large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_143
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
Thomson had suggested, as early as 1898, that the atom was a ball of matter carrying a positive charge and that individual electrons were stuck throughout its substance, like raisins in pound cake. THOMPSON HAD SUGGESTED AS EARLY AS EIGHTEEN NINETY EIGHT THAT THE ATOM WAS A BALL OF MATTER CARRYING A POSITIVE CHARGE AND THAT INDIVIDUAL ELECTRONS WERE STUCK THROUGHOUT ITS SUBSTANCE LIKE RAISINS IN POUND CAKE
The Structure of the Atom Since radioactive atoms gave off either positively charged particles or negatively charged particles, it seemed reasonable to assume that atoms generally were made up of both types of electricity. Furthermore, since the atoms in matter generally carried no charge at all, the normal “neutral atom” must be made up of equal quantities of positive charge and negative charge. It turned out that only radioactive atoms, such as those of uranium and thorium, gave off positively charged alpha particles. Many atoms, however, that were not radioactive, could be made to give off electrons. In 1899 Thomson showed that certain perfectly normal metals with no trace of radioactivity gave off electrons when exposed to ultraviolet light. (This is called the “photoelectric effect”.) It was possible to suppose, then, that the main structure of the atom was positively charged and generally immovable, and that there were also present light electrons, which could easily be detached. E WAS THE STRUCTURE OF THE ATOM SINCE RADIOACTIVE ATOMS GAVE OFF EITHER POSITIVELY CHARGED PARTICLES OR NEGATIVELY CHARGED PARTICLES IT SEEMED REASONABLE TO ASSUME THAT ATOMS GENERALLY WERE MADE UP OF BOTH TYPES OF ELECTRICITY FURTHERMORE SINCE THE ATOMS IN MATTER GENERALLY CARRIED NO CHARGE AT ALL THE NORMAL NEUTRAL ATOM MUST BE MADE UP OF EQUAL QUANTITIES OF POSITIVE CHARGE AND NEGATIVE CHARGE IT TURNED OUT THAT ONLY RADIOACTIVE ATOMS SUCH AS THOSE OF URANIUM AND THORIUM GAVE OFF POSITIVELY CHARGED ALPHA PARTICLES MAN ATOMS HOWEVER THAT WERE NOT RADIOACTIVE COULD BE MADE TO GIVE OFF ELECTRONS IN EIGHTEEN NINETY NINE THOMPSON SHOWED THAT CERTAIN PERFECTLY NORMAL METALS WITH NO TRACE OF RADIO ACTIVITY GAVE OFF ELECTRONS WHEN EXPOSED TO ULTRA VIOLET LIGHT THIS IS CALLED THE PHOTO ELECTRIC EFFECT IT WAS POSSIBLE TO SUPPOSE THEN THAT THE MAIN STRUCTURE OF THE ATOM WAS POSITIVELY CHARGED AND GENERALLY IMMOVABLE AND THAT THERE WERE ALSO PRESENT LIGHT ELECTRONS WHICH COULD EASILY BE DETACHED
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