large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_78
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
Helium, with an atomic number of 2, has a nuclear charge of +2 and 2 electrons, with a total charge of  2, to balance it. (The alpha particle released by radioactive atoms is identical with a helium nucleus.) HELIUM WITH AN ATOMIC NUMBER OF TWO HAS A NUCLEAR CHARGE OF PLUS TWO AND TWO ELECTRONS WITH A TOTAL CHARGE OF MINUS TWO TO TO BALANCE IT THE ALPHA PARTICLE RELEASED BY RADIOACTIVE ATOMS IS IDENTICAL WITH A HELIUM NUCLEUS
ments in this table were sometimes numbered consecutively (“atomic number”) but this was inconvenient since, when new elements were discovered, the list of atomic numbers might have to be reorganized. [Illustration: _Dmitri Mendeléev and Bohuslav Brauner in Prague in 1900. Brauner was a professor of chemistry at the Bohemian University in Prague._] The Danish physicist Niels Bohr (1885-1962) had just advanced a theory of atomic structure that made it reasonable to suppose that the wavelength of the characteristic X rays depended on the size of the nuclear charge of the atoms making up a particular element. Moseley therefore suggested that these X rays be used to determine the size of the positive charge on its nucleus. The atomic number could then be set equal to that charge and be made independent of new discoveries of elements. Hydrogen, for instance, has an atomic number of 1. Its nucleus carries a unit positive charge, +1, and the hydrogen atom possesses 1 electron to balance this.  A PERIODIC TABLE BASED ON THE ATOMIC WEIGHT ORDER IN SUCH A WAY THAT ELEMENTS OF SIMILAR PROPERTIES WERE GROUPED TOGETHER THE ELEMENTS IN THIS TABLE WERE SOMETIMES NUMBERED CONSECUTIVELY ATOMIC NUMBER BUT THIS WAS INCONVENIENT SINCE WHEN NEW ELEMENTS WERE DISCOVERED THE LIST OF ATOMIC NUMBERS MIGHT HAVE TO BE REORGANIZED THE DANISH PHYSICIST NEILSBOR EIGHTEEN EIGHTY FIVE NINETEEN SIXTY TWO HAD JUST ADVANCED A THEORY OF ATOMIC STRUCTURE THAT MADE IT REASONABLE TO SUPPOSE THAT THE WAVE LENGTH OF THE CHARACTERISTIC X RAYS DEPENDED ON THE SIZE OF THE NUCLEAR CHARGE OF THE ATOMS MAKING UP A PARTICULAR ELEMENT MOSELY THEREFORE SUGGESTED THAT THESE X RAYS BE USED TO DETERMINE THE SIZE OF THE POSITIVE CHARGE ON ITS NUCLEUS THE ATOMIC NUMBER COULD THEN BE SET EQUAL TO THAT CHARGE AND BE MADE INDEPENDENT OF NEW DISCOVERIES OF ELEMENTS HYDROGEN FOR INSTANCE HAS AN ATOMIC NUMBER OF ONE ITS NUCLEUS CARRIES A UNIT POSITIVE CHARGE PLUS ONE AND THE HYDROGEN ATOM POSSESSES ONE ELECTRON TO BALANCE THIS
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