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.

When observing how thallium reacts with the air of the earth's atmosphere, its hardness or resistance resembled sodium, it was not investigated further to classify it correctly

Explanation:

Now it is known that they contain different numbers of valence electrons and that thallium is a heavy metal like lead and that they have similar characteristics except for their melting point where thallium is higher.

throwdolbeau throwdolbeau · Answer 2 · 2021-12-23T12:38:32+01:00

Mendeleev might have grouped thallium in the same group as lithium and sodium due to many reasons:

Since, all are metals so they need to be placed in the same group.
The particular group is referred to (group one) is actually the most reactive metals on the periodic table. These are the elements that most easily loose their electron from their outer valence shell. Thus each element can easily give up their valence electron in a reaction to form a positive ion. These are the most easily reactive in this way.
When observing how thallium reacts with the air of the earth's atmosphere, its hardness or resistance resembled sodium, it was not investigated further to classify it correctly.

Now it is known that they contain different numbers of valence electrons and that thallium is a heavy metal like lead and that they have similar characteristics except for their melting point where thallium is higher.

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