http://www.ck12.org Chapter 4. Atomic Structure
4.2 Structure of the Atom
Lesson Objectives
- Describe the properties of cathode rays that led to the discovery of the electron.
- Describe the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus.
- List the properties of protons, neutrons, and electrons.
- Describe the properties of X-rays and how they were discovered.
- Describe the currently accepted model of the atom.
Lesson Vocabulary
- electron: A negatively charged particle that has a very small mass compared to the mass of other subatomic
particles and surrounds the atom. - plum pudding model: An experiment, led by J.J. Thomson, which proposed that the atom was comprised of
negatively charged particles in a disperse field of positively charged particles. - radioactivity: When certain substances emit charged particles.
- gold foil experiment: An experiment, led by Ernest Rutherford, which proposed that atoms consist of a small
positively charged nucleus surrounded by negatively charged electrons. - proton: A positively charged particle that has a relatively large mass compared to electrons. Protons can be
found in the nucleus of the atom. - neutron: An uncharged particle with a mass nearly equal to that of the proton. Neutrons can be found in the
nucleus of the atom.
Check Your Understanding
- Describe Dalton’s atomic theory.
- List two changes that were made to Dalton’s theory based on more recent evidence.
Introduction
After the development of Dalton’s atomic theory, several important discoveries were made that led to a new under-
standing of the atom. A number of experiments revealed that the atom is comprised of subatomic particles called
electrons, neutrons, and protons. Most of the atom’s mass is concentrated in a central nucleus, which contains
protons and neutrons. The nucleus is surrounded by much less massive electrons, which account for most of the
volume occupied by the atom. The electron was the first subatomic particle to be discovered, followed by discoveries
of the nucleus, the proton, and the neutron. In this lesson, we are going to study these important discoveries and how
they led to our current understanding of the atom.