Isotopes and Average Atomic Mass

Isotopes and Average Atomic Mass

High School

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Learning Objectives

• Define isotopes and explain how they differ from one another.

• Differentiate between the atomic number and mass number of an element.

• Calculate the average atomic mass of an element from its isotopes.

• Determine an element's most abundant isotope from its average atomic mass.

Key Vocabulary

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Isotopes

Atoms of an element with a different number of neutrons and therefore a different mass number.

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Atomic Number

The number of protons in an atom's nucleus, which uniquely identifies that specific element.

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Mass Number

The total combined number of protons and neutrons found in the nucleus of an atom.

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Average Atomic Mass

The weighted average mass of all naturally occurring isotopes of an element, reflecting their abundances.

Understanding Isotopes

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Carbon-12 (¹²C)

• ​Contains 6 protons and 6 neutrons in its atomic nucleus.

• ​​Its mass number is 12, the sum of protons and neutrons.

• ​This isotope is stable and makes up most of Earth's carbon.

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Carbon-13 (¹³C)

• ​Has 6 protons and 7 neutrons, one more than carbon-12.

• ​​The mass number is 13, reflecting the extra neutron in it.

• ​Carbon-13 is also a stable isotope but is much less common.

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Carbon-14 (¹⁴C)

• ​Contains 6 protons but has 8 neutrons in its atomic nucleus.

• ​​It has a mass number of 14, the highest of these three.

• ​This isotope is unstable and radioactive, used in carbon dating techniques.

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Solved Example 1
Boron has two isotopes: Boron-10 (mass 10.01 u, abundance 19.9%) and Boron-11 (mass 11.01 u, abundance 80.1%). What is the average atomic mass of Boron?

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Step 1: Analyze and Sketch the Problem

• Goal: Calculate the average atomic mass of boron.

• Knowns: Boron-10 mass = 10.01 u, abundance = 19.9%; Boron-11 mass = 11.01 u, abundance = 80.1%.

• Unknown: Average atomic mass.

• Formula: Avg. Atomic Mass = (Mass₁ × %₁/100) + (Mass₂ × %₂/100)

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Solved Example 1
Boron has two isotopes: Boron-10 (mass 10.01 u, abundance 19.9%) and Boron-11 (mass 11.01 u, abundance 80.1%). What is the average atomic mass of Boron?

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Step 2: Solve for the Unknown

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Solved Example 1
Boron has two isotopes: Boron-10 (mass 10.01 u, abundance 19.9%) and Boron-11 (mass 11.01 u, abundance 80.1%). What is the average atomic mass of Boron?

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Step 3: Evaluate the Answer

• The calculated average atomic mass is 10.811 u.

• This value is between the masses of the two isotopes (10.01 u and 11.01 u).

• Since Boron-11 is more abundant, the average mass is closer to 11.01 u, which confirms the answer is reasonable.

Multiple Choice

Which statement correctly describes isotopes of an element?

• They have the same number of protons but a different number of neutrons.

• They have the same number of neutrons but a different number of protons.

• They have a different number of protons and a different number of electrons.

• They have the same mass number but a different atomic number.

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Calculating Average Atomic Mass

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Multiple Choice

An element has two isotopes. Isotope A has a mass of 10.0 amu and 20% abundance. Isotope B has a mass of 11.0 amu and 80% abundance. What is the average atomic mass?

• 10.8 amu

• 10.2 amu

• 10.5 amu

• 11.0 amu

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Common Misconceptions

Misconception	Correction
All atoms of an element are identical.	Elements have isotopes with different numbers of neutrons and masses.
The mass number and average atomic mass are the same thing.	Mass number is for one atom; atomic mass is an average.
Changing the number of neutrons changes the element.	An element’s identity is determined by its number of protons.

Multiple Choice

An isotope of uranium, U-238, has an atomic number of 92. How many protons and neutrons does it have?

• 146 protons and 92 neutrons

• 92 protons and 146 neutrons

• 238 protons and 92 neutrons

• 92 protons and 238 neutrons

Multiple Choice

The average atomic mass of chlorine is 35.45 amu. The two main isotopes are Chlorine-35 and Chlorine-37. What can be inferred about their abundance?

• Chlorine-37 is more abundant.

• Their abundance cannot be determined.

• They have equal abundance.

• Chlorine-35 is more abundant.

Multiple Choice

Element X has two isotopes: X-63 (mass 62.93 amu, 69.17% abundance) and X-65 (mass 64.93 amu, 30.83% abundance). Predict the approximate average atomic mass of Element X.

• It will be exactly 64 amu.

• It will be closer to 65 amu than to 63 amu.

• It will be the sum of the two masses.

• It will be closer to 63 amu than to 65 amu.

Multiple Choice

If a newly discovered, stable isotope of Carbon (atomic number 6) was found to have a mass number of 15, what would be the primary effect on the average atomic mass of carbon?

• The average atomic mass would increase.

• The average atomic mass would not change.

• The average atomic mass would decrease.

• The atomic number of carbon would change.

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Summary

• Isotopes are versions of an element with different numbers of neutrons.

• The average atomic mass is a weighted average of an element's isotopes.

• The most abundant isotope's mass is closest to the average atomic mass.

• Unstable isotopes can be radioactive, which means they emit radiation.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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