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Chapter 30

30.1 Discovery of the Atom

30.2 Discovery of the Parts of the Atom: Electrons and Nuclei

30.3 Bohr’s Theory of the Hydrogen Atom

30.4 X Rays: Atomic Origins and Applications

Main Ideas

Discovery of atoms

Atom is the basic unit if matter.

Brownian motion is the motion of small particle in a fluid caused by the random thermal motions of fluid molecules colliding with particles in the fluid. This is the first direct evidence of atoms. This motion was discovered in 1827 by Robert Brown.

Structure of an atom: each atom consists of a nucleus (positive electric charge) and electrons (negative electric charge) that moves around it.

Models of an atom

• Plum pudding model of atom: negatively-charged electrons are embedded into positively-charged “soup”.
• Planetary model of atom: a nucleus is in the center of an atom (like the Sun is in the center of our solar system) and electrons orbit the nucleus (like planets orbit the Sun). The re is of force of attraction between the nucleus and the electrons – electrostatic Coulomb’s force (similar to force of gravity between the Sun and the planets).
• Bohr’s model: a nucleus is located in the center of an atom and electrons orbit the nucleus but only certain orbits are allowed. The orbits of electrons are quantized.

Each type of atom (or element) has its own characteristic electromagnetic spectrum.

Physics Simulation

Try this simulation to compare models of atoms.

Additional Resources

OpenStax High School Physics, chapter 22 The Atom