Which of the following best demonstrates that calcium is unique among all elements?

• A. Calcium is the only element that is a solid at room temperature.
• B. Calcium is the only element found in the Earth's crust.
• C. Calcium has an atomic number of 20, which is unique to it.
• D. Calcium is the only element that forms compounds with oxygen.

Answer: (C)

Verified step by step guidance

1. Understand that each element is defined by its atomic number, which is the number of protons in its nucleus. This atomic number is unique to each element and distinguishes it from all others.
2. Evaluate the given options by considering whether the property mentioned is exclusive to calcium or shared by other elements. For example, many elements are solids at room temperature, so that property is not unique to calcium.
3. Recognize that calcium is not the only element found in the Earth's crust; many elements are present there, so this property is not unique either.
4. Consider the statement about calcium forming compounds with oxygen. Many elements form oxides, so this is also not unique to calcium.
5. Conclude that the statement about calcium having an atomic number of 20 is the best demonstration of its uniqueness, because the atomic number is a fundamental and exclusive identifier of an element.