Argon, atomic number 18, has an atomic weight of 39.9. Potassium, atomic number 19, has an atomic weight of 39.1. Which of these statements best explains why potassium has a lower atomic weight than argon? A.
Potassium is reactive, while argon is typically inert.

B.
Potassium atoms lose electrons more easily than argon atoms.

C.
Potassium is typically a solid, while argon is typically a gas.

D.
Potassium atoms usually have fewer neutrons than argon atoms.

D

Explanation:

Atomic weight is measured by adding the number of protons and neutrons in an atom's nucleus. Argon's atomic number is 18 while potassium's is 19. This means that Argon will always have 18 protons while potassium will always have 19 protons.

To make the numbers easier to work with, round each atomic weight. We'll say the atomic weight of potassium is 39 and the atomic weight of argon is 40. To see how many neutrons each one has, I can set up a simple equation for each using the following equation:

Atomic weight = protons + neutrons

Potassium:

39 = 19 + N  -->  N = 20

Argon:

40 = 18 + N  -->  N = 22

An atom is defined by the number of protons it has, but the number of neutrons can vary. We call these isotopes, or atoms with the same number of protons but a different number of neutrons. As the math shows, argon typically has more neutrons per atom than potassium does.