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.