Who am I?

1. Who am I?

2. Democritus • He said you cannot cut a piece of matter infinitely, but at some point you would get the smallest piece of matter possible. • All matter is composed of tiny, indivisible particles called “atomos”. • 450 BC • Greek Philosopher

3. Antoine Lavoisier • France 1872 (1879) • Law of conservation of matter. • Measured the mass of substances before and after a reaction and found that the masses were always equal. No mass was gained or lost in the reaction. • Matter is conserved.

4. John Dalton • 1803-School teacher • Studied the ratios in which elements combine in chemical reactions • Formulated first Atomic theory of matter: • Each element is composed of extremely small particles called atoms. • All atoms of a given element are identical, but they differ from those of any other element • Atoms are neither created nor destroyed in any chemical reaction. • A given compound always has the same composition or ratio of each atom

5. JJ Thomson • England 1897 • He discovered the electron while experimenting with cathode rays. • He determined that the cathode ray was made of negatively charged particles – electrons. • He also was able to estimate that the mass of the electron was equal to about 1/1800 of the mass of a hydrogen atom. • His discovery of the electron won the Nobel Prize in 1906. • He showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes. He concluded that these particles were part of every atom. • He created the “plum pudding model” of the atom.

6. Ernest Rutherford • Born in New Zealand 1871-1937 • Tested Thomson’s theory of atomic structure with the “gold foil” experiment in 1910. • Bombarded thin gold foil with a beam of ‘alpha’ particles. • If the positive charge was evenly spread out, the beam should have easily passed through. • All of the positive charge and most of the mass of an atom are concentrated in a small core, called the nucleus. • Gold Foil Experiment (alpha scattering) • he determined that an atom’s positive charge and most of its mass was concentrated in the core (most of the atom is empty space) • he named the core “the nucleus”

7. James Chadwick • Neutrons- Discovered in 1932 (England 1891-1974). • He bombarded Beryllium with alpha particles. • Neutrons were emitted and in turn ejected protons from paraffin.

8. Robert Millikan • measured the charge of an electron using the oil drop experiment. • x-rays gave the oil a negative electron • 1.60x10^-19 coulomb is the charge of an electron • using Thomson’s charge to mass ratio, he determined the mass of the electron is 9.11x10^-28g

9. Cathode Ray Tube Experiment • Performed by JJ Thomson • Showed the existence of the electron. • The ray is negative and composed of electrons. • The negative end of a magnet attracts the beam.

10. Gold Foil Experiment • Performed by Ernest Rutherford • Positive alpha particles were shot at a thin sheet of gold foil • The deflection of the beam indicated the existence of a small, positively charged nucleus.

11. Oil Drop Experiment • Performed by Robert Millikan • Determined the charge of the electron. • 1909

12. Thomson’s Model of the Atom • Called the “plum pudding” model • The atom was comprised of positively charged matter with negative electrons “stuck” in the positive “pudding”

13. Rutherford’s Model of the Atom • Contained a positive nucleus • Electrons were around the outside of the nucleus

14. Bohr Model of the Atom • Nucleus in center with protons and neutrons • Electrons are orbiting the nucleus in a set pattern • Each orbital has a unique amount of energy

15. Quantum Model of the Atom • Contains a nucleus with protons and neutrons • Electrons move very quickly around the nucleus, not in a circle or orbit, but randomly within an electron cloud.