Task
Draw a timeline to show how ideas about atoms have changed.
Some key ideas to be included are shown below. Note these developments are
not in order.
 John Dalton arranged the known elements in order of their atomic weights.
 Dmitri Mendeleev arranged the known elements in order of increasing atomic
weight. However, he left gaps for elements that hadn’t been discovered and
swapped the order for several elements so that a periodic pattern in their
properties could be seen.
 John Newlands noticed that when the elements were placed in order of their
atomic weight there were sometimes similarities between every eighth
element. He produced a table showing his “Law of octaves”.
 In 1817 Johann Wolfgang Dobereiner attempted to organise the elements by
placing the known elements into groups of three such as chlorine, bromine
and iodine. These groups were called ‘triads’. In each triad the atomic weight
of the middle element is roughly the average of the other two elements.
 In 1862 Alexandre-Emile Beguyer de Chancourtois noticed that elements with
similar properties occurred at regular intervals; a property known as
periodicity. However, he did not use the correct scientific terms and did not
include a diagram in his findings which were largely ignored.