Average Atomic Mass Isotopes & Calculating Average Atomic Mass   

 

Guiding Question: What would happen to a person’s mass if the composition of isotopes on earth changed?  Overview: In this simulation, students explore how percent abundances of different isotopes affect the average atomic mass for that given element. Students form connections between the average atomic mass and a person’s body mass.  Learning Goals:  

● Define an isotope as atoms of an element that contain the same number of protons but different number of neutrons. 

● Identify the structural differences and similarities of an element’s isotopes.  

● Calculate the average atomic mass of an element using naturally occurring percent abundances and mass number.  

● Connect average atomic mass to the mass of atoms in our body (human body). 

 

Key Concepts: Isotopes, Calculating Average Atomic Mass, Atomic Mass Unit  Link to Simulation: https://interactives.ck12.org/simulations/chemistry/average-atomic-mass/app/index.html 

 

 

 

A human body weighing 70 kilograms (154 lbs) is made of over                       27,000,000,000,000,000,000,000,000,000 atoms. Of these atoms, 99% are             either hydrogen, oxygen, carbon or nitrogen. The remaining 1% of atoms                     come from 31 trace elements. For example, carbon is a mixture of carbon                         atoms weighing 12 u and 13 u. The nucleus of a carbon atom contains 6                             protons. However, the number of neutrons varies. Sometimes the nucleus                   contains 6 neutrons other times it contains 7 neutrons. Atoms of the same                         element that contain different numbers of neutrons are called isotopes. In                     nature, elements occur as a mixture of their isotopes. However, it’s not an                         even mixture; some isotopes are found more commonly than others in                     nature. In the case of carbon you are almost 100 times more likely to find a                               carbon-12 atom than a carbon-13 atom in nature. What would happen to a                         person’s mass if the average mixture of carbon isotopes on earth changed? 

   

 

 

Lesson Plan   

Background Video   

  Teacher Guide | Page 1 

 

 

  

Element Sample & Percent Abundance This slider changes the pure element sample. The elements: oxygen, carbon, hydrogen, and nitrogen make up around 99% of all atoms in your body. Phosphorus and calcium atoms make up another 0.5% of the atoms, while the remaining 0.5% come from 29 trace elements 

  

Average Atomic Mass Calculation Average atomic mass is the weighted average of atomic mass of all the atoms (isotopes) in a sample of that element. It is calculated by multiplying the percent abundance of each isotope by its atomic mass and summing these masses. Alternatively the average atomic mass can be calculated by measuring the total mass of the atoms in the sample and dividing it by the sample size. 

 

See the inside of isotopes To see what makes up each isotope, click on the atom of any isotope.  

 

Diagram settings 

The person can be labeled with or .  

 

 

Simulation Overview   

  Teacher Guide | Page 2 

 

Sort Atoms 

Below the center pop-out, students can use the button to arrange the sample of atoms as shown below, making it easier to visualize the ratio of isotopes currently present. Clicking the “unsort” button will return them to the original state. 

 

 

Show Comparison 

Clicking the will take the student to a side-by-side comparison of their model that has a different average atomic mass for one element with the unchanged model.Students can compare the models they have created with a 

modified average atomic mass by clicking on . 

   

 

 

  Teacher Guide | Page 3 

  

 

How can an ancient skeleton talk about food? You know the phrase “you are what you eat?” It                   doesn’t mean you can turn yourself into a carrot,                 but it does mean that most of the matter you                   consume eventually becomes a part of you. It’s               true for humans, and it’s true for the plants and                   animals we eat, too. Plants “eat” carbon dioxide in                 the air, but they have two different ways of doing                   it. C-3 plants, like wheat, berries and vegetables,               use carbon dioxide as they gather it, whereas C-4                 plants, like corn and sugarcane, gather their             carbon dioxide and store it first, then use it later.  All plants use CO2 gas in photosynthesis, however               the organelles in the plant cells can’t keep all of                   the gas in them efficiently, so some of the CO2 gas                     escapes. CO2 made from 12C instead of 13C is                 lighter, so it will more easily escape. So you might                   be thinking? Well then both C-3 and C-4 plants will                   have slightly more heavy CO2 than light CO2, but it turns out that C-4 plants have more 13C. This is because C-4 plants have                                               two times when lighter CO2 gas can escape: when it is stored in the mesophyll and when it is processed in the bundle                                             sheath. Since C-4 plants lose more 12C isotopes, it has a higher concentration of 13C isotopes. Some of the carbon from                                         foods we eat goes into our bons, where it stays even thousands of years after we die. So if an ancient human at mostly corn,                                                 the extra 13C in their skeleton will tell you that.   

How do isotopes help doctors make medical diagnoses? Isotopes, particularly radioisotopes, have many useful           medical applications. As the name suggests,           radioisotopes are radioactive isotopes that decay over             time, emitting particles until they reach a stable state.  Currently, technetium-99 is the radioisotope most           commonly used for medical diagnosis. Technetium-99 is             a versatile radioisotope, used to diagnose heart, brain,               bone, kidney, spleen, liver and thyroid conditions. For               example, technetium-99 allows doctors to identify           tumors and cancerous cells. To do this, doctors               administer antibodies tagged with technetium-99 to a             patient. The antibodies gravitate towards areas of cell               growth and bind mostly to cancerous cells. Because               technetium-99 releases gamma rays that escape the             body, doctors can then use a gamma camera to image                   the target area. High concentrations of gamma ray               emissions identify areas of cancerous growth. The usage               of technetium-99 to identify cancer is non-invasive and               

 

 

Real-World Connections   

  Teacher Guide | Page 4 

 

the short half-life of technetium-99 minimizes patient exposure to radioactivity. Technetium-99 may currently be the most                               commonly used radioisotope for diagnostics; however, it may soon be replaced by a more effective radioisotope as                                 technology develops.    

