Mass Spectroscopy: Identifying Elements
Mass spectroscopy helps chemists identify the isotopes of an element and determine how common each one is. When you see those decimal atomic masses on the periodic table (like chlorine's 35.45), that's actually an average of all its naturally occurring isotopes!
To calculate the average atomic mass from a mass spectrum, multiply each isotope's mass by its relative abundance, then add all these values together. For example, chlorine has two main isotopes: Cl-35 (77% abundant) and Cl-37 (23% abundant). When you do the math: (35 × 0.77) + (37 × 0.23) = 35.46.
You can also identify elements from their mass spectra. For instance, if you see peaks at masses 39, 40, and 41 with relative abundances of 90%, 2%, and 8%, calculating the average mass gives you 39.18—that's potassium! For isotopes like Germanium-74, you can determine the number of neutrons by subtracting the proton number (32) from the mass number (74), giving 42 neutrons.
Pro Tip: On the AP exam, expect three types of mass spectroscopy questions: estimating atomic mass from a spectrum, identifying elements from their spectrum, or identifying specific isotopes. Practice these calculations to build confidence!
