Question

what radioactive element has the lowest atomic number? tc how many of the elements are radioactive? where do the radioactive elements tend to be located on the table?

Explanation:

First Question: What radioactive element has the lowest atomic number?

Hydrogen has some radioactive isotopes, but the first radioactive element with a relatively low atomic number is Technetium (Tc) with atomic number 43, and also Promethium (Pm) with atomic number 61. However, the element with the lowest atomic number that is radioactive (with no stable isotopes) is Technetium (Tc, atomic number 43) or sometimes considered Hydrogen - 3 (tritium) which is a radioactive isotope of Hydrogen (atomic number 1). But among elements (not isotopes), Technetium (Tc) has atomic number 43 and is the first element with no stable isotopes.

There are 38 elements that have no stable isotopes (are radioactive in all their isotopic forms), and many more elements have radioactive isotopes. The elements with no stable isotopes are Technetium (Tc), Promethium (Pm), and all elements with atomic number 84 (Polonium) and above. So the number of elements that are radioactive (either all isotopes are radioactive or have radioactive isotopes) is a large number, but the number of elements with no stable isotopes (radioactive in all forms) is 38.

Radioactive elements (especially those with no stable isotopes) tend to be located at the bottom of the periodic table, specifically in the actinide series (atomic numbers 89 - 103) and also elements with atomic number 84 (Polonium) and above (including all elements after Bismuth, atomic number 83, as Bismuth has a very long - lived isotope but is often considered the last "stable" element). Also, some elements in the transition metals like Technetium (Tc, atomic number 43) and Promethium (Pm, atomic number 61) are radioactive. In general, heavier elements (higher atomic number, especially above atomic number 83) are radioactive, and also some lighter elements have radioactive isotopes or are radioactive in all forms.

Answer:

Technetium (Tc) (atomic number 43) or Hydrogen - 3 (tritium, atomic number 1, radioactive isotope). If considering elements (not isotopes) with no stable isotopes, Technetium (Tc) has the lowest atomic number.

Second Question: How many of the elements are radioactive?