1) Early atomic models proposed atoms as billiard balls (Thomson) or plum puddings (Rutherford) until experiments showed atoms have a small, dense nucleus. 2) Rutherford discovered alpha particles bouncing off a gold foil, indicating a tiny nucleus. 3) Bohr incorporated Rutherford's nuclear model and proposed electrons orbit in distinct energy levels, explaining atomic emission spectra.

1. Atomic Theory Materiale liberamente creato da http://www.visionlearning.com Raffaele Nardella

2. The Early Days - Thomson Model of the atom like a billiard ball In 1897, J. J. Thomson’s discovery of the electron The atom was not "indivisible" as John Dalton suggested J. J. Thomson Raffaele Nardella

3. The Early Days - Cathode Ray Tube An electric current, through a vacuum tube, creates a stream of glowing material bending toward a positively charged electric plate  The stream was made up of electrons carrying a negative charge Cathode Ray Tube Raffaele Nardella

4. The Early Days - Plum Pudding Model Thomson  Atoms like pieces of raisin bread Plum Pudding Model . Raffaele Nardella

5. The Early Days - Ernest Rutherford Experiments with radioactive alpha particles (1908) alpha particles gold foil bouncing straight back positively charged dense core  nucleus tiny solar system . Raffaele Nardella

6. The Early Days – James Chadwick Discovering of a subatomic particle : neutrons Reduction of the repulsion between protons Stabilization of the atom's nucleus. . J. Chadwick Raffaele Nardella

7. The Early Days – Size of an Atom Atoms are very very small: empty space spinning of electrons far from the nucleus (football field) atomic number (z) = number of protons atomic mass = number of protons and neutrons http://facstaff.gpc.edu/~pgore/PhysicalScience/Atoms.html Raffaele Nardella

8. Electron Shells – Niels Bohr Modification to the atomic structure – Line Spectra matter heated  light light bulb: electric current heats a metal filament  absorbing the electrical energy  exciting the electrons white light through a prism  a continuous spectrum of colors light from an excited element through a prism  only specific lines (or wavelengths) of light, called line spectra . Continuous (white light) spectra Raffaele Nardella

9. Electron Shells – Niels Bohr Energy levels (electron shells) emitted light  the movement of electrons  electrons could not move continuously in the atom but only in precise steps electrons occupy specific energy levels (electron shells) atom excited  electrons can jump to higher levels electrons fall back to lower levels, precise quanta of energy are released as specific wavelengths (lines) of light electron shells  concentric circles around the nucleus Niels Bohr Raffaele Nardella

10. Electron Shells – Niels Bohr Key to Bohr's theory  the electron could only "jump" and "fall" to precise energy levels, emitting a limited spectrum of light. electrons in the ground state , the lowest energy level possible (the electron shell closest to the nucleus). electrons excited by adding energy to an atom (for example, when it is heated), electrons absorb energy, " jump " to a higher energy level, and spin in the higher energy level after a short time, electrons " fall " back to a lower energy level, giving off a quantum of light energy levels have limits to the number of electrons Raffaele Nardella