Early chemists attempted to classify the element An element is a substance that consists of only one type of atom and it cannot be broken down into anything simpler by chemical means. according to their atomic weights – what we now call relative atomic mass This is the mass of the atom compared with that of the carbon-12 isotope, which has a mass of exactly 12..
They could see that relative atomic masses of the elements increased by steps.
Many elements were unknown, so these early tables were incomplete. They also contained errors – some elements that were not similar were placed together.