3. Learning Standards Atomic Structure Broad Concept: Atomic models are used to explain atoms and help us understand the interaction of elements and compounds observed on a macroscopic scale.,

4. –Recognize discoveries from Dalton (atomic theory), Thomson (the electron), Rutherford (the nucleus), and Bohr (planetary model of atom) and understand how these discoveries lead to the modern theory. –Describe Rutherford’s “gold foil” experiment that led to the discovery of the nuclear atom. Identify the major components (protons, neutrons, and electrons) of the nuclear atom and explain how they interact. –Write the electron configurations for the first twenty elements of the periodic table.

5. All matter is composed of atoms Atoms cannot be made or destroyed All atoms of the same element are identical Different elements have different types of atoms Chemical reactions occur when atoms are rearranged Compounds are formed from atoms of the constituent elements Atomic theory proposed by John Dalton

6. Atom Definition Atom What is inside the atom?

8. Discovery of Protons Eugene Goldstein noted streams of positively charged particles in cathode rays in 1886. –Particles move in opposite direction of cathode rays. –Called “Canal Rays” because they passed through holes (channels or canals) drilled through the negative electrode.

9. Canal rays must be positive. Goldstein postulated the existence of a positive fundamental particle called the “proton”.

10. nPnPnPnPassing an electric current makes a beam appear to move from the negative to the positive end. Thomson’s Experiment And Discovery of Electrons Voltage source +-

11. Thomson’s Experiment nBnBnBnBy adding an electric field he found that the moving pieces were negative. + -

12. The electron was discovered in 1897 by Thomson. He imagined the atom as a “raisin pudding” with electrons stuck in a cake of positive charge.

13. J.J. Thomson’s Model of Atom Plum Pudding Model, 1896 Thought an atom was like plum pudding –Dough was cloud –Raisins were electrons –Didn’t know about neutrons at this time

14. Rutherford’s experiment and discovery of nucleus English physicist Ernest Rutherford (1911) Shot alpha particles at fluorescent screen. When an alpha particle hits a fluorescent screen, it glows.

15. Lead block Uranium Gold Foil Fluorescent Screen

16. What he expected He Expected The alpha particles to pass through without changing direction very much.

17. What he got

18. He thought the mass was evenly distributed in the atom

19. The Nuclear Atom Since some particles were deflected at large angles, Thomson’s model could not be correct.

20. How Rutherford explained results….. Atom is mostly empty space. Small dense, positive piece at center is (NUCLEUS)

21. RUTHERFORD’S MODEL OF AN ATOM In 1912, Rutherford discovered the nucleus by doing scattering experiments. He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun.

22. The model created by Rutherford had still some serious discordance. According to the classic science, electron moving around the nucleus should emit an electromagnetic wave. Electron should than move not by the circle but helical and finally collide with the nucleus. But atom is stable.

24. Rutherford also realized that the nucleus must contain both neutral and positively charged particles. The neutron was then discovered in 1932 by Chadwick.

25. Bohr’s Model of the Atom Similar to Rutherford’s model Thought atom was mostly empty space Neils Bohr, 1913 –N–Nucleus in center is dense, positively charge –E–Electrons revolve around the nucleus.

26. Following Rutherford’s planetary model of the atom, it was realized that the attraction between the electrons and the protons should make the atom unstable Bohr proposed a model in which the electrons would stably occupy fixed orbits, as long as these orbits had special quantized locations

27. Parts of an Atom Each element has a different number of protons in its nucleus Protons have positive charge Change the number of protons  change elements This is called nuclear physics The element also has the same number of electrons Electrons have negative charge Change the number of electrons  ionize the element This is called chemistry Some elements also have neutrons Neutrons have no charge They are in the nuclei of atoms pn e

28. Subatomic particles Electron Proton Neutron NameSymbolCharge Actual mass (g) e-e- p+p+ n0n0 9.11 x 10 -28 1.67 x 10 -24

29. Bohr’s model Electrons move around the nucleus at stable orbits without emitting radiation. Electron in one of these stable orbit has a definite energy. Energy is radiated only when electrons make transitions from high energy orbit to a low energy orbit.

30. In the Bohr model, the electron can change orbits, accompanied by the absorption or emission of a photon of a specific color of light.

31. Bohr’s Model 1913 – Atomic Model with Fixed Orbits proposed – The Atom has No Charge Number of Protons = Number of Electrons

32. An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2*n 2, Principle energy level ( n ) Maximum number of electrons 2*n 2 12 28 318 432 550 672 798

33. The most electrons possible in the first shell are 2. After the first shell is filled, the second shell starts filling up, according to the number of positive charges in the nucleus. The most allowed in the second shell is 8 electrons. Then the third shell starts to fill.

34. Study Buddy Review Name three subatomic particles. Who is credited with discovering each particle? Describe the Rutherford Gold Foil Experiment.