Periodic Table History

Dobereiner He put elements with similar chemical properties together They went into in groups of 3 He noticed these trends in groups of elements such as: Can you name these elements ? Atomic Weight of “middle” element is average of other two – approx.

Dobereiner’s Law of Triads A triad is a group of three elements with similar chemical properties in which the atomic mass of the middle element is approximately equal to the average of the other two. Only worked for very few elements of the 50 or so that were known at the time BUT He was first to make a link between atomic weight and properties

John Newland Arranged the 60 known elements in order of increasing atomic weight His pattern was… Every 8 th element was a repeat (i.e. had 7 groups) His Law was called…

Law of Octaves An octave is a group of elements arranged in order of increasing atomic weight, in which the first and the eighth element of each group have similar properties. The properties repeat every 8 as noble gases hadn’t been discovered yet! Only worked for 17 out of 60 elements There were several problems such as iron being grouped with oxygen and sulphur. Laughed at but was basically correct

Mendeleev Arranged the known elements in order of increasing atomic weight His Law was called…

Mendeleev’s Periodic Law: When elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements vary periodically.

Mendeleev: Put elements with the same properties in the same vertical group. Reversed the order of some elements (Te/I) so that their properties matched their group. Left gaps to make the elements fit into the proper column (group). Predicted that elements (eg. Germanium and Gallium) would be discovered to fill these gaps. Predicted their properties correctly.

Mendeleev’s Modern Periodic Table V Periodic Table Gaps left for undiscovered elements Group 0 elements missing Elements discovered since 1869 missing D-block elements arranged as subgroups beside each main group Elements arranged in order of increasing atomic weight in almost all cases. Gaps have been filled Group 0 elements included Elements discovered since 1869 included D-block elements arranged as subgroups in a separate block Elements arranged in order of increasing atomic number

Mendeleev’s table

Moseley – Atomic Number The atomic number of an atom is the number of protons in the nucleus of that atom. Moseley used x-rays to find out how much positive charge each nucleus had In other words the difference between the elements is the number of protons in the nucleus. Once the atomic number was known it was seen that Mendeleev’s table was in order of increasing atomic number – not weight.

Modern Periodic Table In order of increasing atomic number. There were 63 elements in Mendeleev’s table – now 109 No gaps The transition elements are listed separately. Today’s table 1940 – Glenn Seaborg

Do I know it??? Who proposed Law of Triads? Who proposed Law of Octaves? Who proposed Periodic Law? Can I explain each ? Can I name 3 triads? What was Moseley’s contribution to the Periodic Table development? Four differences between Mendeleev’s Table and modern table?