Final answer:
The modern Periodic Table arranges elements by atomic number, not atomic mass, due to elements' properties correlating more closely with atomic number. Elements such as technetium and promethium have their atomic mass in brackets, indicating they consist of radioactive isotopes with no stable average atomic mass.
Step-by-step explanation:
The modern Periodic Table does not arrange some elements in order of increasing atomic mass due to the discovery that the properties of elements are better predicted by their atomic numbers. For instance, while tellurium has a greater atomic mass than iodine, it has a lower atomic number (52 compared to iodine’s 53), which places tellurium before iodine in the periodic table according to the modern periodic law. This law states that the properties of elements are periodic functions of their atomic numbers, leading to the current arrangement of the periodic table where elements with similar chemical and physical properties are grouped together in columns known as groups.
Additionally, for some elements like technetium and promethium, and all elements with atomic numbers 84 and higher, the atomic mass is given in square brackets to denote that these consist entirely of unstable, radioactive isotopes. This indicates the mass number of the most stable isotope, rather than an average atomic weight, which cannot be determined due to the significant variation in isotope abundance or because some isotopes do not exist in nature.