We haven't always known as much about Chemistry as we do now (obviously); early chemists looked to try and understand patterns in the elements' properties to get a bit of understanding.
In the Early 1800s they could only go on atomic mass:
Whereas now there are two obvious ways to categories elements:
1. Their physical and chemical properties
2. Their relative atomic mass
REMEMBER - back then they had no idea about atomic structure or about protons and electrons meaning there was no such thing as atomic number.
(Only in the 20th century - after protons and electrons were discovered - that we realised the best arrangement was in order of atomic number)
Back then all they could measure was the relative atomic mass; this meant the known elements were arranged in order of atomic mass.
When this was done, a periodic pattern was noticed in the properties of the elements - it's where the name "Periodic Table" comes from (ta-da)
Newlands' Law of Octaves (the first good effort)
John Alexander Reina Newlands. (Pictured above)
Newlands attempted to arrange things more usefully back in 1864.
- Every eighth (8th) element had similar properties