However this diagram, which is
based on the Bohr model of atomic structure, although
more realistic in terms of the real size of the nucleus compared to the atom
as a whole, so it is not convenient to give a brief diagrammatic picture of the
composition of an atom.
The central
nucleus of
protons and neutrons (most of the mass of an atom) is extremely small even compared to the size of an
atom. The rest of the 'almost empty space' of an atom is occupied by the
negative electrons, held by, and moving around the positive nucleus in their
energy levels or 'shells'.
The electrons are also pretty tiny in
mass too, compared to a proton or neutron, but the volume the electrons and
their energy levels occupy, determines the size of the atom, but an atom mainly
empty space with the nucleus at the centre!
Bohr theorised the negative electrons can
only exist in certain specific energy levels (shells) held in place by the
positive nucleus (see section on the history of development of the atomic
model). These are shown in the above diagram, but fully explained in later
sections.
All of these theories must, and have been, backed up by
repeated and varied experiments.
As each new experiment was/is done, it must
support the current theory or the theory needs to be modified to take into
account new discoveries.
Some of these important experiments are described
further down the page.
Even new experimental findings written up in research
papers should be thoroughly peer reviewed, that is checked by
scientists of at least equal academic ranking to the researchers. That's
how science works!
The size of an atom compared to other
'particles'
The size of an individual atom is around
0.1 nm or 1 x 10-10 m
and the size of the nucleus itself is only
about 1 x 10-14 m, about 1/10000th of the radius of the atom.
(nm = nanometres, 1 nm = 10-9
m).
Most of the mass is in the centre of
the atom, that is the nucleus,
which has a radius of around 1/10000th of the
whole atom!
This means the radius of the nucleus
is about 1 x 10-14 m (0.00001 nm), pretty small !!, but still consists
of most of the mass of an atom!!!
If you look at the table of size
comparison below,
its not until you get to a human hair can we see
clearly something with the naked eye. The width of a human hair is
approximately 106 times that of an atom (a million times
bigger) and 1010 times bigger than a nucleus (ten thousand
million times bigger).
You can of course see cells under examination
with an optical microscope, but these are over 500 000 times bigger than
an individual atom. You can, however, observe atoms using an electron
microscope.
