What was a significant error in Mendeleev's periodic table?

The significant error in Mendeleev's periodic table was that it grouped elements incorrectly by atomic mass. Mendeleev arranged the elements based on increasing atomic mass, which led to the placement of some elements in positions that did not reflect their true chemical properties. For example, he placed iodine (with an atomic mass higher than that of tellurium) before tellurium because of the similarities in chemical behavior despite the irregularity in their atomic masses. This inconsistency highlighted the limitations of using atomic mass as a primary criterion for periodic arrangement, which later was resolved when the periodic table was restructured based on atomic number, leading to a more accurate representation of elemental properties and behaviors.

The options related to atomic number, noble gases, and the inclusion of only metals do not relate to Mendeleev's recognized errors, as these were not considerations in his original table. Noble gases had not been discovered during Mendeleev's time, and his table was not limited exclusively to metals, as it included nonmetals as well.