In the early 1800s they ordered elemnts by physical/chemical properties and thei relative atomic mass.
Newlands law of octaves put them in groups of 7 as every 8th element had similar properties. Pattern was wrong on the third rom as the tranisition metals weren't similar.
Newlans work was criticized because he left no gaps, mixed up metals and non - metals e.g oxygen and iron, and he didn't leave any gaps for undiscovered elements.
Mendeleev put elements in order of atomic mass, but LEFT GAPS in order to keep elements with similar properties in the same vertical row. When new elements were discovered they fitted in the table. Much more trusted than Newlands idea.
The modern Periodic table
At first scientist didn't see much importantce to it because elements didn't really seem to fit. The discovery of protons, neutrons and electrons helped a lot as it put the elements in order of atomic structure which just so happened to fit with relative atomic mass aswell! After this discovery scientists though it was much more important as a summary of the atomic structure.It also meant we can predict the properties of undiscovered elements.
You can use the periodic table to work out the arrangement of electrons.
Electrons are arranged in shells which correspond to energy levels.
To work out the maximum number of electrons in…