Average Atomic Mass Worksheet with Answers (Grade 11 Chemistry)

This free average atomic mass worksheet explains why the numbers on the periodic table aren't whole numbers. Students start by counting protons, neutrons and electrons in atoms and ions, then write isotopes in nuclear and hyphen notation.

The core of the sheet is weighted averages: chlorine, magnesium and copper from real isotope data, a mass spectrum to read and identify, and two problems that work backwards from the atomic mass to the percent abundance. Every answer in the key was calculated and checked.

• Grade 11 Chemistry
• 21 marks
• 35–45 minutes
• Full answer key

What's in This Average Atomic Mass Worksheet

Part	Skill practised	Marks
A	Protons, neutrons and electrons in atoms and ions	3
B	Isotope notation	3
C	Average atomic mass and mass spectra	8
D	Finding percent abundance	3
E	Thinking about isotopes	2
F	Spot the mistake	2

It supports the Ontario Grade 11 chemistry curriculum expectations in the Matter, Chemical Trends and Chemical Bonding unit.

Protons, Neutrons and Electrons

The atomic number (Z) is the number of protons. The mass number (A) is protons plus neutrons, so neutrons = A − Z. In an ion, electrons = protons − charge. Here are the answers to Part A of the average atomic mass worksheet:

Atom or ion	Protons	Neutrons	Electrons
Carbon-14	6	8	6
2311Na+	11	12	10
3517Cl−	17	18	18
5626Fe3+	26	30	23
Uranium-235	92	143	92
168O2−	8	8	10

What Are Isotopes?

Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 (126C) and carbon-14 (146C) both have 6 protons but 6 and 8 neutrons. Isotopes behave the same way chemically but have different masses. For more background, see isotopes on Wikipedia.

How to Calculate Average Atomic Mass

Average atomic mass is a weighted average. Multiply each isotope's mass by its fractional abundance and add:

(34.969 × 0.7578) + (36.966 × 0.2422) = 35.45 u

That is why chlorine's atomic mass is 35.45, not 36: chlorine-35 is about three times as common as chlorine-37. Part C of the average atomic mass worksheet repeats this for magnesium (three isotopes, 24.31 u) and copper (63.55 u). These averages are the molar masses used in the mole conversion worksheet.

Mass Spectra and Percent Abundance

A mass spectrometer separates isotopes by mass. Each bar shows one isotope's mass and abundance. For element X, (50.0 × 0.20) + (52.0 × 0.50) + (53.0 × 0.30) = 51.9 u, which matches chromium on the periodic table.

You can also work backwards. If gallium's atomic mass is 69.72 u, let x be the fraction of Ga-69: 68.926x + 70.925(1 − x) = 69.72, which gives 60.3% Ga-69. Part D practises this with gallium and boron.

Average Atomic Mass Worksheet Practice Questions

These questions match the average atomic mass worksheet PDF, which has the particle table, the mass spectrum and ruled working space.

Part A – Protons, Neutrons and Electrons

Questions 1–6: find the protons, neutrons and electrons in carbon-14, Na⁺, Cl⁻, Fe³⁺, uranium-235 and O²⁻.

Part B – Isotope Notation

7. An atom has 17 protons and 20 neutrons. Write its nuclear symbol and its hyphen notation.
8. An atom has 29 protons and 36 neutrons. Write its nuclear symbol and its hyphen notation.
9. Which pair are isotopes? (a) 126C and 146C   (b) 146C and 147N   (c) 4019K and 4020Ca. Explain.

Part C – Calculating Average Atomic Mass

10. Chlorine has two isotopes: chlorine-35 (34.969 u, 75.78%) and chlorine-37 (36.966 u, 24.22%). Calculate the average atomic mass of chlorine. (2 marks)
11. Magnesium has three isotopes: Mg-24 (23.985 u, 78.99%), Mg-25 (24.986 u, 10.00%) and Mg-26 (25.983 u, 11.01%). Calculate its average atomic mass. (2 marks)
12. Copper is 69.15% copper-63 (62.930 u) and 30.85% copper-65 (64.928 u). Calculate its average atomic mass.
13. Read the mass spectrum. List the mass and percent abundance of each isotope of element X.
14. Calculate the average atomic mass of X and use the periodic table to identify it. (2 marks)

Part D – Finding Percent Abundance

15. Gallium has two isotopes, Ga-69 (68.926 u) and Ga-71 (70.925 u). Its average atomic mass is 69.72 u. Find the percent abundance of each isotope. (2 marks)
16. Boron is made of B-10 (10.013 u) and B-11 (11.009 u). Its average atomic mass is 10.81 u. What percent of boron is B-10?

