1. Development of the Periodic Table
John Dalton
Johann Dobereiner
John Newlands
Dmitri Mendeleev
Henry Moseley

2. John Dalton atomic theory 1803-1805
PROPERTIES
MASS
Dalton’s atomic theory proposed that different elements’ atoms had different masses. This led to the question: what does mass have to do with the different properties of each element? This search for the connection between mass and properties would eventially lead to the Periodic Table.

3. Johann Dobereiner c. 1817 “Triads”
The first attempt at a periodic table was by Doberiner. He grouped elements into threes called “triads”. In each triad (mapped onto a modern P.T. above), the average of the first and third elements’ properties equaled the property of the middle element. This worked for physical properties like boiling point and density as well as chemical properties such as mole, ratios in oxides.
The problem? Dobereiner ran out of or triads before he ran out of elements: not all element fit his pattern.

4. John Newlands 1863-’65 “Law of Octaves”
John Newlands was the first to find a periodic or cyclical pattern. He found that when he lined the elements up from smallest ot biggest atomic mass, properties repeated every 8th element. In the above table you need to go down each column - not across the rows like you’re used to. The 8th element is F, and its properties are similar to H’s. The 15th element is Cl, and it is again similar to F and H, while K is similar to Na and Li.
Newlands ran into two problems. The first was the same as Dobereiner: not all elements fit the pattern. The second was that Newlands compared the periodicity of the elements to the notes on a musical scale (“C” repeats every 8th note) - this made some scientists not take him seriously.

5. Dmitri Mendeleev 1869 Periodic Table by mass
Mendeleev gets the credit for the first sucessful Periodic Table. He took Newlands basic idea and then added two key components. First, he did not try to make ALL the periods the same length. This was very important, but made other scientists doubt the validity of his table. The second innovation was to assume not all the element had yet been found, and to use the gaps in his table to PREDICT the existence and properties of as yet undiscovered elements. He hypothesized for 3 elements, and within five years all three had been found, with the predicted properties.That made everyone more confident his table had meaning.
Medeleev was forced to reverse the order of Co and Ni. He felt the atomic masses were wrong. Actually, something far more fundamental was going on …

6. Henry Moseley 1913 Moseley’s Law
In 1913, while researching something else, Moseley discovers a way to count the number of protons in a atom. When mapped onto Mendeleev’s table, the troublesome Co-Ni fall into place, It becomes apparent that the REAL organizing principle of the Periodic Table is ATOMIC MUMBER, rather than mass.
Because elements with more protons generally have a higher atomic mass, Mendeleev was able to see a pattern. But Moseley’s discovery that atomic number is the key is what led to UNDERSTANDING the pattern.
The properties of elements are actually a result of their electron configuration, which is related to number of protons in the nucleus. This concept had to wait unti the 20th C and the discovery of the parts inside an atom.

7. Moseley’s Modern Periodic Table
MSELEY: