1. Section 1: Introduction to Atoms
How did atomic theory develop and change?
What is the modern model of the atom?

2. Atomic Theory and Models
Dalton thought that atoms were like smooth, hard balls that could not be broken into smaller pieces.

3. Atomic Theory and Models
Thomson suggested that atoms had negatively charged electrons embedded in a positive sphere.

4. Rutherford’s Gold Foil Experiment
Rutherford was surprised that a few particles were deflected strongly. This led him to propose an atomic model with a positively charged nucleus.

5. Later Atomic Models
Through the first part of the twentieth century, atomic models continued to change.

6. Modern Model of an Atom
This model of a carbon atom consists of positively charged protons and neutral neutrons in a nucleus that is surrounded by a cloud of negatively charged electrons.

7. Particles in an Atom
An atom is composed of positively charged protons, neutral neutrons, and negatively charged electrons. Protons and neutrons are about equal in mass. An electron has about 1/2,000 the mass of a proton or neutron.

8. Isotopes
Atoms of all isotopes of carbon contain six protons and six electrons, but they differ in their number of neutrons. Carbon-12 is the most common isotope.

9. More on Atomic Structure
Click the PHSchool.com button for an activity about atomic structure.

10. End of Section: Introduction to Atoms