Evolution of Atomic Theory: From Greeks to Modern Models

1. Studying atoms Ch. 4.1

3. Greek Atom- 2500 years ago • Democritus- Greek philosopher • Atom from atomos – uncut, indivisible • Different types of atoms with different properties • Aristotle's Model :

4. Dalton’s Atomic Theory- 1800’s • Evidence • Measured masses of compounds • Found that the ratio of masses of the elements in a compound is always the same • Fixed composition! • Theory • All elements are composed of atoms • All atoms of the same element have the same mass, and atoms of different elements have different masses • Compounds contain atoms of more than 1 element • In a particular compound, atoms of different elements always combine in the same way

5. Dalton Summed Up: • All matter is made of individual particles, called atoms, which cannot be divided!

6. Thomson’s Experiment • Glass tube without air • 1 side positive, 1 side negative • Glowing beam appears in middle

7. Thomson’s Model of the Atom-1870’s • Evidence • Negative charge attracted to positive charge • 1st evidence atoms are made of even smaller particles • Model • Atom = neutral • Negative and positive evenly mixed • Called Plum Pudding after English dessert

8. Rutherford’s Experiment • Aimed alpha particles at gold screen • Screen flash when struck by alpha particle • Traced path of alpha particles

9. Rutherford’s Atomic Theory-1900’s • Evidence • Alpha particles- fast moving positive charge • Positive is not evenly spread • Nucleus- dense, positively charged mass at atoms center • Theory • All of an atoms positive charge is concentrated in the nucleus

11. The structure of an atom Ch. 4.2

13. Subatomic Particles • Protons- Rutherford • Positively charged particle found in the nucleus • Same as atomic # • Electrons- Thomson • Negatively charged particle found outside the nucleus • Atomic # - charge • Neutrons- Chadwick • Neutral particle found in nucleus • Mass almost exact to the proton • Mass # - Atomic #

14. Comparing Subatomic Particles Proton and neutrons= same ________

15. Atomic Number • Atoms of any given element always have the same # of p+ • Every p+ is balanced by e- for a neutral charge • = # of e- in an atom of that element • = the # of p+ in an atom of that element • Because atoms are neutral!

16. Mass Number • = p+ + n • Ex: Al • 13 p+ • 14 n • Mass # = 27 • n = mass # - atomic # • Ex: Al • Mass # = 27 • Atomic # = 13 • n= 27-13 14

17. Practice

18. Isotopes • Every element does have the same # of protons and electrons • Neutrons can vary • Have the same atomic #, but different mass # due to neutrons

19. Practice

20. Modern Atomic Theory Ch. 4.3

22. Bohr’s Model • Partnered with Rutherford • Focused on electrons • Count the # of electrons, and place in orbit

23. Energy Levels • Energies that electrons can have • Electrons move levels when the atom gains or loses energy • Use with Bohr Models

24. Practice • B • Mg • Si • Ne

25. Electron Cloud Model • Improved Bohr Model • Visual model for most likely locations for electrons • Dense= electrons

26. Atomic Orbital's • Electron cloud = good approximation of how electrons behave in orbitals

27. Electron Configuration • Arrangement of electrons in the orbitals • Most stable = electrons in orbitals with lowest energy • Ground State= all electrons have lowest energies

28. C= 1s22s22p2 Cr= 1s22s22p63s23p64s23d4

29. Practice • Li • Mg • K

30. Lewis Dot Structures • Count valance electrons (outer layer of electrons) • Place around element, no more than 8

31. Examples and rules for compounds

32. Practice • P • Be • Ca • CO2 • HCl • NaCl