Geology Reference
In-Depth Information
It took a while for Soddy to realise that the reason why these
pairs of elements were inseparable was because chemically they
were the same element, despite having di¬erent atomic weights,
since each pair had the same number of orbiting electrons
around the nucleus, determining their chemical identity. In
other words, what appeared to be two separate elements actu-
ally turned out to be two varieties of one element. Soddy termed
the di¬erent varieties of the same element 'isotopes' (from the
Greek isos , same, and topos , place) because, as he now recog-
nised, all varieties of any one element would occupy the same
place in the periodic table - a classification which summarises
the major properties of the elements and enables predictions to
be made about their behaviour.
Today we know that the nucleus itself is comprised of two
types of particle - protons and neutrons - and that an element's
position in the periodic table is dictated not by its atomic weight
but by its atomic number, which is the number of protons in
the nucleus (and the number of electrons orbiting in the sur-
rounding field). It is the number of neutrons in the nucleus that
varies from one isotope to another, and it is this that determines
an isotope's atomic weight. The neutrons and protons added
together give the isotope number.
For example, an atom of lead derived from the decay of
uranium (238) is called 'lead 206' because it contains 82 pro-
tons (and 82 electrons) and 124 neutrons (82+124 = 206), hence
'206' is the isotope number. Similarly, the isotope derived from
the decay of thorium (232) also has 82 protons but 126 neu-
trons, and so it is called 'lead 208'. Since all the isotopes of lead
have 82 orbiting electrons they cannot be distinguished from
each other chemically, but because each has a di¬erent number
of neutrons, their atomic weight is di¬erent and so they can be
separated physically.
The concept of isotopes as put forward by Soddy was yet
another radical departure from accepted understanding about
the atom and it was some while before he was brave enough to
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