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CHAPTER 10
POSITIVE RAYS AND ISOTOPES
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10—1.
Introduction.—In preceding chapters of this book we
have seen that the atom contains a massive, positively charged ;
,
nucleus around which negative electrons are conned in a number ‘
of known energy levels. The electrons in the outer or valence shell
are of importance in the chemical combination of various elements,
_
“
in metallic and heat conduction and in the emission of light.
The electrons in the inner shells, near the nucleus, are responsible
'
for the radiation of the characteristic x—rays.
It has also been
'
pointed out that the nucleus contains most of the mass of the atom
—
.
and determines whether the atom is one element or another. In
…
,
this chapter we shall delve into the properties of
Early measurements of the atomic weights of various elements, …
in terms of oxygen as 16, yielded very nearly whole number
»
values. 'Thus Prout was led to_ the belief that all substances
are made up
of hydrogen, whose atomic weight is nearly equal to
_
unity. The hydrogen atom was considered as the unit b10ck out
of which the heavièr atoms were built. More accurate atomic
.
:
weight determinations, h0wever, showed that they were not exact
.
,
multiples of a unit amount. Still later, by methods to be presented
in this chapter, it was shown that for those cases in which the
'__ atomic number was not a whole number, the actual chem1cal
—
substances consisted of a mixture of groups
atoms, eachof
Wh1€hhadthe same chefniCal property as the other buta
Thèse isof0Pe—f of an _elementwere found
:t0haveatomlcwelghtswhmhwere exceedingly Close to whole
lstemptedwacœpthuts hypothes1s and
‘
cons1derthe
butatleaStfasmallnumberofelementary units
__
oîf ofstabl€