Which one of the following statements for D.I. Mendeleeff, is incorrect?

Let us examine each statement about Mendeleev.

Statement (1): He authored the textbook "Principles of Chemistry" — this is correct. Mendeleev did write this famous textbook in which he developed the periodic law.

Statement (2): "At the time he proposed the Periodic Table, the structure of atom was known" — this is incorrect. Mendeleev proposed his Periodic Table in 1869. The internal structure of the atom (discovery of the electron by J.J. Thomson) was not established until 1897, and Rutherford's nuclear model came in 1911. Therefore, atomic structure was completely unknown when Mendeleev proposed his table.

Statement (3): Element with atomic number 101 is named after him (Mendelevium) — this is correct.

Statement (4): He invented the accurate barometer — this is incorrect as a factual matter (the barometer was invented by Torricelli), but this is a distractor. The question asks for a statement about Mendeleev that is "incorrect," and statement (2) is the most clearly incorrect claim.

The incorrect statement is option (2): atomic structure was not known at the time Mendeleev proposed the Periodic Table.