Worked example: using the mass number equation | High school chemistry | Khan Academy

Hi everyone. In this video, we're going to practice using the mass number equation. This equation represents the fact that the mass number of an atom is equal to its number of protons plus its number of neutrons.

Let's use the mass number equation to answer the following question: How many neutrons are in an isotope of sodium with the following notation? To answer this question, we'll first need to rearrange our mass number equation to solve for the number of neutrons. Pause the video and take a moment to try this for yourself.

The rearranged equation is as follows: The number of neutrons equals the mass number minus the number of protons. Based on this notation, we know that this sodium isotope has a mass number of 23 and an atomic number of 11. If we subtract the atomic number 11 from the mass number 23, we're left with 12, which is the number of neutrons in this sodium isotope.

Okay, more practice! This time with an isotope of platinum: How many neutrons does this isotope have based on the following notation: PT-195? Again, we'll need to use the rearranged mass number equation to solve for the number of neutrons. This particular notation provides the chemical symbol of platinum and its mass number.

We can look up platinum on the periodic table to find its atomic number, which is 78, meaning that every platinum atom has 78 protons. We can subtract the atomic number 78 from the mass number 195 to get 117. So this isotope of platinum has 117 neutrons.