An atom of phosphorus has an atomic number of 15 and a mass number of 31. How many neutrons does it contain?

The number of neutrons in an atom can be found by subtracting the atomic number from the mass number. In this case, the atomic number of phosphorus is 15 and the mass number is 31.

Step 1: Identify the atomic number. For phosphorus, it is 15. This number represents the number of protons in the atom.

Step 2: Identify the mass number. For phosphorus, it is 31. This number represents the total number of protons and neutrons in the atom.

Step 3: Subtract the atomic number from the mass number to find the number of neutrons.

So, 31 (mass number) - 15 (atomic number) = 16.

Therefore, a phosphorus atom contains 16 neutrons.