper sits ontop of silver and silver sits on top of gold because of their chemical affinities as metals； whilehelium； neon； and argon are in a column made up of gases。 （the actual； formal determinant inthe ordering is something called their electron valences； for which you will have to enroll innight classes if you wish an understanding。） the horizontal rows； meanwhile； arrange thechemicals in ascending order by the number of protons in their nuclei鈥攚hat is known as theiratomic number。
the structure of atoms and the significance of protons will e in a following chapter； sofor the moment all that is necessary is to appreciate the organizing principle： hydrogen hasjust one proton； and so it has an atomic number of one and es first on the chart； uraniumhas ninety…two protons； and so it es near the end and has an atomic number of ninety…two。
in this sense； as philip ball has pointed out； chemistry really is just a matter of counting。
（atomic number； incidentally； is not to be confused with atomic weight； which is the numberof protons plus the number of neutrons in a given element。） there was still a great deal thatwasn鈥檛 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鈥攊ts existence hadn鈥檛 even been suspectedbefore that鈥攁nd 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鈥檛 beisolated until 1895。 even so； thanks to mendeleyev鈥檚 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。 鈥渨ithout adoubt； the periodic table of the chemical elements is the most elegant organizational chartever devised；鈥潯rote robert e。 krebs in the history and use of our earth鈥檚 chemicalelements； and you can find similar sentiments in virtually every history of chemistry in print。
today we have 鈥�120 or so鈥潯nown elements鈥攏inety…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鈥檚 dayjust sixty…three elements were known； but part of his cleverness was to realize that theelements as then known didn鈥檛 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 鈥減urely speculative；鈥潯ut what is certain is thatanything that is found will fit neatly into mendeleyev鈥檚 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 茅migr茅 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鈥檛 know鈥攚hat no one could know until einstein explained things the followingdecade鈥攚as that the rocks were converting mass into energy in an exceedingly efficient way。
marie curie dubbed the effect 鈥渞adioactivity。鈥潯n the process of their work； the curies alsofound two new elements鈥攑olonium； 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鈥揵orn 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鈥檚 warmth。
they also discovered that radioactive elements decayed into other elements鈥攖hat 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鈥攖he celebrated half…life鈥攁nd 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 ag