4.2 Defining the Atom
Since the ancient Greeks, atoms were
once thought to be solid and indivisiblethe smallest matter possible.
In 1897, English physicists Sir William
Crookes and J.J. Thomson discovered a
smaller, negatively charged particle that
is part of an atom – the electron.
For detailed information about Thomson’s experiments, please visit:
http://www.aip.org/history/electron/jjhome.htm
Thomson’s model (the Plum Pudding model):
Negatively charged
are surrounded by a
an area of positive
charge.
His claims were met with considerable
skepticism, but Thomson was right about the
electrons.
Scientists are not any more open-minded
about new ideas than the rest of us. New
ideas, especially if they are very different
from well-accepted ones, are often resisted.
He also speculated that electrons were the only
subatomic particles, a claim soon proven
incorrect by one of his students.
In 1908, one of Thomson’s former students,
Ernest Rutherford, discovered that a small
area of extremely dense, positively charged
material was located at the center of the
atom.
How did Rutherford, in 1908,
discover something no one could see?
Rutherford’s Gold Foil Experiment
Rutherford
bombarded a thin
sheet of gold foil with
positively charged
radioactive particles.
As they passed the
foil and impacted the
screen behind the foil,
he could see little
bursts of light on the
screen.
Rutherford’s Gold Foil Experiment
Much to his surprise,
some of the
particles did not
pass straight
through but were
deflected. Clearly,
something very
dense, and positively
charged, was in the
foil.
Rutherford’s Gold Foil Experiment
Rutherford called the area the nucleus, and the
small, positively charged particles protons.
While he couldn’t see the nucleus itself, he
deduced its existence and properties from the
way it affected the radioactive particles.
Rutherford and others suspected there was
another particle besides the proton in the
nucleus because atoms had about twice the
expected mass.
It wasn’t until 1932 that English physicist
James Chadwick, a student of Rutherford’s,
identified neutrons.
The Rutherford Model or nuclear model
• very dense, positively charged center
(nucleus)
•electrons (negative) surrounding nucleus
but at a great distance from the nucleus.
How were the electrons arranged?
Were they moving, and if so, how?
Why didn’t they fall into the nucleus?
No one knew.