MCAT Mnemonics: Atomic Mass vs Atomic Weight

Ken Tao is the MedSchoolCoach expert on MCAT, and will discuss this general chemistry topic, and show you how atomic mass is the mass of an atom, whereas atomic weight is the weighted average of the naturally occurring isotopes.

Full Transcription

Welcome back to another MCAT Mnemonic Monday. I’m Ken, and I’m an expert with MedSchoolCoach. Today’s mnemonic is on an MCAT general chemistry topic: atomic mass versus atomic weight.

These two terms are often confused by students. We often use them interchangeably, but they’re actually different. Atomic mass is the mass of an atom. That’s a good mnemonic for memorizing it. Mass and mass.

So as an example, Chlorine 35 has an atomic mass of 35 AMU or atomic mass units. Chlorine 37 has an atomic mass of 37 AMU. These two atoms have different masses because they are different atoms.

Atomic weight is the weighted average of the naturally occurring isotopes. Again, the mnemonic you can see: weight goes with weighted.

Chlorine has an atomic weight of 35.5 AMU. And the reason why is because 75% of chlorine in the world is Chlorine 35 and 25% of chlorine in the world is Chlorine 37. So since we have more Chlorine 35 than Chlorine 37, the weighted average is certainly closer to 35 than 37,So that’s how we end up with 35.5 as the atomic weight of chlorine.

So again, the mnemonic for memorizing the difference between atomic mass and atomic weight is: atomic mass is the mass of an atom, whereas atomic weight is the weighted average of the naturally occurring isotopes.