Chemistry - 2 The Periodic Table - 2.1 Development of the Periodic Table

John Dalton - periodic table

he organised elements in order of their atomic weight

John Newlands - periodic table [3]

- also organised in order of atomic weight but noticed that the properties of every eighth element were similar.
- created his 'law of octaves' table
- assumed that all elements had been found

Dmitri Mendeleev - periodic table [3]

- organised in order of atomic weight
- switched elements so that properties matched
- left gaps for undiscovered elements

The modern periodic table [2]

- ordered by atomic number (number of protons)
- has group 0 (these elements were undiscovered when Mendeleev made his)

atomic mass vs. atomic weight

atomic mass represents the mass of a single isotope or atom, while atomic weight is the weighted average of an element, taking into account all its isotopes and their abundances