|| ≈330 BC
|| Four element theory:
earth, air, fire and water
| Antoine Lavoisier
|| Wrote the first extensive list of elements containing 33 elements.
Distinguished between metals and non-metals.
|Some of Lavoisier's elements were later shown to be compounds and mixtures.
| Jöns Jakob Berzelius
|| Developed a table of atomic weights.
Introduced letters to symbolize elements.
| Johann Döbereiner
|| Developed 'triads', groups of 3 elements with similar properties.
Lithium, sodium and potassium formed a triad.
Calcium, strontium and barium formed a triad.
Chlorine, bromine and iodine formed a triad.
| Forerunner to the notion of groups.
| John Newlands
|| The known elements (>60) were arranged in order of atomic weights and observed similarities between the first and ninth elements, the second and tenth elements etc.
He proposed the 'Law of Octaves'.
| Newlands' Law of Octaves identified many similarities amongst the elements, but also required similarities where none existed.
He did not leave spaces for elements as yet undiscovered.
Forerunner to the notion of periods.
| Lothar Meyer
|| Compiled a Periodic Table of 56 elements based on the periodicity of properties such as molar volume when arranged in order of atomic weight.
|| Meyer and Mendeleev (see below) produced their Periodic Tables simultaneously.
| Dmitri Mendeleev
|| Produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other.
Gaps were left for elements that were unknown at that time and their properties predicted (the elements were gallium, scandium and germanium).
The order of elements was re-arranged if their properties dictated it, eg, tellerium is heavier than iodine but comes before it in the Periodic Table.
| Mendeleev's Periodic Table was important because it enabled the properties of elements to be predicted by means of the 'periodic law' which states that:
properties of the elements vary periodically with their atomic weights.
| William Ramsay
|| Discovered the Noble Gases.
|| In 1894 Ramsay removed oxygen, nitrogen, water and carbon dioxide from a sample of air and was left with a gas 19 times heavier than hydrogen, very unreactive and with an unknown emission spectrum. He called this gas Argon.
In 1895 he discovered helium as a decay product of uranium and matched it to the emission spectrum of an unknown element in the sun that was discovered in 1868. (helios is the Greek for Sun).
He went on to discover neon, krypton and xenon, and realised these represented a new group in the Periodic Table.
Ramsay was awarded a Nobel Prize in 1904.
| Henry Moseley
|| Determined the atomic number of each of the elements.
He modified the 'Periodic Law' to read that the properties of the elements vary periodically with their atomic numbers.
| Moseley's modified Periodic Law puts the elements tellerium and iodine in the right order, as it does for argon and potassium, cobalt and nickel.
|| Predicted that there were 3 unknown elements between aluminium and gold and concluded there were only 92 elements up to and including uranium.
|| Synthesised transuranic elements (the elements after uranium in the periodic table)
|| In 1940 uranium was bombarded with neutrons in a cyclotron to produced neptuniun (Z=93).
Plutonium (Z=94) was produced from uranium and deuterium.
These new elements were part of a new block of the Periodic table called Actinides (actinoids).
Seaborg was awarded a Nobel Prize in 1951.