Atomic Mass and Mass Number

Atomic Mass and Mass Number

High School

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Learning Objectives

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

• Define an isotope and explain how isotopes of an element differ.

• Calculate the number of protons, neutrons, and electrons in an atom or isotope.

• Use the periodic table to determine the subatomic particles of an element.

Key Vocabulary

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Atomic Number

The number of protons in an atom's nucleus, which uniquely identifies an element and its properties.

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Mass Number

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

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Isotopes

Atoms of the same element that have a different number of neutrons, resulting in different mass numbers.

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Atomic Mass Unit (amu)

A standard unit of mass that is used to express the masses of atoms and subatomic particles.

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What is Atomic Mass?

• Most of an atom's mass is concentrated in the nucleus.

• Protons and neutrons are much more massive than electrons.

• Atomic particle masses are measured in atomic mass units (amu).

• Protons and neutrons each have a mass of about 1 amu.

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Solved Example 1
A neutral sodium atom has an atomic number of 11 and a mass number of 23. Calculate the number of protons, neutrons, and electrons in this atom.

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Step 1: Analyze and Sketch the Problem

• Goal: Find the number of protons, neutrons, and electrons.

• Knowns: Atomic Number = 11; Mass Number = 23; The atom is neutral.

• Unknown: Number of protons (p⁺), neutrons (n⁰), and electrons (e^-).

• Formula:
  #p⁺ = Atomic Number
  #n⁰ = Mass Number - #p⁺
  #e^- = #p⁺ (for a neutral atom)

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Solved Example 1
A neutral sodium atom has an atomic number of 11 and a mass number of 23. Calculate the number of protons, neutrons, and electrons in this atom.

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Step 2: Solve for the Unknown

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Solved Example 1
A neutral sodium atom has an atomic number of 11 and a mass number of 23. Calculate the number of protons, neutrons, and electrons in this atom.

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Step 3: Evaluate the Answer

• Check the mass number: 11 protons + 12 neutrons = 23. This matches the given mass number.

• Check the charge: 11 positive protons and 11 negative electrons result in a neutral charge of 0. The answer is reasonable and correct.

Multiple Choice

What components of an atom contribute the most to its overall atomic mass?

• Electrons and neutrons

• Only the electrons

• Protons and electrons

• Protons and neutrons

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What Defines an Element?

• An element’s atomic number is the number of protons in its nucleus.

• Each element has a unique and unchanging atomic number that identifies it.

• For example, carbon’s atomic number is 6 as it has 6 protons.

• In a neutral atom, the number of electrons equals the number of protons.

Multiple Choice

An electrically neutral atom has an atomic number of 13. How many protons and electrons does it have?

• 13 protons and 0 electrons

• 26 protons and 13 electrons

• 13 protons and 13 electrons

• 0 protons and 13 electrons

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What Is Mass Number?

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Multiple Choice

An atom has a mass number of 40 and an atomic number of 18. How many neutrons are in its nucleus?

• 40

• 18

• 22

• 58

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What Are Isotopes?

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

• They have the same number of protons, so they share the same atomic number.

• Different numbers of neutrons result in different mass numbers for each isotope.

• The average atomic mass on the periodic table is a weighted average of isotopes.

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Multiple Choice

Boron has an atomic number of 5. One isotope of Boron has a mass number of 10, and another has a mass number of 11. What is the key difference between these two Boron isotopes?

• They have a different number of electrons.

• They have a different number of protons.

• They have different atomic numbers.

• They have a different number of neutrons.

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Putting It All Together: Calculations

• For a neutral atom, the number of protons equals the atomic number.

• The number of electrons is the same as the number of protons.

• Find the mass number by rounding the atomic mass to a whole number.

• Then, subtract the atomic number from the mass number to find the neutrons.

Multiple Choice

Using the rules provided, how many neutrons would a typical atom of Silicon (Si), with an atomic number of 14 and an average atomic mass of 28.086, have?

• 42

• 15

• 14

• 28

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Common Misconceptions about Atomic Structure

Misconception	Correction
Atomic mass and mass number are the same thing.	Mass number is a count. Atomic mass is a weighted average of isotope masses.
All atoms of the same element are identical.	Isotopes are atoms of an element with different numbers of neutrons and masses.
The number of protons always equals the number of neutrons.	In most atoms, the number of neutrons is greater than the number of protons.

Multiple Choice

An atom has an atomic number of 25 and a mass number of 55. How many protons, neutrons, and electrons does this neutral atom have?

• 25 protons, 55 neutrons, 25 electrons

• 25 protons, 30 neutrons, 30 electrons

• 30 protons, 25 neutrons, 30 electrons

• 25 protons, 30 neutrons, 25 electrons

Multiple Choice

Krypton (Kr) has an atomic number of 36. Its most common isotope is Krypton-84. How do the subatomic particles in Krypton-84 differ from another isotope, Krypton-86?

• Krypton-86 has two more electrons than Krypton-84.

• Krypton-86 has two more protons than Krypton-84.

• Krypton-86 has two fewer neutrons than Krypton-84.

• Krypton-86 has two more neutrons than Krypton-84.

Multiple Choice

The average atomic mass of chlorine is 35.45 amu. Its two major isotopes are Chlorine-35 and Chlorine-37. What can you infer about the abundance of these two isotopes in nature?

• Chlorine-37 is significantly more abundant than Chlorine-35.

• Chlorine-35 is significantly more abundant than Chlorine-37.

• They exist in roughly equal amounts.

• Chlorine-35 and Chlorine-37 have the same number of neutrons.

Multiple Choice

An unknown element, X, is found to have 94 protons and 150 neutrons in its most stable form. Analyze this information to determine the atomic number and mass number of this element.

• Atomic Number = 150, Mass Number = 244

• Atomic Number = 94, Mass Number = 244

• Atomic Number = 244, Mass Number = 94

• Atomic Number = 94, Mass Number = 150

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Summary

• An atom's mass is concentrated in its nucleus, holding its protons and neutrons.

• An element is defined by its atomic number, which is its proton count.

• Mass number is the sum of protons and neutrons in an atom's nucleus.

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

• A neutral atom has an equal number of electrons and protons.

• Number of Neutrons = Mass Number – Atomic Number.

Poll

On a scale of 1-4, how confident are you about calculating the subatomic particles of an atom?

• 1

• 2

• 3

• 4