Chapter 4: Atoms and Elements
Date:____________
Section 2: Atomic Discoveries - Notes
Objectives:
 Explain how the experiments of Thomson, Rutherford, and Chadwick led to the development of
the nuclear theory of the atom.
 Describe early atomic models.
Solid Sphere:

Solid Sphere Model:
Solid Sphere Model
o
o
J. J. Thomson (1856–1940):

In 1897, Thomson performed his famous __________________________________ experiments.

______________________ was passed through a ________________________ sealed tube.

When charged plates were brought close to the tube, the beam of electricity reacted.

o
it moved _________________________ the positively charged plate
o
it moved _________________________ the negatively charged plate
Based on his experiments, Thomson discovered the following:
o
_________________________ are negatively charged
o
Electrons are much _________________ and ________________________ than atoms.
o
Electrons are uniformly _____________________ in many different kinds of substances.
o
He proposed that atoms must contain a _____________________________________
that balances the negative charge of electrons.

Plum Pudding Model
Plum Pudding Model
o
o
Ernest Rutherford (1871-1937):

In 1909, Rutherford performed his famous _________________________________ experiment.

Tiny particles called ____________________________________ were directed at a thin sheet of
gold foil.
o
Most particles passed ___________________________________ the foil, but some
were deflected at ___________________________________.

Expected result of Rutherford’s gold foil experiment:
o
If the plum pudding model were correct, the alpha-particles
would pass right through the gold foil with
_______________________________________.

Actual result of Rutherford’s gold foil experiment:
o
A small number of alpha-particles were _______________________
or ___________________________.

Based on his experiments, Rutherford discovered the following:
o
Most of the atom’s ________________ and all of its positive charge are contained in a
small core called the ________________________.
o
Most of the _______________________ of the atom is ___________________________
through which the tiny, negatively charged electrons are dispersed.
o
The number of negatively charged electrons outside the nucleus is _________________
to the number of positively charged particles (protons) inside the nucleus, so that the
atom is electrically ____________________.

Planetary Model:
Planetary Model
o
o
James Chadwick (1891-1974):

In 1919, Chadwick and his mentor, Rutherford, kept finding that the _______________________
of an atom was larger than the __________________________________ of that atom.

Chadwick suggested that there could be an ___________________________________________
in the nucleus that had mass, but no charge.
o

These particles were called __________________________.
Nuclear Model
Nuclear Model
o
o


The _______________ nucleus makes up more than ______________ of the mass of the atom
o

but the nucleus occupies only a small fraction of an atoms ____________________
The electrons are distributed through a much _______________ region but don’t have much
_____________.

A single grain of sand composed of solid atomic nuclei would have a mass of _______________.
o
Astronomers believe that ________________________ and _______________________
________________ are composed of this kind of incredibly dense matter.