Relative atomic mass (Aᵣ)

When explaining a pair reversal, link the ideas explicitly: relative atomic mass is a weighted mean affected by isotope abundance, so strict mass order can conflict with the order supported by chemical properties.

Do not confuse mass number with relative atomic mass. A mass number belongs to one isotope and is a whole number; relative atomic mass is a weighted mean for an element and is often not a whole number.

For tellurium and iodine, state the direction correctly: tellurium has the greater relative atomic mass, but Mendeleev placed tellurium before iodine because that kept each element with others having similar properties.