Key Scientists in the construction of the periodic timetable

This doesn't work for six elements, however. These being: iron, manganese, nickel, cobalt, zinc and copper

In 1817, Johann Wolfgang Dobereiner thought that he had made a great observation about elements. He thought that there was a relationship between the atomic weights of elements and their properties. His theory was that, in groups of three (triads), the middle element had the mean atomic mass of the two elements to either side of it. This works with many elements, like Lithium, Sodium and Potassium. (Li + K) / 2 = Na (7 + 39) / 2 = 23

Dobereiner's Traids

In 1864, John Newlands ordered the periodic table by relative atomic mass. After doing so, he regarded that each element was similar to the element 8 places further on. This is true for some many elements, such as Lithium and Sodium The name comes from musical instruments like the piano. There are 8 different notes in a piano, and moving up 8 notes is called an octave

Newlands' Octaves

There are many exceptions to this rule, however, including iron being put in the same group as sulfur and oxygen - explaining why it was disregarded by other scientists

In 1869, Dmitri Mendeleev put forward another periodic table. This table was published only 5 years after Newlands' and also organised the elements in order of relative atomic mass. Mendeleev took Newlands' ideas and furthered them, making the connection between both chemical and physical properties and their atomic masses. This let him see that these properties were related in a 'periodic' way and ordered the table using periods - aka vertical columns

Mendeleev's Periodic Table

This was very accurate and precise and, in places where there were exceptions, allowed for him to predict where new elements would be placed and their relative atomic mass, to a great degree of accuracy