There were times when Mendeleev didn't completely follow his rules when placing elements in his periodic table. For example, he switch iodine and tellurium on the periodic table, in spite of the fact that tellurium has a larger atomic mass than iodine. Why did he do this?

The properties of iodine more closely matched the properties of elements like fluorine, chlorine, and bromine.

There are always exceptions to rules, and he made this exception because he could.

He had no good reason to make this switch.

The properties of tellurium more closely matched the properties of elements like fluorine, chlorine, and bromine.

There were times when Mendeleev didn't completely follow his rules when placing elements in his periodic table. For example, according to his rules, arsenic should come immediately after zinc. However, he put arsenic in the same column as nitrogen and phosphorus because its properties more closely matched these elements. As a result, he put two blank spaces in his periodic table Why did he do this?

He realized that there were two elements that had not been discovered yet.

So his periodic table would be easier to publish.

He had no good reason to put these gaps in the periodic table. He just wanted to do it.

Sometimes there are just gaps in the periodic table.

He was able to make predictions about the existence and properties of elements that had not been discovered yet.

His table was prettier to look at than other scientists' ways of organizing elements.

His periodic able was not able to make any predictions about any elements that had not been discovered yet.

He was the OG periodic table guy, so his periodic table had to be good.

This periodic table is difficult to publish.

The elements in yellow don't alloy well with the other elements, resulting in their separation from the rest of the periodic table.

There are gaps in our knowledge of the elements, which is reflected in

The elements in yellow are radioactive, which must be separated from the rest of the elements.