Mendeleev arranged the elements into a periodic table. Explain two reasons why Mendeleev's arrangement of the elements was not fully accepted by other scientists at the time.

Mendeleev’s table contained empty slots where elements had not yet been discovered, which made other scientists sceptical because there was no experimental evidence for those elements.

He also rearranged elements that did not follow the strict order of increasing atomic weight so that chemically similar elements were grouped together. This contradicted the prevailing belief that atomic weight should determine the order of the table.

The use of atomic weight as the ordering principle was problematic: different isotopes give different masses, so atomic weight is not a reliable criterion for arranging elements.

Because of these gaps and the unconventional ordering, many contemporaries rejected Mendeleev’s arrangement as speculative and unscientific.