### Video Transcript

Phosphorus-31 has an atomic number of 15. How many neutrons are there in an atom of phosphorus-31?

Atomic number, represented by 𝑍, can be used to distinguish one element from another as each element has its own unique atomic number. When looking at a cell on the periodic table, the atomic number is frequently written above the element symbol. The atomic number also represents the number of protons in the nucleus of all atoms or ions of a particular element. We are told that phosphorus-31 has an atomic number of 15. This means that an atom of phosphorus-31 will have 15 protons.

The question asked us how many neutrons there are in an atom of phosphorus-31. The name phosphorus-31 provides useful information for answering this question. 31 is the mass number of a phosphorus-31 atom. Mass number, represented by a capital 𝐴, indicates the number of nucleons in an atom or ion. Nucleons collectively refer to protons and neutrons. So the mass number can be expressed as the sum of the number of protons and the number of neutrons.

We can use this relationship to determine the number of neutrons in an atom of phosphorus-31. We can substitute the mass number and number of protons into the equation. Then, we can subtract 15 from both sides of the equation to determine that the number of neutrons is 16. Therefore, the number of neutrons in an atom of phosphorus-31 is 16.