2. Atomic Structure

3. Dalton’s Atomic Theory 1. All matter is made of tiny indivisible particles called atoms. 2. Atoms of the same element are identical, those of different atoms are different. 3. Atoms of different elements combine in whole number ratios to form compounds 4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

4. Crooke’s Tube + - vacuum tube metal disks voltage source magnet William Crookes Crooke’s Tube + - vacuum tube metal disks voltage source magnet William Crookes

5. J. J. Thompson did experiments with modified Crooke’s Tubes.

6. J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these negative particles. He knew that atoms did not have a net negative charge and so there must be balancing the negative charge. J.J. Thomson

7. William Thomson (Lord Kelvin) In 1910 proposed the Plum Pudding model –Negative electrons were embedded into a positively charged spherical cloud. Spherical cloud of Positive charge Electrons

8. Ernest Rutherford (1871-1937) Learned physics in J.J. Thomson’ lab. Noticed that ‘alpha’ particles were sometime deflected by something in the air. Gold-foil experiment

9. Rutherford’s Apparatus beam of alpha particles radioactive substance gold foil circular ZnS - coated fluorescent screen

10. What he expected… Because he thought the mass was evenly distributed in the atom.

11. What he got…

12. The Rutherford Atom (a working model, but not the final answer for the structure of an atom) n +

13. Discovery of the Neutron James Chadwick bombarded beryllium-9 with alpha particles, carbon-12 atoms were formed, and neutrons, particles with no charge, were emitted. + +

14. Atomic Structure

15. Electrons (-) charge no mass located outside the nucleus Protons (+) charge 1 amulocated inside the nucleus Neutron no charge 1 amulocated inside the nucleus Particles in the Atom

16. Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ nono +1 0 1/1840 1 1 9.11 x 10 -28 1.67 x 10 -24

17. Structure of the Atom There are two regions The nucleus With protons and neutrons –Positive charge –Almost all the mass Electron cloud –Most of the volume of an atom –The region where the electron can be found

18. Symbols Contains the symbol of the element, the mass number and the atomic number. The atomic number is specific for an element! X Mass number Atomic number # protons + # neutrons mass number

19. Symbols Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass number F 19 9 = 9 = 10 = 9 = 19 +

20. Symbols Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass number Na 23 11 Sodium atom = 11 = 12 = 11 = 23

21. Symbols If an element has 60 protons and 84 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol Nd 144 60 = 60 = 144 = 60

22. Isotopes Atoms of the same element with different mass numbers. Mass # Atomic # Nuclear symbol: Hyphen notation: carbon-12 12 6 C

23. Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = AVERAGE ATOMIC MASS Boron is 20% B-10 and 80% B-11. That is, B-11 is 80 percent abundant on earth. For boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

24. Naming Isotopes Put the mass number after the name of the element carbon- 12 carbon -14

25. Review Atomic Number –The number of protons; specific for an element. Atomic Mass –The combined number of protons and neutrons. Isotope –Atoms of the same element (same Atomic Number), but with different Atomic Masses.