Mendeleev placed thallium (Tl) in the same group as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). However, the modern periodic table doesn’t place thallium in this group. Use the periodic table to find the properties of thallium. Explain why Mendeleev might have grouped thallium in the same group as lithium and sodium. Also explain how thallium is different from the other five elements.