The Changing Model of the Atom

1. The Changing Model of the Atom

2. J.J. Thomson • Thomson developed the first model of the atom that contained electrons • Before Thomson, other models had been developed that only had positive charge • Electrons are the subatomic particles with a negative charge

3. Thomson’s model of the atom was a positive sphere with electrons evenly distributed and embedded in it. • Later a scientist by the name of Nagaoka modeled the atom as a large positively charged sphere surrounded by a ring of negative electrons

4. Ernest Rutherford and the Gold Foil Experiment • Rutherford wanted to test Thomson’s model of the atom. • He used alpha particles and shot them like missiles towards a piece of gold foil. • Most of the alpha particles passed through the foil, but occasionally one would bounce back as if it had hit something.

5. The conclusion of this experiment was that there must be tiny places in the piece of gold foil that contained a lot of charge and a lot of mass. • Rutherford concluded that almost all of the mass and all of the positive charge of the atom must be concentrated in an extremely small part in the center of the atom. • He called this the nucleus • He also used the term proton to name the smallest unit of positive charge in the nucleus. • The number of protons in the nucleus is different for each element and is called the atomic number. • All previous models had shown the atom as a solid sphere and Rutherford’s model was mostly empty space.

6. Homework • P.382 #1-4 • P.392 #3-4