1. Early Models of the Atom Democritus Universe was made of empty space and tiny bits of stuff Called these tiny bits of stuff atomos Atoms could not be divided

2. Early Models of the Atom Lavoisier Demonstrated that burning wood caused no change in mass Law of Conservation of Matter

3. Early Models of the Atom Dalton All elements are composed of indivisible particles. Atoms of the same element are the same Atoms of different elements are different. Compounds consisted of atoms of different elements combined together

4. Early Models of the Atom Thomson Plum pudding model Atom made of a positively charged material with the negatively charged electrons scattered through it.

5. Early Models of the Atom Rutherford Mostly empty space Small, positive nucleus Contained protons Negative electrons scattered around the outside

6. Early Models of the Atom Bohr Electrons move in definite orbits around the nucleus

7. Early Models of the Atom Chadwick Discovered the neutron

8. Atomic Structure Nucleus –Protons –Neutrons Electrons

9. Atomic Structure Electrons Tiny, very light particles Have a negative electrical charge (-) Move around the outside of the nucleus

10. Atomic Structure Protons Much larger and heavier than electrons Protons have a positive charge (+) Located in the nucleus of the atom

11. Atomic Structure Neutrons Large and heavy like protons Neutrons have no electrical charge Located in the nucleus of the atom

12. Describing Atoms Atomic Number = number of protons In a neutral atom, the # of protons = the # of electrons

13. Describing Atoms Atomic Mass Number - equal to the number of protons plus neutrons.

14. Ions An atom that carries an electrical charge is called an ion If the atom loses electrons, the atom becomes positively charged (because the number of positively charged protons will be more the number of electrons)

15. Ions An atom that carries an electrical charge is called an ion If an atom gains electrons, the atom becomes negatively charged (more negative charges than positive charges)

16. Isotopes The number of protons for a given atom never changes. The number of neutrons can change. Two atoms with different numbers of neutrons are called isotopes

17. Electron Configuration and Lewis Dot Structures

18. Atomic Orbitals Energy levels (shells) = n –n = 1, 2, 3….etc. –The higher the energy level, the farther away from the nucleus Sublevels –Designated by s, p, d, f –Refers to the shape (s) of the area in which the electron can be located –Also designates an energy level within the shell –Represents the region in space where the electron is most likely to be – show 90% probability –Cloud is more dense where probability of finding an e - is high –Where are the e - within each sublevel? We don’t know! Can’t determine the exact position of an e - in an atom

19. Electron Configurations Def: the ways in which electrons are arranged around the nucleus of an atom Show the approximate location of electrons Represent in which orbitals electrons are located

20. Electron Configurations cont. Aufbau Principle Electron configuration determined since electrons tend to be in lowest energy orbitals. Rules: The Aufbau principle (“building up”) guides us in the filling of orbitals: –Fill lowest energies first. –Maximum of two electrons with opposite spins in each orbital. (Pauli’s Exclusion Principle) –Degenerate orbitals (orbitals with same energy) follow Hund’s rule

21. Electron Configurations cont. Hund’s Rule Hund’s rule: If two or more orbitals have the same energy, fill each orbital with one electron before pairing electrons.

22. Periodic Table and Electron Configurations Build-up order given by position on periodic table; row by row. Elements in same column will have the same outer shell electron configuration.

23. Examples Fill according to the Energy Level Diagram Ex: –C–C 1s 2 2s 2 2p 2 –Ne 1s 2 2s 2 2p 6 –Na 1s 2 2s 2 2p 6 3s 1

24. Electron Dot Diagrams (aka Lewis Dot Structures) How do we draw a picture for the number of electrons an atom has? If you want to see how atoms of one element will react, it is handy to have an easier way to represent the atoms and the electrons in their outer energy levels. You can do this with electron dot diagrams. An electric dot diagram is the symbol for the element surrounded by as many dots as there are electrons in its outer energy level.

25. How to Write E.D.D.’s The dots are written in pairs on four sides of the element symbol. Start by writing one dot on the top of the element symbol, then work your way around adding dots to the right, bottom, and left.

26. How to Write E.D.D.’s Add a fifth dot to the top to make a pair. Continue in this manner until you reach eight dots to complete the level.

27. QUESTION The _______ an energy level is from the nucleus, the _______ electrons it can hold. A. closer, more B. closer, less C. farther, less D. farther, more

28. ANSWER The answer is D. The farthest shells contain the greatest number of electrons.