An isotope has an atomic number of 15 and a mass number of 31. The number of protons it contain 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 fundamental definitions of atomic structure:

1. Atomic Number ($Z$): This represents the number of protons in the nucleus of an atom. It is the identity of the element (e.g., Phosphorus has an atomic number of 15).
2. Mass Number ($A$): This is the sum of the number of protons and the number of 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 values from the question. • 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 at the atomic number provided.

Conclusion

The number of protons is 15. Looking at the options provided, this corresponds to option B.

Exam Tip

Always remember the acronym "APE":

• Atomic number = Protons = Electrons (in a neutral atom). If you are ever confused, just look for the atomic number—it is the "ID card" of the element and tells you the proton count immediately!