An atom of phosphorus has an atomic number of 15 and a mass number of 31. How many neutrons does it contain?
The correct answer and explanation is:
Correct Answer: 16 neutrons
To find the number of neutrons in an atom, you subtract the atomic number from the mass number. The atomic number of an atom represents the number of protons it contains. In a neutral atom, the number of protons is equal to the number of electrons. The mass number is the total number of protons and neutrons combined in the nucleus of the atom.
For phosphorus:
- Atomic number = 15
- Mass number = 31
Using the formula:
Number of neutrons = Mass number − Atomic number
Number of neutrons = 31 − 15 = 16
So, a phosphorus atom contains 16 neutrons.
Phosphorus is a non-metal element located in group 15 of the periodic table. Its atomic number of 15 tells us it has 15 protons in its nucleus. In a stable and electrically neutral atom, it also has 15 electrons orbiting the nucleus. These protons and neutrons reside in the dense central region of the atom called the nucleus, while the electrons move around in the electron cloud.
The mass number is not found directly on the periodic table because elements can exist as different isotopes. An isotope is an atom with the same number of protons but a different number of neutrons. The given mass number of 31 for phosphorus refers to its most common isotope, phosphorus-31. This isotope is stable and naturally abundant.
Knowing the number of neutrons is important because neutrons contribute to the stability of the nucleus. If the number of neutrons is too high or too low compared to the number of protons, the atom may become unstable or radioactive. In this case, 16 neutrons provide the right balance for phosphorus-31 to be a stable and naturally occurring isotope.