2. Atomic Discovery

3. Early Models of the Atom 400 B.C. – Democritus proposed the existence of fundamental particles of matter that were indivisible and indestructible - “atomos”. Aristotle thought all matter was continuous; he did not believe in atoms. Neither idea was supported by any experimental evidence – speculation only.

4. Foundations of Atomic Theory  The late 1700’s –definitions and basic laws had been discovered and accepted by chemists.  Element – substance that cannot be broken down by ordinary chemical means.  Chemical Reaction – transformation of substance or substances into one or more new substances.

5.  Law of Conservation of Mass – mass cannot be created or destroyed just changed from one form to another. Mass is conserved in reactions. (Antoine Lavosier-Father of Chemistry 1778))  Law of Definite Proportions – a chemical compound contains exactly the same elements in the same proportion regardless of sample size. (Joseph Proust from work of Gay-Lussac & Amadeo Avogadro – 1802/1804)  Law of Multiple Proportions – If two or more different compounds are composed of the same two elements, then the ratio of the masses of those elements will always exist as a ratio of small whole numbers. (John Dalton - 1808)

6. Dalton’s Atomic Theory 1808  All elements are composed of tiny indivisible particles called atoms.  Atoms of the same element are identical. The atoms of one element are different from the atoms of another element.  Atoms cannot be subdivided, created or destroyed.  Atoms combine in simple whole-number ratios.  Atoms are separated, joined or rearranged in chemical reactions. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

7. Other Contributors to the Atomic Theory  Amadeo Avogadro-discovered the relationship between volume of gases and the number of particles in them (1811)

8. Discovery of Electrons  1897 – J.J. Thomson – “Cathode Ray Tube Experiment”  Showed existence of first know sub-atomic particle  Determined charge to mass ratio of the electron  1909 – Robert Millikan found the charge of the electron – “Millikan’s Oil Drop Experiment”

9. Cathode Ray Tube High Voltage Cathode Ray (electrons) Metal disk (cathode) Metal disk (anode) Gas at very low pressure

10. Cathode Ray Tube High Voltage Cathode Ray (electrons) Metal disk (cathode) Metal disk (anode) Gas at very low pressure Negative plate Positive plate

11. Observations from Cathode Ray  A glow (cathode ray) was formed between the cathode and anode.  A paddle wheel between the cathode and anode rolled toward the anode. (had enough mass)  Cathode ray was deflected from a magnetic field.  Cathode ray was deflected by a negative charge.

12. Discovery of Charge of Electron  Milikan’s Oil Drop Experiment  Negative charge  Mass of electron  Concluded electrons are present in all atoms  Concluded that atoms are divisible.

13. Assumptions based on Discovery of Electron  Because atoms are neutral, there must be a positive charge to balance the electrons.  Because electrons have very small mass compared to an atom, there must be other particles in an atom to account for the other mass.

14. Rutherford’s Gold Foil Experiment  Rutherford, Geiger & Marsden (1912) -showed that most of the atom was empty space, but that atoms had a solid, positive core. Alpha Particles Radioactive source Lead shield

15. Discovery of Protons  1919 -J.J. Thomson & James Chadwick– discovered particles traveling opposite of the cathode rays.  Determined existence, mass and charge of protons  Idea had actually been previously proposed by Goldstein in 1886.

16. Cathode Ray Tube High Voltage Cathode Ray (electrons) Metal disk (cathode) Metal disk (anode) Gas at very low pressure Negative plate Positive plate protons

17. Neutrons  James Chadwick 1932 - confirmed the existence of the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.  Walter Bothe had first reasoned the existence of a third subatomic particle in 1930.  Bothe’s work was based in part on that of Henry Mosely who showed by X-ray analysis that not all atoms of the same element were identical. (Isotopes – 1907)

18. Radioactivity  Mosely’s X-ray analysis of atoms was an attempt to explain radioactivity.  1896 – Henri Becquerel – Uranium spontaneously emits energy.  1898 – Marie & Pierre Curie – first isolated a radioactive element - Radium

19. “Planetary” Model of the Atom  Niels Bohr (1913) – developed the “planetary” model of the atom based upon the following:  Rutherford’s Gold Foil Experiment  E = mc 2 – Albert Einstein (1905)  Quantum Theory – Max Planck (1910)

20. Properties of Subatomic Particles Particles Symbol Charge Relative Mass Mass Mass Electrone - 1- 1/1840 amu 9.11 x 10 -28 g Proton p + 1+ 1 amu 1.67 x 10 -24 g Neutronn º 0 1 amu 1.67 x 10 -24 g

21. Atom 10 -13 cm 10 -13 cm electrons electrons protons protons neutrons neutrons 10 -8 cm 10 -8 cm nucleus

22. Size of the Atom Puncher Dome e-e- Aluminum Atom e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- nucleus - size of a marble 1 mm Outside edge of Al atom 150 m e-e- goal post stands