History of the Periodic Table

In the 1800s only the relative atomic mass could be measured so known elements were ordered by atomic mass.

1828, Döbereiner began to put list of elements in groups based on chemical properties.

He put them in groups of three called triads e.g. Cl, Br, I and Li, Na and K.

Middle element had average relative atomic mass of the other two.

1864, noticed every 8th element had similar properties so he listed them in rows of seven. 

Sets of 8 called Newland's Octaves but pattern broke on third row with transition metals like Titanium and Iron messing it up.

He left no gaps in the table so work was ignored.

Ideas presented to Chemical Society in 1865 but criticised because:

• Groups contained elements with unsimilar properties e.g. Carbon and Titanium
• Mixed metals and non-metals e.g. Oxygen and Iron
• Didn't leave gaps for undiscovered elements

1869 Russia, 50 known elements, in order of atomic mass like Newland.

Had to leave very large gaps to keep elements with similar properties in the same vertical groups.

Gaps predicted properties of undiscovered elements - found and fitted pattern.

Made even more sense when later discoveries on atomic structure.

Each element has one more atomic number than previous.

Pattern in periodic table (two in first, eight in second, eight in third) matches electron arrangement.