1. H ISTORY OF THE A TOM

2. A TOMISM 5 th century BC, Ancient Greece – Leucippus, Democritus Both from the Ionian school of naturalistic philosophy The earliest proponents of the concept of atomism “All matter is composed of indivisible particles called atoms.” Proposed the earliest views on the shapes and connectivity of atoms. “They reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials.”

3. B ILLIARD B ALL M ODEL – “C HEMICAL A TOMISM ” ~ 1800, Great Britain – John Dalton Elements are made of tiny particles (small solid spheres) called atoms. All atoms of a given element are identical. Compounds are composed of atoms in specific ratios. Chemical reactions are rearrangements of atoms (mass is conserved).

4. D ISCOVERY OF C ATHODE R AYS ~ 1860, Germany A heated cathode of an electric circuit in a vacuum with a large potential difference applied between the electrodes emitted a strange beam that was thought to be an electromagnetic wave.

5. D ISCOVERY OF THE E LECTRON ~ 1900, Great Britain – J. J. Thomson Investigated cathode rays. Can be deflected in their path by magnetic and electric fields → carry a charge. Concluded that these rays were actually particles, later called electrons. Able to measure their charge-to-mass ratio.

6. T HOMSON ’ S P LUM P UDDING M ODEL The atoms consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification. Atoms are neutral → the total neg. charge = the total pos. charge.

7. ~ 1910, USA – R. A. Millikan Charged droplets of oil between two metal electrodes State of mechanical equilibrium between the downward gravitational force and the upward electric force The charges were all multiples of a fundamental value, calculated to be 1.602×10 −19 C → charge is quantized. M ILLIKAN ’ S O IL D ROP E XPERIMENT

8. R UTHERFORD ’ S G OLD FOIL EXPERIMENT ~ 1910, New Zealand/Great Britain – E. Rutherford Beam of alpha particles (helium atoms without electrons) was directed onto a sheet of very thin gold foil.

9. R UTHERFORD ’ S G OLD FOIL EXPERIMENT Expected ResultsObserved Results

10. Expected ResultsObserved Results Thomson’s ModelRutherford’s Model Conclusion: The atom is mostly empty space with a small, dense, positively charged nucleus surrounded by negative electrons.

11. D ISCOVERY OF THE P ROTON ~ 1920, E. Rutherford Alpha particles were shot into nitrogen gas → scintillation detectors showed the signatures of hydrogen nuclei. This hydrogen could only have come from the nitrogen → nitrogen must contain hydrogen nuclei. о The hydrogen nucleus is present in other nuclei as an elementary particle named proton. (Greek πρ  τον = first)

12. D ISCOVERY OF THE N EUTRON ~ 1930, Great Britain – James Chadwick Bombarded beryllium with alpha particles. Radiation emitted by beryllium stroke into paraffin wax. Protons shot out from the paraffin wax. What was in the “beryllium radiation”?

13. The radiation not affected by electric or magnetic field → neutral. Photons would be too “weak” to shoot out protons. These new neutral particles have approximately the mass of protons. Named neutrons. D ISCOVERY OF THE N EUTRON

14. A TOMIC N UCLEUS – S UMMARY a very dense region at the center of an atom consisting of nucleons: protons and neutrons accounts for almost all the mass of an atom ~ 10 -15 m in diameter

15. The number of protons in an atomic nucleus identical to the charge number of the nucleus uniquely identifies a chemical element in a neutral atom, equal to the number of electrons The number of nucleons in an atomic nucleus different for each different isotope of a chemical element e.g., common has 6 n 0 while radioactive has 8 n 0 Atomic (Proton) Number ZMass (Nucleon) Number A

16. Chemical Element Is composed of atoms with the same Z Naturally a mixture of different isotopes Nuclide Is composed of atoms with the same Z and A

17. P ERIODIC T ABLE OF THE E LEMENTS

18. E LEMENTARY (?) P ARTICLES Electron is not known to be made up of smaller particles Mass kg, charge C BUT Protons and neutrons ARE known to be made up of QUARKS (though these do not exist separated or isolated) Mass, charge C

19. Q UESTIONS 1) What is the charge of an electron? 2) What did Millikan find out in his experiment? 3) How was an atomic nucleus discovered? 4) What is the charge of a proton? 5) What is the charge of a neutron? 6) What is the difference between a chemical element and a nuclide? 7) What particles are atoms composed of? 8) What do you know about quarks?