Describe two differences between Mendeleev's periodic table and the modern periodic table

There's a massive improvement in the layout, to begin with. Elements are frequently colour-coded to differentiate between their physical properties.

Mendeleev's periodic table has no place for isotopes, and also placed dissimilar atoms together in groups.

Mendeleev also left gaps for elements he suspected would be discovered, but he did not have the equipment to discover. These were later discovered through things like electrolysis. These days these unknown elements are not so evenly spread, but are mostly found around unununium.