Answer:
Mendeleev placed thallium (Tl) in the same group as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs)
Explanation:
Properties of Thallium :
Thallium is a heavy metal denoted by symbol Tl. It is placed in 13th group and 6th period of modern periodic table. It is very soft in nature. It has 3 valency. It does not occur freely in environment.
Mendeleev grouped thallium in the same group as lithium and sodium because of he found that as Li, Na etc. have compound with one atom of chlorine(Cl), Thallium(Tl) also forms TlCl. Therefore he placed Tl with these elements.
Thallium is different from the other five elements
Tl has 3 valency whereas lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) has only 1 valency.
