1. History of the Atom John Dalton J.J. Thomson Robert Milikan Ernest Rutherford James Chadwick Neils Bohr Quantum Mechanical/Wave Mechanical Model

2. John Dalton Known as the “Father of the Atom” Founded the atomic theory Based his 3 laws off the atomic theory

3. John Dalton (Beliefs) He believed that there were 100 different kinds of atoms from which all matter was formed. He said elements were composed of only 1 type of atom and that compounds were composed of 2 or more different kinds of atoms. He proposed that the properties of the elements differ because their atoms differ. He recognized that compounds bear NO resemblance of the elements from which they are composed.

4. Dalton’s 3 Laws!! Law of Definite Composition— A compound contains the SAME elements in the SAME PROPORTIONS (AMOUNT) by mass no matter the size or source of the compound. (SAME composition)

5. Dalton’s Laws (continued) Law of Conservation of Mass— The mass of the reactants are equal to the mass of the products. Mass is not created or destroyed!! When elements combine to form compounds, elements are simply combined, separated, or rearranged.

6. Dalton’s laws (cont’d) Law of Multiple Proportions— Happens when different compounds are formed from the same 2 elements. The mass ratios of all elements in the compound are expressed in small whole numbers.

7. Dalton’s 5 principles: All matter is made up of indivisible and indestructible atoms. All atoms of a given element are identical in their physical and chemical properties. Atoms of different elements differ in their physical and chemical properties. Atoms of different elements combine in simple, whole number ratios to form compounds. Chemical reactions consist of rearranging, combining, and separating atoms.

8. John Dalton Which of Dalton’s 5 principles in his atomic theory are NOT true as of today? Dalton proved existence of atoms. His theory led to the idea that atoms consisted of smaller particles which were founded by other atomic scientists.

9. J. J. Thomson Performed a cathode ray tube experiment. The experiment consisted of the following items:  Cathode tube with a paddle wheel  2 metal plates (electrodes) placed at each end of the glass tube.  One plate had a positive charge (anode) and the other one had a negative charge (cathode).  He applied voltage (electric current) and used a magnet

10. J. J. Thomson (experiment ) Pumped out some of the air (gas) in a glass tube Applied voltage and found that rays emerged from the cathode (negative end) and were attracted to the anode (positive end). These “rays” were called cathode rays since they emerged from that end of the tube. Since the cathode rays were attracted to the anode, Thomson concluded that the rays must have a negative charge. The cathode rays proved the existence of ELECTRONS!!

11. He found that no matter what type of metal or gas he used in the tube that the cathode rays were always identical. The paddle wheel proved the electrons had a small mass. This proved that electrons must be in all types of atoms!! His model of the atom was called the “Plum Pudding Model” or the “Raisin Bun Model”. J. J. Thomson

12. Robert Millikan Performed the “Oil Drop” experiment after Thomson completed the cathode ray tube experiment. He calculated the mass to charge ratio in his experiment. He performed his experiment by dropping small drops of oil (which had picked up extra electrons) between 2 electrically charged plates.

13. Robert Millikan He monitored the drops, measured how the voltage on the plates affected their fall, and then calculated the charges on all the drops. After continuous experimentation, this proved the charge of the electron was definitely negative. Using complex calculations from his data, he also concluded that the mass of the electron is about 2,000 times smaller than that of hydrogen (which is the lightest atom).

14. Ernest Rutherford He discovered the nucleus and the positive charge inside the nucleus (protons)!!! He discovered this by performing his famous “Gold Foil Experiment ”.

15. Ernest Rutherford’s Experiment He used lead box from which he fired fast moving alpha particles (positive particles) at thin gold foil. He placed a fluorescent screen behind the foil. After completing his experiment he found out that about 99% of the alpha particles went straight through the gold foil. Only about 1% did not go through and were deflected backwards.

16. Ernest Rutherford Therefore, he said most of the atom was empty space since most of the particles went straight through. Since there were a few (1%) that did not go straight through and were located in the middle, he reasoned that the atom must have a small positive charge concentrated in the middle of the atom.

17. Ernest Rutherford So… Rutherford hypothesized that this “localized region” in the atom ( the NUCLEUS) must have a large mass compared to the alpha particle and it must contain positively charged particles (PROTONS) or else incoming particle (alpha particle) would have knocked the positive charge out of the way!!

18. James Chadwick Partner : Irene Curie Discovered the NEUTRON in 1932!! Came about after the discovery of protons and electrons Performed an experiment with the element Beryllium

19. James Chadwick Experiment : He and Irene bombarded Beryllium with alpha particles. The outcome??? A great penetrating power was produced that was made up of particles that were not deflected by neither electric nor magnetic fields.

20. James Chadwick His conclusion??? He concluded that since these particles carried no electric charge, then they must be NEUTRAL!! Thus, proving the existence of NEUTRONS!!!! This accounted for the rest of the mass in the atom’s nucleus!!!

21. Neils Bohr Discovery : Electrons reside on specific energy levels NOT between them!! Experiment : Used line- emission spectra to show electrons can exist in many different energy levels.

22. Neils Bohr Pictures associated with Bohr!!

23. Neils Bohr Things he found out through his experiment : Electrons can move from a low energy state (ground level) to a higher energy level (excited state) by absorbing energy!!! Electrons can also “fall” back from a higher energy level to a lower energy level by releasing energy!!

24. Neils Bohr When an electron is in the “excited state” and jumps to the “ground state” (to become more stable), it releases excess energy in the form of light!!! This light has specific colors and wavelengths on a line spectrum. Each different atom emits a specific color associated with a certain wavelength when the electron “falls” back to the ground state.

25. Quantum Model Involved several scientists: Heisenberg Louis de Broglie Schrodinger Max Planck Wolfgang Pauli

26. Quantum Model (cont’d) Louis de Broglie said that matter would act as a wave. Schrodinger said that matter acts like waves AND particles! This means that electrons would possess a lot of energy and pass right through each other like waves do!!!

27. Quantum Mechanical (cont’d) They said that electrons are still on specific energy levels (like Bohr said), but that the electron’s path cannot be EXACTLY determined (due to it constantly moving!). The region of the electron cloud that is the “most dense” is where there is the highest probability of finding the electron.

28. Quantum Mechanical Heisenberg Uncertainity Principle States that it is impossible to find BOTH the position and velocity at the SAME time!! You can either know the position OR the velocity, but not both simultaneously!!! This model is also called the Wave Mechanical Model. This model involves quantum numbers!!

29. Quantum Model Wolfgang Pauli—he came up with quantum numbers specifically the spin of the electron! Max Planck– he said that energy was held in discrete set called a “quanta” of energy.