History of the Atom

1. History of the Atom

2. Atomism • 5th 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. Billiard Ball Model – “Chemical Atomism” • ~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. Discovery of Cathode Rays • ~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. Discovery of the Electron • ~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. Thomson’s Plum Pudding Model • 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. Millikan’s Oil Drop Experiment • ~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.

8. Rutherford’s Gold 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. Rutherford’s Gold foil experiment Expected Results Observed Results

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

11. Discovery of the Proton • ~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. • The hydrogen nucleus is present in other nuclei as an elementary particle named proton. (Greek πρτον = first)

12. Discovery of the Neutron • ~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. Discovery of the Neutron • 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.

14. Atomic Nucleus – Summary • 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. Atomic (Proton) Number Z Mass (Nucleon) Number A • 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 n0 while radioactive has 8 n0

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

17. Periodic Table of the Elements

18. Elementary (?) Particles • 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. Questions • What is the charge of an electron? • What did Millikan find out in his experiment? • How was an atomic nucleus discovered? • What is the charge of a proton? • What is the charge of a neutron? • What is the difference between a chemical element and a nuclide? • What particles are atoms composed of? • What do you know about quarks?