Give one drawback of Mendeleev's periodic table.

Mendeleev's periodic table was revolutionary because it classified elements based on atomic mass and helped predict the existence of undiscovered elements. However, it was not free from drawbacks. One major drawback was the incorrect positioning of certain elements due to the dependence on atomic mass instead of atomic number.

For example:

When arranged by atomic mass, iodine (127) was placed before tellurium (128), but iodine has properties similar to halogens (chlorine, bromine, fluorine), while tellurium has properties similar to oxygen and sulfur. To preserve chemical similarities, Mendeleev had to place iodine after tellurium despite its higher atomic mass, which created a confusion.

This happened because Mendeleev did not consider atomic number, which is the true basis of periodic classification discovered later by Moseley. When scientists realized that the properties of elements depend primarily on the number of protons (atomic number) rather than atomic mass, the modern periodic table was created, resolving these issues.

Despite this drawback, Mendeleev's version was extremely important as it laid the framework for modern chemistry. His table also left gaps for elements that were undiscovered at that time, like gallium and germanium, which were later found and matched his predictions.

Therefore, one crucial drawback of Mendeleev's periodic table was the misplacement of certain elements due to dependence on atomic mass, highlighting the need for a more accurate system based on atomic number.