Historical Development of the Atomic Theory

1. Historical Development of the Atomic Theory

2. Democritus • Ancient Greek philosopher (circa 400 BC) • Matter is made of tiny particles that we can’t see • Atomos means indivisible • Thought these spheres of matter were solid and could not be broken down further

3. John Dalton (1766 – 1844) • English scientist and teacher • Performed experiments • 4 points to his atomic theory:

4. Dalton’s atomic theory: • All elements are made of tiny indivisible atoms • Atoms of the same element are the same as each other and different from other elements • Atoms of different elements can mix together in whole-number ratios • Chemical reactions occur when atoms change how they are arranged.

5. J.J. Thompson (1856 – 1940) • English Physicist • Passes electric current through a cathode ray tube • The cathode ray is a stream of negatively charged electrons • Electrons are a sub-atomic particle • Must be a corresponding positive part to the atom • “discovers” the electron

6. Credited with the “Plum Pudding” Model

7. Ernest Rutherford (1871 – 1937) • New Zealander who worked in England • Gold Foil experiment

8. Diagram the gold foil experiment: An alpha particle (α)is a Helium nucleus (2 protons, 2 neutrons)

9. What was learned from the gold foil experiment: • Small, massive center called the nucleus • Nucleus has a positive charge (+) • Electrons (-) are outside the nucleus • Most of atom is empty space

11. Neils Bohr (1885 – 1962) • Danish Physicist • Used Spectral Line Data from hydrogen • Organized electrons into discrete shells or orbitals. • The Bohr Model is what is used to explain electron configurations in Regents Chemistry

12. Modern Atomic Theory

13. All matter is composed of atoms • An element is composed of only one type of atom • Compounds are different kinds of elements bonded together in definite proportions

14. Atoms can be broken down into smaller units called subatomic particles: • Proton • Neutron • Electron

15. Proton • positively charged • Mass of 1 amu • Found in the nucleus • Atomic # = protons

16. Finding Protons: • Drag out periodic table

17. Neutron • No charge (electrically neutral) • Mass of 1 amu • Also in the nucleus • Atomic mass – atomic #

18. Calculating neutrons:

19. Electron • Negatively charged • Mass of 0 (zero) amu • Found outside the nucleus • “Shells” or energy levels • Usually = protons

20. Wave-Mechanical Model • AKA (Also Known As): • Quantum Mechanical Model • Electron Cloud • Deals with the probability of finding an electron in a certain area of the atom outside the nucleus.

22. Calculating Electrons:

23. Electron Shells(or orbitals or energy levels): • (crack out your Periodic Table) • Row # = # of electron shells.

25. Valence Electrons • electrons in the outermost energy level • Column # = # of valence electrons sort of…

27. Column 1 = 1 valence electron • Column 2 = 2 Valence electrons • Columns 3 – 12 there is no obvious pattern

29. Columns 13 -18 = 10 fewer valence electrons than column number • e.g. • Column 14 = 4 Valence electrons • Columns 15 = 5 valence electrons • Exception, He has 2 valence electrons.