1. Theory of Atomic Structure

2. Greeks – Democritus, Leucippus
Over 2000 years ago
All matter is composed of tiny particles
These particles are so small that they cannot be broken down (indestructible)
Named these particles atoms (from the Greek work for indestructible)
Common Greek theory was that all matter consisted of four "elements" - earth, air, fire, and water

3. John Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass
All elements are composed of atoms, which are indivisible and indestructible particles
All atoms of the same element are exactly alike; in particular they all have the same mass
Joining atoms of two or more elements forms compounds. In any compound, the atoms are joined in a definite whole number ratio (H2O, 2 to 1 ratio)

4. Dalton Did account for the following laws:
Law of conservation of mass (a chemical change is simply a rearrangement of atoms)
Law of definite proportions (atoms are combined in definite ratios)
Law of multiple proportions (elements can combine in different ratios to form new compounds – H2O, H2O2
Did not account for isotopes

5. J.J Thompson Discovered electrons
Through the use of cathode rays, Thompson was able to discover a negatively charged particle with almost no mass
Discovered electrons
See Cathode Ray Tube

6. “Plum Pudding” Model (1890)
Negative charges were scattered throughout the positive atom

7. Ernest Rutherford
Discovered that atoms have a positive, dense nucleus surrounded by negative “empty” space (the electrons are in the “empty” space)
Gold Foil Experiment – 1909 (chem ASAP)

8. Gold Foil Experiment What was used What was done
Alpha (α) particles – positively charged
Same as Helium nuclei (42He2+)
Gold Foil – only a few atoms thick
What was done
Shot a beam of α particles through the gold foil

9. Gold Foil Experiment What happened?
The majority of the particles went straight through the foil and exposed the film behind it
A few particles were deflected back from the foil and scattered

10. Gold Foil Experiment What does this mean?
Since the α particles are (+) charged, they must have been pulled through the foil by a strong (-) charge
The atom was mostly negatively charged “empty” space
Since some α particles bounced back, they must have hit a positive part of the foil
The atom must have a small positive part located in the center

11. Gold Foil Experiment Atomic Theory (Conclusion)
An atom consists of a very small, positively charged nucleus and the rest is negatively charged empty space. This empty space must contain the electrons.
Lets see the experiment again!

12. Bohr Model – Neils Bohr 1919 Electrons resemble our solar system
Electrons revolve (orbit) around the nucleus
Principle energy levels (PEL) = rings around the nucleus

13. Principle Energy Levels
The only regions in which electrons can be found
Each level can only contain a specific number of electrons (2 in the 1st, 8 in the 2nd)
Electrons farther away from the nucleus have higher energy
Number of PEL’s is given by the period number (found on the periodic table)

14. James Chadwick 1932 – discovered neutrons
Using alpha particles he discovered a neutral atomic particle with a mass close to a proton.

15. Valence Electrons – outer electrons
Outermost electrons give an atom its properties
These are the electrons that are involved in reactions
Last number in the electron configuration
Kernel electrons – inner electrons

16. Electron Dot Symbols / Lewis Dot Diagrams
Dots represent valence electrons
Examples: K Ca S Ar
Ca2+
S2-

17. Electron Configurations
Distribution of electrons
Given in the lower left hand corner of the periodic table (below the atomic number)

18. Rules 1st PEL can hold 2 electrons 2nd PEL can hold 8 electrons
3rd PEL can hold 18 electrons
4th PEL can hold 32 electrons
No more than 8 electrons are in the outermost principle energy level
This equal 8 valence electrons, the maximum amount of valence electrons possible
The electrons are added one at a time to the unfilled principle energy levels

19. Examples Ca: 2-8-8-2 What does this mean?
4 principle energy levels contain electrons
1st PEL contains 2e-, 2nd and 3rd PEL contain 8e-, and the 4th PEL contains 2e-
1st and 2nd are filled, 3rd and 4th are not completely full
Ca has 2 valence electrons, and 18 kernel electrons

20. Examples: C Li F

21. Detailed Electron Configurations
Within each principle energy levels there are sublevels
Each s sublevel contains 2 electrons
Each p sublevel contains 6 electrons
Each d sublevel contains 10 electrons
Each f sublevel contains 14 electrons

22. PEL
Sublevels
# of electrons 1 s 2
s, p
2 + 6 = 8 3
s, p, d
= 18 4
s, p, d, f
= 32

23. Example Calcium: What does this mean??? 1s22s22p63s23p64s2
The number in front of the sublevel indicates the PEL (4s = 4th PEL, s sublevel)
The number of electrons is given by the superscript (4s2 = 4th PEL, s sublevel, 2 e-)

24. Rules Fill the lowest energy sublevels first
See energy chart and diagonal method
No more than 8 electrons can be in the outermost principle energy level

25. Examples C Al Ca Ni

26. Ground State Electrons are in the lowest available energy levels
This is how the electron configuration is written on the periodic table
Normal configuration
Stable

27. Excited State
When an electron gains energy, the electron is at a higher energy state (excited state)
Electrons absorb energy and move to new higher energy levels
UNSTABLE

28. Excited State
When an electron returns from the excited state to the ground state, energy is released
The energy is often emitted in the form of light
The wavelength (color) of the light can be used to identify the element because each element absorbs and releases a specific amount of energy

29. Quantum Theory
Electrons can only absorb (or release) specific amounts of energy called quanta
This energy corresponds to the differences between energy levels
Energy is always absorbed (or released) in definite amounts rather than in a continuous flow
A quantum of radiation is called a photon

30. Flame Tests
Every element absorbs and hence emits different amounts of energy (different colors)
Can be used for identification

31. Spectra/Spectroscope
The photons of light that are given off can be broken down into lines (spectral lines)
Spectroscope
An instrument that breaks light into colored bands
Every element has a unique band length (arrangement)

32. Modern Orbital Theory (Wave Mechanical/Electron Cloud Model)
Electrons do not have a specific path or location – they have probabilities of being located there
Regions of high probabilities are called orbitals
Electrons are located in orbitals