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Modern Physics
The Atom
What is physics really about
Mechanics we look at
interactions between
bodies
Electricity and
magnetism we look at
interactions between
charges
Waves and light are
about the transfer of
energy
Mass
Charge
Energy
What is matter (the stuff around us)
If you took a lump of
matter, could you
keep on dividing it in
two or would you get
to a piece so small
that you couldn’t
divide any further?
The greeks deicded
you couldn’t – they
called the smallest
particle an atom (the
greek word for
indivisible)
Aristotle
The greeks thought
all matter made of
earth, air, fire and
water
Eg lead was a mixture
of these so if you
could change the
mixture
You could turn gold
into lead!
1800
Compunds were
known about
Dalton came up with
the idea that
compounds were
made up of atoms
Lead and gold atoms
different so could not
change lead to gold
1910
Thompson came up
with plum pudding
model for the atom
Knew about electrons
(the plums) and said
the dough was
positive
Matter is mass,
charge and energy!
1911 – NZ’s Rutherford
Gold foil experiment
using alpha (+)
particles (He nuclei)
fired through very thin
gold foil onto a zinc
sulfide screen
Most particles went
straight through but
some scattered
through a large angle
The details Rutherford considered
The large scattering of the alpha (+)
particles suggested there was a large +
charge at the centre of a gold atom
Gold is neutral so the electrons must be
small and orbiting
The atom must be mostly empty space
1913 - Bohr
Bohr put the electrons
into energy shells
around the nucleus
Chadwick discovered
the neutron in the
nucleus
The model of the
atom took 2000 years
to design and we
have had it about 70
years!
The atom is really more complex
But for Year 12 Physics – we don’t care!
Atomic notation
Element symbol
Atomic number – give
the number of protons
in the nucleus and
therefore the number
of electrons in neutral
atom
Mass number – how
many particles in the
nucleus
Isotopes – very important to
nuclear physics
They are atoms that
have the same
number of protons but
different numbers of
neutrons
Heavier isotopes of
an atom are unstable
and are the most
likely to decay or be
involved in nuclear
reactions