Brownian motion was discovered by the
Scottish botanist Robert Brown
(1773–1858) in the course of his
microscopical investigations in 1827.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
It is customary to start the names of the
Greek elements with capital letters since
they were meant to represent ‘essences’
rather than actual samples of air, earth, fire
and water.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
Even though oxygen and nitrogen are
elements, samples of either taken from the
atmosphere will mainly consist of
diatomic molecules1—1a diatomic molecule
of oxygen consists of two atoms of oxygen
bound together (and similarly for
nitrogen). It is not uncommon for samples
of elements to occur in molecular forms
under a range of conditions.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
This formula is true whatever units are
used for the drop volume.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
This process is analogous to the diffraction
of visible light by a diffraction grating.
A crystalline solid is, in effect, a threedimensional ‘diffraction grating’ for
X-rays.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
In the kinetic theory of gases, (explained
elsewhere in FLAP) it is assumed that
atoms are hard spheres, but this is a
deliberate simplification. The fact that
atoms are not really like this is one of the
factors that limits the applicability of
kinetic theory.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
The pascal (Pa) is the SI unit of pressure.
11Pa = 11N1m−2.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
Joseph John Thomson was the director of
Cambridge University’s Cavendish
Laboratory from 1884 to 1919 (when he
was succeeded by Rutherford). It was
largely due to Thomson and his students
that Britain maintained its pre-eminence in
subatomic physics throughout the early
decades of the 20th century. He was
awarded the Nobel Prize for Physics in
1906 and knighted in 1908.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
Thomson actually determined the
mass/charge ratio but it is now much more
common to quote its reciprocal: the
charge/mass ratio.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
Evidence of Thomson’s insight is to be
found in the fact that other researchers,
who performed very similar experiments at
about the same time, came to quite
different and entirely erroneous
conclusions.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
Note that e is a positive quantity and that
the charge on the electron is −e. It is a
common error to call e the charge on the
electron; it is in fact the magnitude of the
charge on the electron.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
A number of individuals played an
important role in the development of the
mass spectrometer. Particularly notable
were J. J. Thomson and Francis Aston
(1877–1945).
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
This expression for the force magnitude F
is derived from the general expression
F = q0(v · B) for the force F on a particle
of charge q travelling with velocity v
through a point at which the magnetic field
is B. See Lorentz force law in the Glossary
for further details.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
This is the magnitude of the centripetal
force required to keep the particle moving
in a circular orbit. See the Glossary for
details.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
The atomic mass unit is also referred to as
the dalton.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
Carbon-12 is an isotope of carbon. We will
discuss isotopes in the next subsection.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
The great advantage of using the relative
atomic mass is that it can be determined by
measuring the masses of macroscopic
samples of matter, without knowing the
mass of any individual atom. So many
relative atomic masses, or atomic weights
as they used to be called, were reasonably
well known before the first atomic masses
were accurately determined.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
It is always possible to distinguish between
isotopes in nuclear reactions which take
place at much higher energies.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
He imagined it to be like plums in a
pudding.
FLAP P8.1
Introducing atoms
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THE OPEN UNIVERSITY
S570 V1.1
α is the Greek letter alpha.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
Born in New Zealand, and destined to
succeed Thomson as Cavendish Professor
at Cambridge, Ernest Rutherford played a
central role in revealing the nature of the
atom. A scholarship took him to
Cambridge in 1895, after which he moved
first to McGill University in Canada, then
to the University of Manchester and finally
back to Cambridge in 1919. He assembled
around himself a team of brilliant and
industrious young men, many of whom
made major contributions to physics.
He was awarded the Nobel Prize for
Chemistry (!) in 1908, knighted in 1914
and created Baron Rutherford of Nelson in
1931.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
It is interesting to note that the atomic
nucleus was so called in analogy with the
cellular nucleus of living organisms.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
The same instability was present in
Thomson’s model, but in that case the
effect was less dramatic and it was hoped
that it might explain the phenomenon of
radioactivity that had been discovered in
1896.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
Chadwick graduated from Manchester
University in 1911 and then worked there
with Rutherford. He subsequently received
a scholarship which took him to Berlin to
work with Rutherford’s former assistant
Geiger. Unfortunately he was in Germany
at the time of the First World War and was
interned for the duration as an enemy alien.
After the War he was invited to rejoin
Rutherford, who had by that time
succeeded Thomson at Cambridge.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
The recognition that the chemical elements
correspond to a continuous sequence of
integer atomic numbers, 1, 2, 3, etc., grew
out of a number of pieces of work.
Particularly notable were the studies of
X-ray scattering by Henry J. G. Moseley
(1887–1915), which indicated the
existence of seven previously
unrecognized elements. Moseley’s
untimely death at Gallipoli in the First
World War is widely regarded as one of
the great tragedies in the history of
physics.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
After discovering the neutron, Chadwick
became Professor of Physics at the
University of Liverpool. He was awarded
the Nobel Prize for Physics in 1935 and,
following war work on the atomic bomb,
he was knighted in 1945.
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
The film appears to have a roughly
constant thickness. As the number of drops
increases, the area increases
proportionately, maintaining the thickness
of the film.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
The process is one in which an electron
(e−) collides with an atom of neon (Ne)
and knocks an electron out of the atom.
The outcome is a positive ion of neon
(Ne+) and two free electrons (2e −).4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
Cl + 3e − → Cl03−4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
19.9921u0/11u = 19.992. It is just the
numerical value of the mass in atomic
mass units, without the units.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
No, the mass number is simply the nearest
whole number to the relative atomic mass
in each case. The masses will be very
similar but they need not be identical.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
Because these elements each have only one
common isotope.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
The ratio is roughly
(5 × 10−101m)/(10−141m) = 501000, though
either or both of the values in the ratio
might legitimately be reduced by a factor
of five or so.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
The charge is +79e, where e is the
magnitude of the charge of the electron
(e = 1.602 × 100−191C). Hence the charge of
a gold nucleus is
79 × 1.602 × 10−191C = 1.3 × 100−171C.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1
✧
The atomic number of argon is Z = 18, and
the nuclear charge is 18e.4❏
FLAP P8.1
Introducing atoms
COPYRIGHT © 1998
THE OPEN UNIVERSITY
S570 V1.1