Which of the following isotopes is the heaviest?
This question is asking us to compare the masses of different isotopes. Isotopes are versions of an element with the same number of protons but a different number of neutrons. And by heaviest we mean the one that has the largest mass number.
In order to parse the possible answers to this question, we need to understand nuclide notation. Nuclide notation is a way of writing an isotope that includes some valuable information. We include the chemical symbol of the element. To the bottom left, we include the atomic number of that element. And in the top left, we include the mass number of the specific isotope we’re dealing with.
So if we take a look at the nuclide notation for choice (A), we can see that the K indicates that the element is potassium. The 19 indicates an atomic number of 19, which also means there are 19 protons in potassium atoms. The mass number is 39. And if we know that there are 19 protons and that the mass number is the number of protons and the number of neutrons added together, it follows that there are 20 neutrons in this isotope of potassium. Since isotopes are named with their element and mass number, we can call this isotope of potassium potassium-39.
This question is asking which isotope is the heaviest. The heaviest isotope will have the highest mass number. So the number we wanna look at in each of the choices is the number in the top left, the mass number. Of the five choices, choice (E) argon-44 has the highest mass number, 44. Choice (E) is the correct answer.
The atomic number of argon-44, 18 to the bottom left, is lower than the atomic number of some of the other choices. However, the atomic number simply refers to the number of protons. While this does contribute to the mass number, the increased number of neutrons in argon-44 more than makes up for the lower number of protons, resulting in a larger mass number and the heaviest isotope of the five.
Note that the information you see in nuclide notation is similar to yet different from the information you’ll see on the periodic table. In both cases, you will find the atomic number to the bottom left of the chemical symbol in nuclide notation and above the chemical symbol on the periodic table. The mass number that we see in the top left of nuclide notation refers to the number of protons and neutrons for that specific isotope. Different isotopes will have different mass numbers.
The number on the periodic table that has to do with mass but is not the mass number is the atomic mass. This number, often found below the chemical symbol on the periodic table, is the average mass of an atom of that element in a sample. For example, the atomic mass of argon is 39.95. This is very close to 40 because in a sample of argon atoms, the vast majority of them are argon-40 atoms.
There’s also a small abundance of argon isotopes with a mass number lower than 40 that lowers the atomic mass or the average mass of an atom in a sample a little bit below 40. Argon-40, listed in choice (C), is by far the most common isotope of argon, with over 99 percent of argon atoms being argon-40. Choice (E) argon-44 is both extremely unstable and extremely rare. Among these isotopes, argon-44, choice (E), is the heaviest.