An isotope has an atomic number of 15 and a mass number of 31. The number of proton it contains is

Analysis

The question asks for the number of protons in an atom given its atomic number and mass number.

Concept Explanation

To solve this, you need to understand the basic structure of an atom:

1. Atomic Number ($Z$): This represents the number of protons in the nucleus of an atom. It is the unique identifier for every element (e.g., Phosphorus has an atomic number of 15).
2. Mass Number ($A$): This is the sum of the number of protons and neutrons in the nucleus ($A = \text{protons} + \text{neutrons}$).
3. Neutral Atom: In a neutral atom, the number of protons is equal to the number of electrons.

Step-by-Step Solution

• Step 1: Identify the given information. • Atomic number ($Z$) = 15 • Mass number ($A$) = 31
• Step 2: Apply the definition of atomic number. • By definition, the atomic number is exactly equal to the number of protons. • Therefore, Number of protons = Atomic number = 15.
• Step 3: (Optional) Calculate neutrons for extra practice. • Neutrons = Mass number - Atomic number • Neutrons = 31 - 15 = 16.

Since the question specifically asks for the number of protons, we look directly at the atomic number provided.

Conclusion

The number of protons is 15. This matches option B.

Exam Tip

Always remember the acronym "APE": Atomic number = Protons = Electrons (in a neutral atom). If you ever get confused, just remember that the atomic number is the "identity card" of the element, and it always tells you the proton count!