Which of the following is considered the rarest naturally occurring element on Earth?

• A. Promethium
• B. Rhenium
• C. Astatine
• D. Francium

Answer: (C)

Verified step by step guidance

1. Understand that the question asks for the rarest naturally occurring element on Earth, which means the element must be found in nature, not just synthesized in labs.
2. Review the options given: Promethium, Rhenium, Astatine, and Francium, and recall their natural occurrence and abundance.
3. Recall that Promethium is mostly synthetic and not found naturally in significant amounts, so it is not considered naturally occurring in measurable quantities.
4. Recognize that Rhenium is rare but is found naturally in the Earth's crust in small amounts.
5. Know that Astatine and Francium are both extremely rare and radioactive elements, but Astatine is considered the rarest naturally occurring element because it is produced in trace amounts by the decay of heavier elements and has a very short half-life, making it almost impossible to accumulate.