4.2. The Nuclear Model of the Atom http://www.ck12.org
4.2 The Nuclear Model of the Atom
Lesson Objectives
- Distinguish between the three main subatomic particles.
- Understand the contributions of J. J. Thomson, Robert Millikan, and Ernest Rutherford to atomic theory.
- Describe the structure of the nuclear atom.
Lesson Vocabulary
- atomic model
- cathode ray
- cathode ray tube
- electron
- neutron
- nucleus
- proton
Introduction
Dalton’s atomic theory represented an improvement over the idea of Democritus because the theory was based on
experimental findings and the scientific method. However, his theory did have its shortcomings. He believed that
atoms were indivisible, meaning that the atom was the smallest possible component of matter. Further investigations
in the late 1800s proved that atoms can indeed be broken down into smaller particles. It is the unique number
and arrangement of these subatomic particles that makes atoms of one element different from those of every other
element. The three fundamental particles are called the proton, the neutron, and the electron.
Discovery of the Electron
The first discovery of a subatomic particle resulted from experiments into the nature of the relationship between
electricity and matter.
Cathode Rays
In 1897, English physicist J. J. Thomson (1856-1940) experimented witha device called acathode ray tube,in
which an electric current was passed through gases at low pressure. A cathode ray tube (Figure4.6) consists of
a sealed glass tube fitted at both ends with metal disks called electrodes. The electrodes are then connected to a
source of electricity. One electrode, called the anode, becomes positively charged while the other electrode, called