he factory burned down and thefamily was reduced to penury。 determined to get her youngest child an education； theindomitable mrs。 mendeleyev hitchhiked with young dmitri four thousand miles to st。
petersburg鈥攖hat鈥檚 equivalent to traveling from london to equatorial guinea鈥攁nd depositedhim at the institute of pedagogy。 worn out by her efforts； she died soon after。
mendeleyev dutifully pleted his studies and eventually landed a position at the localuniversity。 there he was a petent but not terribly outstanding chemist； known more forhis wild hair and beard； which he had trimmed just once a year； than for his gifts in thelaboratory。
however； in 1869； at the age of thirty…five； he began to toy with a way to arrange theelements。 at the time； elements were normally grouped in two ways鈥攅ither by atomic weight（using avogadro鈥檚 principle） or by mon properties （whether they were metals or gases；for instance）。 mendeleyev鈥檚 breakthrough was to see that the two could be bined in asingle table。
as is often the way in science； the principle had actually been anticipated three yearspreviously by an amateur chemist in england named john newlands。 he suggested that whenelements were arranged by weight they appeared to repeat certain properties鈥攊n a sense toharmonize鈥攁t every eighth place along the scale。 slightly unwisely； for this was an ideawhose time had not quite yet e； newlands called it the law of octaves and likened thearrangement to the octaves on a piano keyboard。 perhaps there was something in newlands鈥檚manner of presentation； but the idea was considered fundamentally preposterous and widelymocked。 at gatherings； droller members of the audience would sometimes ask him if he couldget his elements to play them a little tune。 discouraged； newlands gave up pushing the ideaand soon dropped from view altogether。
mendeleyev used a slightly different approach； placing his elements into groups of seven；but employed fundamentally the same principle。 suddenly the idea seemed brilliant andwondrously 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 age。 he tested a piece of pitchblende； the principal ore of uranium； and found it to be 700million years old鈥攙ery 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鈥攖he 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鈥攆or the aging kelvin was present； if not always fully awake鈥攔utherford 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鈥攁nd self…evidently was鈥攎uch older than the twenty…four million yearskelvin鈥檚 calculations allowed。
kelvin beamed at rutherford鈥檚 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鈥攆ar greater than his work onthermodynamics。
as with most scientific revolutions； rutherford鈥檚 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 billion years or so of earth鈥檚 actual age。 science was on the right track； but still wayout。
kelvin died in 1907。 that year also saw the death of dmitri mendeleyev。 like kelvin； hisproductive work was far behind him； but his declining years were notably less serene。 as heaged； m