1 The Periodic Table Theoretical Chemistry

2 Origin of Periodic Table Triads - groups of 3 elements with similar properties (Dobereiner – 1817) Law of Octaves – properties of elements repeat every 8 elements (Newlands – 1863)

3 Origin of Periodic Table Dimitri Mendeleev – 1869 –Properties of elements are periodic functions of their atomic masses. –Developed 8 column table –Left spaces for undiscovered elements –Columns contained elements with similar properties –Kept elements with similar properties together even if atomic mass out of order

4 Origin of Periodic Table X-ray experiments by Mosley (1913) led to discovery of atomic numbers. Modern Periodic Law – when elements arranged by increasing atomic number, chemical and physical properties show a periodic pattern

5 Organization of Periodic Table Rows are called periods. Columns are called families or groups. All elements in a family have similar properties. Octet Rule – elements with 8 valence electrons are unreactive

6 Practice Aluminum is in what group? What period? Iodine is in what group? What period? What element is in Group 2, period 5? What element is in period 6, group 12?

7 Nuclear Decay  Alpha Emission parent nuclide daughter nuclide alpha particle Numbers must balance!! Courtesy Christy Johannesson

8 Nuclear Decay  Beta Emission electron  Positron Emission positron Courtesy Christy Johannesson

9 Nuclear Decay  Electron Capture electron  Gamma Emission  Usually follows other types of decay.  Transmutation  One element becomes another. Courtesy Christy Johannesson

Neutrons (A-Z) Protons (Z) Nuclear Decay  Why nuclides decay…  need stable ratio of neutrons to protons DECAY SERIES TRANSPARENCY Courtesy Christy Johannesson  P = N e - capture or e + emission  stable nuclei

Neutrons (A-Z) P = N Protons (Z) stable nuclei e - capture or e + emission   Neutrons (A-Z) P = N Protons (Z) stable nuclei  Why nuclides decay…  need stable ratio of neutrons to protons Nuclear Decay