1. ATOMIC STRUCTURE Modern theory of matter

2. LET’S REVIEW Dalton’s atomic theory – Elements are made of atoms ; all atoms of an element are identical, atoms of one element are different from atoms of other elements, atoms can combine to form compounds. Compounds always have same relative numbers and types of atoms. Atoms are indivisible in chemical processes. Chemical reactions simply rearrange atoms. Questions arose from Dalton’s theory: What causes atoms to “stick together” to form compounds? What is an atom like? What is its internal structure?

3. SCIENTISTS NEEDED EVIDENCE TO HELP THEM UNDERSTAND ATOM’S INTERNAL STRUCTURE JJ Thomson – 1890s Cathode ray tube Evidence: any element can be made to emit negatively charged particles Conclusion: all types of atoms must contain these negative particles. He called them electrons

4. JJ THOMSON – OTHER CONCLUSIONS Thomson knew that atoms were neutral – they didn’t have a charge This meant that there also had to be a positive particle in the atom to counterbalance the negative charge of the electron Negative Positive

5. PLUM PUDDING (OR CHOCOLATE CHIP COOKIE) MODEL Diffuse positive “pudding” (or cookie dough0 Negative charges inside like raisins (or chocolate chips)

6. NO MORE PLUM PUDDING! Ernest Rutherford Shot positively charged α-particles at thin gold foil Evidence : most alpha particles went through foil, but some got deflected straight back Conclusion : something concentrated and positive in center of atom – rest of atom mostly empty space

7. THE NUCLEAR ATOM Atom with dense center of positive charge (the nucleus) around which tiny electrons move in a space that is otherwise empty

8. BOHR MODEL Electrons revolve around positive nucleus (which also contains neutrons) Electron charge (-) same same magnitude (but opposite) of proton charge (+)

9. MORE CURRENT VIEW Tiny nucleus (10 -13 cm diameter) Electrons move about nucleus (10 -8 cm or so away) Neutron and proton have similar mass Electron about 2000 times smaller than neutron and protons Electrons “intermingle” to bond to other atoms

10. NUMBER OF ELECTRONS AN ATOM HAS AFFECTS HOW IT INTERACTS WITH OTHER ATOMS Atoms of different elements, with different numbers of electrons, behave differently

11. ISOTOPES Neutral atom has equal numbers of protons and electrons All atoms of particular element contain same number of protons and electrons E.g. Na (Sodium) – 11 protons, 11 electrons Some atoms of Na contain 12 neutrons; some contain 13 neutrons ISOTOPES = same type of atoms – same protons and neutrons – but different number of neutrons

12. HOW TO SPECIFY ISOTOPES For Example: 12 neutrons13 neutrons

13. HOW TO DETERMINE NUMBER OF NEUTRONS IN AN ATOM Mass number – Atomic Number = # Neutrons 12 – 6 = 6 Neutrons Mass number (number of protons + number of neutrons) Atomic Number (number of protons)

14. YOU DO IT Mass number – Atomic Number = # Neutrons Mass number Atomic Number 13 – 6 = 7 Neutrons

15. YOU DO IT Mass number – Atomic Number = # Neutrons Mass number Atomic Number 201 – 80 = 121 Neutrons

16. THE END