Which one of the following elements or isotopes is not radioactive?

Introduction / Context:
This question tests your basic knowledge of radioactivity and the difference between radioactive and stable elements or isotopes. Several elements near the bottom of the periodic table are highly unstable and radioactive, whereas many lighter elements have at least one stable isotope. You are asked to choose which of the given species is not radioactive.

Given Data / Assumptions:

• Astatine and francium are heavy elements in the periodic table.
• Tritium is an isotope of hydrogen.
• Zirconium is a transition metal.
• The question expects you to know which among these do not undergo spontaneous radioactive decay under normal conditions.

Concept / Approach:
Radioactive substances have unstable nuclei that spontaneously emit particles or radiation. Astatine and francium are both very heavy, highly unstable elements with no stable isotopes; all of their isotopes are radioactive. Tritium is a radioactive isotope of hydrogen with one proton and two neutrons, used in luminous paints and nuclear fusion research. Zirconium, however, is a transition metal with several stable isotopes, such as zirconium 90 and zirconium 91. Ordinary zirconium metal used in industry is not radioactive. Therefore, zirconium is the only species in the list that is not naturally radioactive.

Step-by-Step Solution:
Step 1: Consider astatine. It is a halogen in group 17 and is very heavy. All its known isotopes are radioactive and it occurs only in trace amounts in nature. Step 2: Consider francium. It is an alkali metal heavier than caesium. All isotopes of francium are also highly radioactive with very short half lives. Step 3: Consider tritium. It is an isotope of hydrogen with atomic number 1 but mass number 3, consisting of one proton and two neutrons, and it is well known to be radioactive. Step 4: Consider zirconium. It is a transition metal used in alloys and nuclear reactors for cladding fuel rods, and it has several stable isotopes. Step 5: Since zirconium has stable forms and is not inherently radioactive, it is the only non radioactive choice among the options.

Verification / Alternative check:
Textbooks of nuclear chemistry and general chemistry often list astatine and francium among elements with no stable isotopes, meaning that every isotope of these elements is radioactive. Tritium is specifically mentioned as a radioactive hydrogen isotope with a half life of about twelve years. Zirconium, however, is widely used in industry and nuclear technology specifically because of its corrosion resistance and low neutron absorption cross section; its normal isotopic composition is stable. If it were strongly radioactive, it would not be used so commonly in structural applications. This confirms that zirconium is not radioactive in its common forms.

Why Other Options Are Wrong:
Astatine is always radioactive and decays quickly, making it extremely rare in nature. Francium is another element with only radioactive isotopes, and it decays into other elements rapidly. Tritium emits beta particles and is widely used as a radioactive tracer and in self luminous devices. These three options therefore all describe radioactive species. Zirconium, in contrast, has stable isotopes and is not radioactive under normal conditions, so the other options cannot be correct answers to the question asked.

Common Pitfalls:
Some students may think that any element associated with nuclear reactors, like zirconium, must be radioactive, but in reality zirconium is chosen precisely because it is non radioactive and stable. Another source of confusion is between tritium and deuterium; deuterium is a stable isotope of hydrogen, while tritium is radioactive. The general rule that very heavy elements near the bottom of the periodic table are often radioactive also helps you remember that astatine and francium do not have stable isotopes.

Final Answer:
Among the given options, the element that is not radioactive is zirconium.