1a.
The Structure of Atoms
- the three fundamental
particles
- Atoms are the smallest particles of
matter whose properties we study in Chemistry. However from
experiments done in the late 19th and early 20th
century it was deduced that atoms were made up of three fundamental
sub-atomic particles (listed below).
- More on the
Atomic Structure page and
the use of radioactivity 'bullet' experiments to deduce the
structure of an atom (Rutherford and Marsden scattering experiment).
- Earlier theories of atomic
structure, eg the 'plum pudding' model in which 'protons' and
'electrons' were scattered or arranged evenly across the atom, were
superceded by the model described in the picture below.
- It was the only model that could
explain the scattering of alpha particles by a small dense and
positive atomic centre. Later experiments showed that the out bits
could be knocked off atoms and these had a very tiny mass and a
negative charge, in other words the electron!
- It should be emphasised right from
the start that radioactivity is due to energy changes in the
nucleus and the surrounding electrons are not usually involved.
1b.
A
Portrait of an Atom
- what is it like?
The diagram below gives some idea on the
structure of an atom, it also includes some important definitions and notation
used to describe atomic structure. The atomic number (Z) is also known as
the proton number. the mass number (A) is also known as the
nucleon number.
The neutron number (N) = mass number (A) - atomic number (Z).
Protons and neutrons are the 'nucleons' present in
the nucleus
and the negative electrons are held by the positive nucleus in
'orbits' called energy levels or shells.
RADIOACTIVITY IS DUE TO
CHANGES IN ATOMIC STRUCTURE IN THE NUCLEUS OF AN ATOM
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