How many protons are present in an isotope of sodium with a mass number of 25?

• A. 11
• B. 25
• C. 14
• D. 12

Answer: (A)

Verified step by step guidance

1. Recall that the number of protons in an element is equal to its atomic number, which is unique for each element.
2. Identify the element in question: sodium (Na). Sodium has an atomic number of 11, meaning it has 11 protons.
3. Understand that the mass number (given as 25) is the sum of protons and neutrons in the nucleus of the isotope.
4. Since the question asks for the number of protons, focus on the atomic number rather than the mass number.
5. Conclude that the isotope of sodium with a mass number of 25 still has 11 protons, as the number of protons does not change between isotopes.