1. The Atom Aim: How has the model of the atom change over the years?

2. Evolution of the Atom The earliest idea of an atom came from the Ancient Greeks Atom is derived from the word “atomos” – ‘a’ = not, unable – ‘tomos’ = cut, slice, section The idea of the atom was in existence since 460BC The model of the atom, however, has evolved since 1900CE

3. Thomson’s Model Plum Pudding Model J.J. Thomson realized that electrons are relatively small, negatively charged particles He concluded that part of the atom must possess a positive charge equal to the total charge of the electrons

4. Rutherford’s Model Ernest Rutherford’s famous gold foil experiment Rutherford shot massive, positively charged alpha (α) particles at a thin (0.00004cm) layer of gold foil Most of the alpha particles passed through disproving the Thomson’s Plum Pudding Model However, some of the alpha particles were deflected at large angles causing Rutherford to conclude that there must be an even larger positively charged nucleus at the center of each atom with the electrons orbiting it like a miniature solar system

5. Bohr Model Two years after Rutherford, Niels Bohr attempted to explain why electrons don’t crash into the nucleus He made the following assumptions: – All forms of energy is QUANTAIZED; the electrons can gain/lose energy in certain fixed amounts – Electrons can only occupy certain specific orbits of a fixed radius – Electrons can jump from one orbit to another depending if it gains/loses quantized energy from a photon – Each orbit corresponds to a specific amount of energy

6. Bohr Model Each individual orbit is called an ENERGY LEVEL The first energy level (n = 1) that is closest to the nucleus is called the GROUND STATE If the electron gains energy by absorbing a photon it gets EXCITED causing the electron to jump to the next orbit If the atom is able to absorb enough energy to knock off an electron then the atom has become IONIZED If the electron loses energy, it will drop down to the lower energy level and give off RADIATION in the form of photons

7. Cloud Model There are no specific electron orbits There are areas around the nucleus that the electron has the greatest probability of being found; this is called the ELECTRON CLOUD This model is the current accepted model

8. Calculating Energy Levels Photon Energy Units: eV -or- J Initial Energy Level Units: eV Final Energy Level Units: eV (+) Value = Emission (–) Value = Absorption

9. Energy Level Diagrams AbsorbAbsorb EmitEmit

10. Example #1 An electron initially in the n = 5 state of a hydrogen atom emits a photon when it transitions to the ground state. Determine the amount of energy the photon has

11. Example #2 An electron initially in the n = 3 state of a hydrogen atom gets ionized when it absorbs a photon. How much energy did the photon have in (a) eV and (b) Joules

12. Example #3 A mercury atom has an electron in the n = c state that drops down to the ground state. How many different photon energies can be emitted? 3

13. Summary Describe the evolution of the model of the atom How do you calculate the photon energy that is either absorbed or emitted from an atom?