Which elements are considered radioactive?

The correct answer identifies that all elements larger than atomic number 84 are considered radioactive. This stems from the stability of atomic nuclei; elements on the periodic table that have atomic numbers greater than 84 tend to have unstable isotopes, meaning they undergo radioactive decay.

Elements with atomic numbers above 84 include polonium, astatine, radon, and francium, all of which are known to emit radiation as they decay into more stable forms. This concept is grounded in the principles of nuclear physics, where elements with heavier nuclei face a higher likelihood of instability due to the imbalance of protons and neutrons.

While other answer choices suggest various thresholds or types of elements, they do not capture the comprehensive nature of radioactivity as effectively. For instance, elements with atomic numbers over 74 or over 100 include both stable and unstable isotopes, and focusing on only synthetic elements excludes natural radioactive isotopes, which are also present among elements larger than atomic number 84. Thus, the classification of all elements larger than 84 as radioactive is the most accurate representation of their behavior in terms of nuclear stability and decay.