Atomic Mass

Atomic Mass

High School

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Learning Objectives

• Define the subatomic particles of an atom: protons, neutrons, and electrons.

• Differentiate between atomic number, mass number, and average atomic mass.

• Explain what isotopes are and how they relate to an element's average atomic mass.

• Calculate the average atomic mass of an element using the mass and natural abundance of its isotopes.

Key Vocabulary

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Atom

The smallest unit of matter that retains the identity and properties of an element.

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Isotope

Atoms of the same element with different numbers of neutrons and thus different mass numbers.

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Atomic Number

The number of protons in an atom's nucleus, which serves to identify the element.

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Mass Number

The total count of protons and neutrons within a single atom's nucleus.

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Atomic Mass Unit (amu)

A unit of mass equal to exactly one-twelfth the mass of a carbon-12 atom.

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Average Atomic Mass

The weighted average mass of all naturally occurring isotopes of a specific element.

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The Structure of the Atom

• The atom is the fundamental building block of all matter.

• Its center, the nucleus, has positive protons and neutral neutrons.

• Protons and neutrons are made of smaller particles called quarks.

• Tiny, negative electrons orbit the nucleus in a cloud.

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Solved Example 1
Naturally occurring boron is 19.9% Boron-10 (mass 10.013 amu) and 80.1% Boron-11 (mass 11.009 amu). What is the average atomic mass of boron?

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Step 1: Analyze and Sketch the Problem

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Solved Example 1
Naturally occurring boron is 19.9% Boron-10 (mass 10.013 amu) and 80.1% Boron-11 (mass 11.009 amu). 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
Naturally occurring boron is 19.9% Boron-10 (mass 10.013 amu) and 80.1% Boron-11 (mass 11.009 amu). What is the average atomic mass of boron?

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Step 3: Evaluate the Answer

• The calculated average atomic mass, 10.81 amu, correctly falls between the masses of the lighter and heavier isotopes (10.013 amu and 11.009 amu).

• The average is closer to the mass of Boron-11, which is logical because it is the more abundant isotope (80.1%).

Multiple Choice

Which combination of subatomic particles is found in the nucleus and accounts for almost all of an atom's mass?

• Protons and electrons

• Only electrons

• Protons and neutrons

• Neutrons and electrons

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Atomic Number and Mass Number

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Solved Example 2
A specific atom of gold has an atomic number of 79 and a mass number of 197. How many neutrons are in the nucleus of this gold atom?

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Step 1: Analyze and Sketch the Problem

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Solved Example 2
A specific atom of gold has an atomic number of 79 and a mass number of 197. How many neutrons are in the nucleus of this gold atom?

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Step 2: Solve for the Unknown

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Solved Example 2
A specific atom of gold has an atomic number of 79 and a mass number of 197. How many neutrons are in the nucleus of this gold atom?

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Step 3: Evaluate the Answer

Multiple Choice

An atom has an atomic number of 17 and a mass number of 35. How many protons and neutrons does it contain?

• 17 protons and 18 neutrons

• 18 protons and 17 neutrons

• 17 protons and 35 neutrons

• 35 protons and 17 neutrons

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Isotopes and Their Symbols

• Isotopes are atoms of the same element with different numbers of neutrons.

• Their chemical properties are identical because they have the same number of protons.

• Different neutron numbers mean isotopes have different mass numbers and atomic masses.

• Symbols show mass number (top) and atomic number (bottom) like ⁶³₂₉Cu.

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Solved Example 3
The symbol for an isotope of titanium is ⁴⁸₂₂Ti. How many protons, neutrons, and electrons are in a neutral atom of this isotope?

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Step 1: Analyze and Sketch the Problem

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Solved Example 3
The symbol for an isotope of titanium is ⁴⁸₂₂Ti. How many protons, neutrons, and electrons are in a neutral atom of this isotope?

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Step 2: Solve for the Unknown

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Solved Example 3
The symbol for an isotope of titanium is ⁴⁸₂₂Ti. How many protons, neutrons, and electrons are in a neutral atom of this isotope?

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Step 3: Evaluate the Answer

Multiple Choice

What is the defining difference between two isotopes of the same element, such as Carbon-12 and Carbon-14?

• They exhibit different chemical properties.

• They have a different number of neutrons.

• They have a different number of electrons.

• They have a different number of protons.

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What Is an Atomic Mass Unit (amu)?

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Multiple Choice

What is the standard used to define the atomic mass unit (amu)?

• The mass of a single hydrogen-1 atom

• One-sixteenth the mass of a single oxygen-16 atom

• The mass of a single proton

• One-twelfth the mass of a single carbon-12 atom

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Calculating Average Atomic Mass

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Multiple Choice

If an element's most abundant isotope has a mass of 35 amu, but the element's average atomic mass on the periodic table is 35.45 amu, what does this indicate?

• The element must have at least one other isotope with a mass less than 35 amu.

• The element must have at least one other isotope with a mass greater than 35 amu.

• The element has only one isotope.

• A calculation error must have occurred.

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Common Misconceptions about Atomic Structure

Misconception	Correction
Mass number and atomic mass are the same thing.	Mass number is for one isotope. Atomic mass is an element's average.
All atoms of the same element are identical.	Isotopes are atoms of an element with different numbers of neutrons.
An element's atomic number changes between its isotopes.	The atomic number (number of protons) is always constant for an element.

Multiple Choice

Titanium (atomic number 22) has an isotope, Titanium-48. Based on this information, how many protons and neutrons does this isotope have?

• 22 protons and 48 neutrons

• 26 protons and 22 neutrons

• 48 protons and 22 neutrons

• 22 protons and 26 neutrons

Multiple Choice

Why do isotopes of the same element, such as Copper-63 and Copper-65, share the same chemical reactivity?

• Because their chemical behavior is determined by their mass number, which is similar.

• Because they have the same number of quarks in their nuclei.

• Because their chemical behavior is determined by the number of neutrons, which is the same.

• Because their chemical behavior is determined by their electron configuration, which is the same.

Multiple Choice

The average atomic mass of Boron is 10.81 amu. Boron has two primary isotopes, Boron-10 and Boron-11. What can you infer about the relative abundance of these two isotopes?

• They are found in nearly equal 50/50 abundance.

• Their abundance cannot be inferred from the average atomic mass.

• Boron-10 is significantly more abundant.

• Boron-11 is significantly more abundant.

Multiple Choice

An element consists of two isotopes: 75.0% has a mass of 35.0 amu, and 25.0% has a mass of 37.0 amu. Analyze this data to determine the average atomic mass of the element.

• 36.0 amu

• 35.5 amu

• 35.2 amu

• 36.5 amu

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Summary

Poll

On a scale of 1-4, how confident are you about calculating average atomic mass from isotope data?

• 1

• 2

• 3

• 4