How do smoke detectors use radioisotopes? Most smoke detectors rely on the radioactive isotope,               americium-241, in order to trigger an alarm in the presence of                     smoke. Inside of a smoke detector sits a small sample of                     americium-241 next to two metal plates, both connected to the                   battery. Because of its radioactivity, americium-241 releases             alpha particles. Inside of a smoke detector is an ionisation                   chamber that contains a very small piece of Americium-241.                 The top and bottom of the chamber are made of metal plates                       that are attached to a battery.   When clean air enters the chamber, the alpha particles cause                   the nitrogen and oxygen particles in the air to ionize. The ionized                       particles allow electricity to flow between the metal plates                 completing the electrical circuit. However, when smoke particles enter the detector, they absorb the alpha particles released                                 by americium-241. Without alpha particles to ionize the air, electricity cannot flow. The circuit breaks and triggers the alarm.   

How can isotopes indicate the migratory patterns of birds? The stable isotope analysis of bird feathers and rainwater                 across the world can be used to track the migratory patterns of                       birds. Like all living things, birds are what they eat. The ratio of                         hydrogen, carbon, and nitrogen isotopes in a bird’s feather,                 correlates to the food/water they consume while growing               feathers on their breeding ground.  Birds consume rainwater, which naturally contains both             hydrogen-1 and hydrogen-2 isotopes. When a water molecule               contains a hydrogen-2 isotope, it is heavier which makes it                   harder for it to flow over mountains. It also affects the                     precipitation and evaporation rates of the water molecules. This                 causes variation in the presence of ratio of hydrogen-2 isotopes                   to hydrogen-1 isotopes in the water across the content.   

As birds migrate across the country they lose feathers or their feathers are collected by scientists. The scientists then use                                       isotope analysis to determine the amount of hydrogen-2 in the feathers, which they can then trace back to the birds’ origin.                                         Scientists have used stable isotope analysis to investigate the migratory patterns of many bird species, such as the                                   Black-Throated Blue Warblers, Wilson’s Warblers, and Red Knots. 

 

 

  Teacher Guide | Page 5 

Name: ___________________________________________ Date: ______________   

Average Atomic Mass Isotopes & Calculating Average Atomic Mass 

Challenge Me Questions 1-7: As you explore the simulation, answer the following questions. 

 1. What element has the greatest percent by mass in the human body? 

 

 

2. Which element makes up the most atoms in the human body? 

 

 

3. Click on the atoms to see a zoomed in view. How are the atoms of carbon alike and different? Explain it in terms of their structure.  

 

 

4. Using the words average atomic mass, mass number, and atomic number. How would you explain the similarities and differences between the carbon atoms? 

 

 

5. What is the most abundant isotope of carbon found on earth? How does this relate to the average atomic mass of carbon? 

 

 

6. What is the heaviest isotope of oxygen that naturally occurs on earth? 

 

 

7. There are only two naturally occurring isotopes of helium on earth. If you increase the % abundance of helium-3, does the % abundance of helium-4 change? Why or why not?   

 

 

Exploration Questions   

  Exploration Worksheet | Page 1 

Name: ___________________________________________ Date: ______________   

 

 Questions 8-15: Circle the choice that best answers each question.  

8. In an isotope of an element, the number of which subatomic particle changes? 

a. proton 

b. neutron 

c. electron 

 

9. The human body is primarily composed of 4 elements. Which element makes up the greatest proportion of the human body by number of atoms? 

a. oxygen 

b. nitrogen 

c. hydrogen 

d. carbon 

 

10. Which element makes up the greatest proportion of the human body by mass? 

a. oxygen 

b. nitrogen 

c. hydrogen 

d. carbon 

 

11. How many neutrons are in a nucleus of oxygen-18? 

a. 18 

b. 10 

c. 8 

d. 6 

 

   

12. If all the oxygen atoms in the human body were oxygen-18.Would the human have gained or lost weight? How much? 

a. gained 5.2 kg 

b. lost 4.7 kg 

c. gained 4.7 kg 

d. lost 5.2 kg 

 

13. Which isotope of carbon occurs most often in nature? 

a. carbon-12 

b. carbon-13 

c. carbon-14 

 

14. Hypothetically, which of the following nitrogen isotopes can exist? Select all that apply. 

a. nitrogen-5 

b. nitrogen-6 

c. nitrogen-7 

d. nitrogen-15 

e. nitrogen-16 

f. nitrogen-22 

 

15. Neon does not occur in the human body, but it is in the air we breathe. Neon has two naturally occurring isotopes neon-20 and neon-22. If neon-20 has a natural abundance of 90% and neon-22 has a natural abundance of 10%, what is the average atomic mass of neon? 

a. 15.8 u 

b. 20.2 u 

c. 21.8 u 

d. 22.0 u 

 

 

Check My Understanding   

  Exploration Worksheet | Page 2