Part E – Thinking About Isotopes

17. Why is the average atomic mass of most elements not a whole number?
18. Copper’s average atomic mass is 63.55 u. Without calculating, which isotope (Cu-63 or Cu-65) is more abundant? Explain.

Part F – Spot the Mistake

19. A student writes: “Isotopes of an element have different numbers of protons.” Explain the error and correct it.
20. A student calculates chlorine’s average atomic mass as (35 + 37) ÷ 2 = 36 u. Explain the error and correct it.

Average Atomic Mass Worksheet Answer Key

Open each part of the average atomic mass worksheet answers to check your work. The printable key is at the end of the PDF.

Part A answers (Q1–6)

1. 6 protons, 8 neutrons, 6 electrons
2. 11 protons, 12 neutrons, 10 electrons
3. 17 protons, 18 neutrons, 18 electrons
4. 26 protons, 30 neutrons, 23 electrons
5. 92 protons, 143 neutrons, 92 electrons
6. 8 protons, 8 neutrons, 10 electrons

Part B answers (Q7–9)

7. 3717Cl, chlorine-37
8. 6529Cu, copper-65
9. (a): same number of protons (6), different numbers of neutrons (6 and 8).

Part C answers (Q10–14)

10. (34.969 × 0.7578) + (36.966 × 0.2422) = 35.45 u
11. (23.985 × 0.7899) + (24.986 × 0.1000) + (25.983 × 0.1101) = 24.31 u
12. (62.93 × 0.6915) + (64.928 × 0.3085) = 63.55 u
13. 50 u: 20%; 52 u: 50%; 53 u: 30%
14. (50.0 × 0.20) + (52.0 × 0.50) + (53.0 × 0.30) = 51.9 u, closest to chromium (52.00 u)

Part D answers (Q15–16)

15. Let x = fraction of Ga-69: 68.926x + 70.925(1 − x) = 69.72 → x = 0.6028. Ga-69 = 60.3%, Ga-71 = 39.7%
16. 10.013x + 11.009(1 − x) = 10.81 → x = 0.200, so 20.0% B-10

Part E answers (Q17–18)

17. It is a weighted average of several isotopes with different masses and abundances.
18. Cu-63: the average is much closer to 63 than to 65.

Part F answers (Q19–20)

19. Isotopes have the same number of protons but different numbers of neutrons. Changing the protons changes the element.
20. The average must be weighted by abundance. Chlorine-35 is about three times as common, so the average is 35.45 u.

5 Common Average Atomic Mass Mistakes

1. Using percentages instead of fractions. Divide each percent by 100 before multiplying.
2. Taking a simple average. Always weight by abundance.
3. Mixing up mass number and atomic mass. Mass number is a whole number for one isotope; atomic mass is the average.
4. Forgetting the ion's charge. A 2− ion has two extra electrons; a 3+ ion has three fewer.
5. Abundances that don't add to 100%. Check your answer to a "find the abundance" problem.

How to Use This Average Atomic Mass Worksheet in Class

• Lesson 1: Parts A–C of the average atomic mass worksheet, after a "beanium" or penny isotope lab.
• Lesson 2: Parts D–F, then link isotopes to radioactive dating with carbon-14.
• Extension: Students research medical uses of isotopes such as iodine-131.

Continue with the electron configuration worksheet, and review with the electron configuration quiz.

Frequently Asked Questions

Is this average atomic mass worksheet free?

Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.

How do you calculate average atomic mass?

Multiply each isotope's mass by its fractional abundance (percent ÷ 100) and add the results. For chlorine, 34.969 × 0.7578 + 36.966 × 0.2422 = 35.45 u.

What is the difference between mass number and atomic mass?

Mass number is the whole-number count of protons and neutrons in one isotope. Atomic mass is the weighted average mass of all of an element's isotopes, so it usually isn't a whole number.

How do you find the percent abundance of an isotope?

Let x be the fraction of one isotope and (1 − x) the other. Set the weighted average equal to the known atomic mass and solve for x.

What grade is this average atomic mass worksheet for?

It is written for Grade 11 chemistry and also works for Grade 10 science and introductory college chemistry.

More Free Grade 11 Chemistry Resources

• Electron Configuration WorksheetOrbitals and diagrams
• Periodic Trends WorksheetAtomic radius and more
• Mole Conversion WorksheetMolar mass
• Periodic Table Trends Quiz20 multiple-choice questions
• All Grade 11 ResourcesWorksheets and quizzes

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