narrow beam of alpha particles targeted thin piece of
•
at a
gold foil
•
alpha particles scattered by foil + detected on zinc sulfide
screen in front of a microscope .
•
microscope moved to count number of alpha particles through
different angles
Observations t conclusions :
•
Most alpha particles passed straight through the foil with
very little
scattering
o most of the atom is
empty space ,
mass concentrated in small
region (nucleus)
•
Very few alpha particles were deflected through angles 90° (1 in 10000 )
nucleus is
positively charged ( repels +ve alpha particles)
The distance d of closest approach between an alpha particle and the gold
nucleus be calculated the idea of At this
can
using conservation of energy .
distance ,
the alpha particle momentarily stops therefore ,
:
initial kinetic
energy of alpha particle electrical potential energy at distance d
-
_
F- n 1.2×10
"
J 1-2×10-12 QQ
_
: =
atomic
41T Eod gold number
=
79%2 ✗ (1-60×10-19) '
41T 8-85×10 "
Xd
_
✗
"
d
"
=
3. 0×10 m
this gives an upper limit for the radius of a
gold nucleus as higher energy
alpha particles may get closer
radius of nucleus 10 5m
'
-
a :
radius of an atom : to -10m
°o° nucleus is ~
105 × smaller than the atom
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