Give one difference between Mendeleev's periodic table of the elements and the modern periodic table. - Leaving Cert Chemistry - Question 4(a) - 2018

One significant difference between Mendeleev's periodic table and the modern periodic table is that Mendeleev's table did not include noble gases, whereas the modern periodic table includes noble gases as a distinct group. Additionally, Mendeleev's arrangement was based primarily on atomic mass, whereas the modern periodic table is arranged by atomic number. Mendeleev left gaps for undiscovered elements, indicating that he anticipated more elements would be found, while the modern table accounts for all known elements and their properties.