Where is the periodic table arranged to begin with hydrogen?

Hydrogen occupies the top-left position in the modern periodic table because it has one proton and one electron, giving it atomic number 1. The table is arranged by atomic number, not by atomic mass, which was a major step in making the table chemically consistent. Hydrogen’s placement is unusual because it behaves like alkali metals in some ways and like halogens in others. The periodic table was first published by Dmitri Mendeleev in 1869, though the modern arrangement was refined later after the discovery of atomic number. Today, hydrogen remains one of the most important and least easily classified elements in chemistry.