1. Atomic Models

2. Dalton Model (1803) “Billiard Ball” model
All matter is made up of tiny indivisible particles called atoms .
All atoms of the same element are the same in size, mass, etc. (see next slide for isotope determination)
Atoms of different elements are different in size and mass.
Atoms of different elements combine in small whole number ratios to form molecules or compounds.
In chemical reactions, atoms are not created or destroyed. They are rearranged or recombined.
Ist model after Democritus (400 BCE) proposed atoms and Void
Newton (1704) mechanical universe with small solid masses in motion.

3. Isotopes and Mass Spectroscopy
1832 Faraday - studied electricity in solution, electrolyte
1869 Mendeleev - created the periodic table based on similar chemical properties
1873 Maxwell - proposed that magnetic and electric fields filled the void
1879 Crookes - discovered negative charges using Cathode ray tubes
1886 Goldstein – used a CRT to discover “canal rays” that were opposite to electrons
1887 J.J.Thomson – modified the CRT for low pressures and created a mass spectral graph

4. Mass spectra graphs can be used to determine the number and relative amounts of each isotope in an element.
Atoms and molecules are ionized and then passed through a magnetic field.
Lighter ions are defected more than the heavier ones and so the ions are separated.
The intensity of the signal is used to determine the relative abundance, where the largest peak is assigned a value of 100, or % abundance.
This data is used to calculate the relative atomic mass for the element.

5. eg. Mass spectra for zirconium , Zr atomic number 40. Where:
Zr - 90 has 51.5 %
Zr - 91 has 11.2 %
Zr - 92 has 17.1 %
Zr - 94 has 17.4%
Zr - 96 has 2.8%
Calculate the relative atomic number for Zr:
atm = (90 x 0.515) + (91 x 0.112) + (92 x 0.171) + (94 x 0.174) + (96 x 0.028)
= amu

6. Thomson Model (1904) “Raisin Bun” Model
The atom is a spherical cloud of positive charge with negatively charged electrons embedded in it.
The amount of positive charge equals the amount of negative charge, so the entire atom is neutral.
1897 Thomson determined the charge to mass ratio of the electron to be x 108 C/g
1898 Rutherford and Curie study radioactivity
1900 Planck used the idea of “quanta” to explain hot glowing matter
1909 Millikan uses the oil drop experiment to determine the charge (1.602 x C) and the
mass of the electron (9.11 x g)

7. Rutherford Model (1911) “Planetary or Solar System Model”
Discovered using the gold foil experiment.

8. The atom is mostly empty space.
Most of the mass is concentrated in a small, dense nucleus.
Electrons move around the nucleus like planets around the sun.
Problem with this model is that the e- revolving around should emit radiation and lose NRG, slow down and spiral down, ultimately falling into the nucleus. UNTRUE!

10. Bohr Model - Energy Level Model
Electrons travel around the nucleus in specific energy levels .
If an atom is excited, electrons absorb a certain amount of energy, hv and are boosted to a higher energy level.
Electrons later drop back to a lower state releasing energy of a colour equal to the amount of energy released.
The number of e- in each energy level, n, is equal to 2n2.
Recent X-ray PES data gives evidence of the energy levels within an atom.

11. Emission and Absorption Spectrum