The Structure of the Atom

The Structure of the Atom

1. Key Terms

   • Atom: the smallest unit of an element that maintains the chemical properties of that elements
     • Atoms contain positive and negative particles. The first discovery of a subatomic particle came in the late 1800s. At the time, mny experiments were performed in which electric current was passed through various gases at low pressures
   • Nuclear force: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus

2. Cathode Rays and Electrons

   • Investigators noticed that when current was passed through the tube, the surface of the tube directly opposite the cathode glowed. They hypothesized that the glow was caused by a stream of particles, which they called a cathode ray. The experiments devised to test this hypothesis revealed the following observations
     • 1. Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge
     • 2. The rays were deflected away from a negatively charged object

3. Charge and Mass of the Electron

   • Cathode-ray experiments provided evidence that atoms are divisible and that one of the atom's basic constituents is the negatively charged electron.
     • The electron has a mass of 9.109 x 10^-31 kg
       • Inferences were made about the atomic structure through electrons. (1) because atoms are electrically charged, they must contain a positive charge to balance the negative electrons. (2) Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

4. A nucleus contains protons and neutrons

   • Proton: 1.673 x 10^-27 kg
   • Neutron: 1.675 x 20^-27 kg

5. The unit used for the sizes of atoms is the picometer (pm)