Grade 10 · Science · Chemistry

10th grade Average atomic mass from isotopes worksheet

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Average atomic mass from isotopes

Grade 10 Science Worksheet - Challenge Me

NameDate

Circle the best answer, or write your answer on the line.

Magnesium is 78.99% Mg-24 (23.985 u), 10% Mg-25 (24.986 u) and 11.01% Mg-26 (25.983 u) (NIST). What is its average atomic mass, to 0.01 u, Ari?
Silicon is 92.223% Si-28 (27.977 u), 4.685% Si-29 (28.976 u) and 3.092% Si-30 (29.974 u) (NIST). What is its average atomic mass, to 0.01 u, Bo?
3.
Before calculating, about where should magnesium's average mass fall, Jun?
- Exactly at 25 u
- Near 26 u
- A little above 24 u
4.
A student gets 24.98 u for magnesium. What likely went wrong, Kira?
- The masses were rounded
- Magnesium has no isotopes
- A plain average was used
5.
Oxygen is 99.757% oxygen-16. Why is its average so close to 16, Luz?
- Oxygen-16 is almost all of it
- The masses are equal
- Oxygen has one isotope
6.
Milo says every chlorine atom has a mass of 35.45 u. Agree or disagree?
- Agree: chlorine atoms match exactly
- Disagree: it is an average
- Agree: the table says so
7.
Pia says a sample is gone after two half-lives. Agree or disagree?
- Disagree: a quarter is left
- Agree: two halves make a whole
- Agree: it drops to zero
8.
An ancient leaf buried in mud has half the carbon-14 of a living leaf. About how old is it, Ren?
- About 11,400 years
- About 2,850 years
- About 5,700 years
9.
A piece of old wood has one quarter of its carbon-14 left. About how old is it, Tala?
- About 11,400 years
- About 5,700 years
- About 22,800 years
10.
Why does carbon-14 dating work only on things that were once alive, Zee?
- Rocks have no atoms
- Carbon-14 is made in fossils
- They took in carbon while living

10th grade Average atomic mass from isotopes worksheet – printable — Print: 1 2 3 4 5 6

About this worksheet

Average atomic mass from isotopes. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

I can find an element's average atomic mass as a weighted average of its isotopes, for two or three isotopes.
I can work back from an average to an isotope's abundance.
I can explain why table masses aren't whole numbers and lean toward the most common isotope.
I can do half-life steps and simple carbon-14 dating (half-life about 5,700 years).

Standard: none (TX)

Words to know

isotope: Atoms of one element with different numbers of neutrons

abundance: The percent of an element's atoms that are one isotope

weighted average: An average that counts each value by how common it is

atomic mass unit (u): One-twelfth of a carbon-12 atom's mass

half-life: The time for half of a radioactive isotope's atoms to change

Common mistakes

Your child took a plain average. Isotopes aren't equally common, so weight each mass by its abundance.

Your child found the heavier isotope's percent. The question asks for the lighter one: (heavier mass − average) ÷ (mass gap) × 100.

Your child thought isotopes have different numbers of protons. Isotopes of one element have the same protons and different neutrons.

Questions parents ask

What is the most common mistake with average atomic mass from isotopes?

Your child took a plain average. Isotopes aren't equally common, so weight each mass by its abundance.

Are these 10th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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