Average Atomic Mass: Definition and Examples

📚 Quick Study Guide

• ⚛️ Average atomic mass is the weighted average of the masses of all the isotopes of an element.
• 🔢 It's calculated by multiplying the mass of each isotope by its natural abundance (expressed as a decimal) and then summing these products.
• ⚖️ The formula is: Average Atomic Mass = $ (Mass_{isotope1} \times Abundance_{isotope1}) + (Mass_{isotope2} \times Abundance_{isotope2}) + ... $
• 🧪 Atomic mass is usually expressed in atomic mass units (amu).
• 📈 Natural abundance is the percentage of each isotope that occurs naturally on Earth.
• 💡 Remember to convert percentages to decimals before using them in the calculation (e.g., 75% = 0.75).
• 📝 The average atomic mass is the number shown below the element's symbol on the periodic table.

🧪 Practice Quiz

1. What is the definition of average atomic mass?
   1. A) The mass of the most abundant isotope of an element.
   2. B) The sum of the masses of all isotopes of an element.
   3. C) The weighted average of the masses of all the isotopes of an element.
   4. D) The mass of a single atom of an element.
2. Chlorine has two isotopes: Chlorine-35 (34.969 amu, 75.77% abundance) and Chlorine-37 (36.966 amu, 24.23% abundance). What is the average atomic mass of chlorine?
   1. A) 35.00 amu
   2. B) 36.00 amu
   3. C) 35.45 amu
   4. D) 36.97 amu
3. Which of the following is necessary to calculate the average atomic mass of an element?
   1. A) The number of neutrons in each isotope.
   2. B) The mass and natural abundance of each isotope.
   3. C) The number of electrons in each isotope.
   4. D) The density of each isotope.
4. Magnesium has three isotopes: Mg-24 (23.985 amu, 78.99%), Mg-25 (24.986 amu, 10.00%), and Mg-26 (25.983 amu, 11.01%). Calculate the average atomic mass of magnesium.
   1. A) 24.00 amu
   2. B) 24.31 amu
   3. C) 25.00 amu
   4. D) 26.00 amu
5. What unit is typically used to express average atomic mass?
   1. A) Grams (g)
   2. B) Moles (mol)
   3. C) Atomic mass units (amu)
   4. D) Kilograms (kg)
6. Boron has two naturally occurring isotopes: Boron-10 (10.013 amu) and Boron-11 (11.009 amu). If the average atomic mass of boron is 10.81 amu, what is the approximate natural abundance of Boron-10?
   1. A) 20%
   2. B) 50%
   3. C) 80%
   4. D) 10%
7. What does 'natural abundance' refer to in the context of average atomic mass?
   1. A) The number of isotopes of an element.
   2. B) The percentage of each isotope found naturally on Earth.
   3. C) The rate at which an isotope decays.
   4. D) The mass of the most common isotope.