Chemists still argue about the best way to arrange the elements. The periodic table orders them by the number of protons in the nucleus and stacks elements with the same number of outer-shell electrons into columns, but several elements don't fit neatly.
Hydrogen sits above lithium and sodium because it has one outer electron, yet it is a reactive gas that behaves more like the halogens. Mercury is a liquid while the rest of its group are solid metals: its inner electrons move at about 58% of the speed of light, and that relativistic effect pulls them in so tightly they barely form bonds. The same effect gives gold its colour. Chemists also disagree on which elements belong at the start of the f-block in group 3.
Proposed fixes include Theodor Benfey's 1964 spiral (pictured), a 3D "Christmas tree" table, and a long 32-column table that keeps the atomic numbers in one unbroken sequence.
Three reasons why the periodic table needs a redesign (New Scientist)