The Early 1800's
In the early 1800's they could only go on atomic mass. Until quite recently, there were only two obvious ways to categorise elements:
- Their physical and chemical properties
- Their relative atomic mass
Remember, they had no idea of atomic structure or of protons and electrons, so there was no such thing as atomic number to help them, It was only in the 20th century after protons and electrons were discovered that it was realised the elements were best arranged in order of atomic number. Back then, the only thing they could measure was relative atomic mass, and so the known elements were arranged in order of atomic mass. When this was done, a perioidc pattern was noticed in the properties of the elements.
Newlands had the first good attempt at arranging things more usefully in 1864. he noticed that every eight element had similar properties, and so he listed some of the known elements in rows of seven.
These sets of eight were called Newlands' Octaves. Unfortuanately, the pattern broke down on the third row, with transition metals like titanium and iron messing it up.
It was because he left no gaps that his work was ignored. However he was getting very close.
Newlands presented his ideas to the chemical society in 1865. But his work was criticised because:
- His groups contained elements that didn't have similar properties, such as carbon and titanium.
- He mixed up metals and non-metals e.g. oxygen and iron.
- He didn't leave any gaps for elements that hadn't been discovered yet.
In 1869, Mendeleev in Russia, armed with about 50 known elements, aranged them into his Table of Elements. He put them in order of atomic mass (like Newlands did). However he found that he had to leave gaps in order to keep elements with similar properties in the same vertical groups. He was prepared to leave some very big gaps in the first two rows before the transition metals come in on the third row.
The gaps were the really clever bit because they predicted the properties of so far undiscovered elements. When they were found and fitted the pattern it was good evidence for his table.
What did scientists think of the periodic table?
When the periodic table was first released, many scientists thought it was just a bit of fun. At that time, there wasn't much evidence to suggest that the elements really did fit together in that way.
After Mendeleev released his work, newly discovered elements fitted into the gaps he left. This was convincing evidence in favour of the periodic table.
Once there was more evidence, many more scientists relaised that the periodic table could be a useful tool for predicting properties of elements.
In the early 20th century, electrons, protons and neutrons were discovered.This led to scientists accepting the periodic table as it seemed to match up with what was discovered about the structure of the atom.
The modern periodic table
The modern periodic table can be seen as an arrangement of the elements in terms of their electronic structures. Elements in the same Group have the same number of electrons in their highest occupied energy level (outer shell)