Developing Models of Matter

1. Developing Models of Matter Section 3.1

2. What did the early Greeks think about the world around us? • Matter was composed of four elements: • Earth • Air • Fire • Water In about 400 B.C. Democritus suggested that matter was made up of ATOMS, but his idea was not accepted at that time even though he was right!!!

3. What did John Dalton contribute to our understanding of matter in 1808? Dalton’s Atomic Model states: • All matter is made up of atoms, which are particles too small to be seen. • Each element has its own kind of atom, with its own particular mass. • Compounds are created when atoms combine in fixed proportions. • Atoms cannot be created or destroyed in chemical changes.

4. J.J Thomson (1904) • J.J. Thomson developed a “raisin-bun” model, which states that: • Atoms contain particles called electrons • Electrons have a small mass and a negative charge • The rest of the atom is a sphere of positive charge • The electrons are embedded in this sphere

5. Ernest Rutherford (1911) • Rutherford aimed a type of radiation called alpha particles (positively charged) at a thin sheet of gold foil • He predicted that the particles would pass straight through, BUT a few of them bounced back. WHY???? • An atom has a tiny, dense positive core called the nucleus, which contains positive charge, so the alpha particles bounced off the protons.

6. Gold Foil Experiment

7. Bohr Model of the Atom • Bohr proposed a “planetary” model of the atom • He suggested that electrons move around the nucleus in nearly circular paths • Each electron in an orbit has a definite amount of energy

9. Electron Shells

10. Excited Electrons • When electrons are energized, they jump to higher energy level (excited electrons) • When they fall back to the ground state, they emit light