The element in the periodic table that hasn't been arranged according to its increasing atomic number is hydrogen (H). Hydrogen is typically placed at the top left of the periodic table, separate from the other elements, due to its unique properties. It has an atomic number of 1, making it the lightest element. While hydrogen usually appears in Group 1, it is not a member of the alkali metal family and does not exhibit typical alkali metal characteristics. Instead, it is placed separately to indicate its distinctive properties and behavior.