How many neutrons does an atom of phosphorus-31 have? (The symbol for phosphorus is P.)

• A. 31
• B. 16
• C. 14
• D. 15

Answer: (D)

Verified step by step guidance

1. Identify the atomic number of phosphorus (P), which is the number of protons in the nucleus. For phosphorus, the atomic number is 15.
2. Understand that the mass number (given as 31 in phosphorus-31) represents the total number of protons and neutrons in the nucleus.
3. Use the formula to find the number of neutrons: \(\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}\).
4. Substitute the known values into the formula: \(\text{Number of neutrons} = 31 - 15\).
5. Calculate the difference to find the number of neutrons in phosphorus-31.