The lead that is found in uranium and has presumably
descended from uranium, behaves like other lead but is lighter. Its
atomic weight is only 206, while ordinary lead weighs 207. It appears
then that the same chemical element may have different atomic weights
according to its ancestry, while on the other hand different chemical
elements may have the same atomic weight. This would have seemed
shocking heresy to the chemists of the last century, who prided
themselves on the immutability of the elements and did not take into
consideration their past life or heredity. The study of these
radioactive elements has led to a new atomic theory. I suppose most of
us in our youth used to imagine the atom as a little round hard ball,
but now it is conceived as a sort of solar system with an
electropositive nucleus acting as the sun and negative electrons
revolving around it like the planets. The number of free positive
electrons in the nucleus varies from one in hydrogen to 92 in uranium.
This leaves room for 92 possible elements and of these all but six are
more or less certainly known and definitely placed in the scheme.
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