How many protons are present in an atom of strontium-90?

• A. 38
• B. 90
• C. 28
• D. 52

Answer: (A)

Verified step by step guidance

1. Identify the element in question, which is strontium (Sr). Each element is defined by its atomic number, which is the number of protons in its nucleus.
2. Recall that the atomic number of strontium (Sr) is 38. This means every atom of strontium has 38 protons regardless of its isotope.
3. Understand that the number 90 in strontium-90 represents the mass number, which is the sum of protons and neutrons in the nucleus.
4. Since the question asks for the number of protons, focus on the atomic number, not the mass number.
5. Conclude that the number of protons in an atom of strontium-90 is equal to the atomic number of strontium, which is 38.