Final answer:
Mendeleev organized elements into a periodic table based on atomic weight, finding that elements in the same group exhibited similar chemical properties. He sometimes listed elements out of atomic mass order to maintain grouping by similar properties, as seen with tellurium and iodine. This organization reveals patterns in several properties, including atomic size and metallic character.
Step-by-step explanation:
Dmitri Mendeleev noticed that if he organized the elements by atomic weight, certain patterns emerged. For instance, elements in the same group or family appeared to have the same chemical properties.
Mendeleev, while writing a chemistry textbook, found that placing elements in order of increasing atomic mass demonstrated periodic similarities in chemical behavior at regular intervals.
These similarities are because elements in a group have the same number and distribution of electrons in their valence shells.
Mendeleev adjusted some elements out of atomic mass order to align them with other elements sharing similar properties. For example, even though tellurium has a higher atomic mass compared to iodine, Mendeleev placed iodine with elements like fluorine, chlorine, and bromine due to their similar properties, suggesting a pattern governed by more than just atomic mass.
The periodic table is a result of such organization, reflecting the similarities and trends in element properties, like atomic size, ionization energy, and electron affinity, as well as changes in metallic character within a group.