Notes: Isotopes and Average Atomic Mass



Lab: Beanium Inquiry

In this lab you will not have a formal write-up. Start by placing your name, date, and class in the upper right corner. Then write the name of lab at top of paper. Then, start answering the questions and completing the activity.

Small Group Review:

( What makes one isotope different from another?

( Write the hyphen notation for 2 isotopes of Nitrogen: Nitrogen with a mass of 12 and Nitrogen

with a mass of 14

( Write the nuclear symbol for the two isotopes above

( Write the nuclear symbol for an isotope with 16 protons, 18 electrons, and 19 neutrons.

Small Group Inquiry:

( Imagine that you are given a sample of carbon atoms. Within this sample, you have

carbon-12, carbon-13, and carbon-14.

1) If each isotope made up the same percentage of the sample (each accounted for 33.33%), how would you find the average mass of the sample? Show this below and calculate the average.

2) If each isotope made up different percentages of the sample (lets say: carbon-12 makes up 83%, carbon-13 makes up 12%, and carbon-14 makes up 5%) - can you calculate the average mass the same way you did in #1? Explain.

3) What should your average mass be closest to given the percentages in #2. Why?

4) Why do you think the masses in the periodic table are not whole numbers?

Activity: Working w/ Lab Partner Finding %

( How do you find a general %? Like say, how would you calculate the % of girls in your class?

( Fill in your data table below and calculate the percentage of each bean in your sample of the element “Beanium”

|Beanium Isotope |# of each Beanium Isotope in Sample |% of each Beanium Isotope in Sample |

|Lima Beanium | | |

|Black-eyed Pea Beanium | | |

|Kidney Beanium | | |

|Pinto Beanium | | |

|Total | | |

( Do your isotopes of Beanium each make up an equal % of the total?

( Can you just add up their masses and divide by the total # to find the average mass?

( Will each pinto bean have the exact same mass as all of the other pinto beans? How about lima? How about Kidney?

(This is where our example falls short of representing isotopes…all isotopes that are the same in mass would have the same mass…but we have to factor this in since we are in fact using beans and not real isotopes!)

Activity: Finding an Average Mass for a sample of beans

( How could you find the average mass of one pinto or of one black-eyed pea?

(There are 2 ways…a really long way, and a rather short way)

( Fill in the data table below with the average mass of just one of each bean type:

|Beanium Isotope |# of each Beanium Isotope in Sample |Mass of Entire Sample |Average Mass of 1 bean in the sample (Called Relative Mass) |

|Lima Beanium | | | |

|Black-eyed Pea Beanium | | | |

|Kidney Beanium | | | |

|Pinto Beanium | | | |

( To find the Average mass of this entire sample of Beanium, can you just add all of the averages together and divide by 4? Explain.

Activity: Finding a Weighted Mass

( We must factor in the % each isotope makes up of the sample.

( If the Kidney bean (rather large mass) only makes up 5% of the sample, will it have that much control on the overall average mass of the sample?

( To find a weighted mass: % isotope makes up X mass of the isotope ( Add all of these together to get a total!

*Since we are representing isotopes with beans (which due to nature all have slightly different masses even tough they are the same type of bean) we will use the “average mass of 1 bean” to represent the “mass of the isotope”

*This is not something you have to factor into a real situation involving real isotopes!

( Remember that you are using %, which has 100 factored into it ( make sure you divide the total by 100 to factor out the %!

( Fill in the data table below to find the weighted mass of your sample of beanium.

|Beanium Isotope |Average mass of 1 bean from the sample (Relative Mass) |% of each Beanium isotope in the sample |% of each X Mass of each |

|Lima Beanium | | | |

|Black-eyed Pea Beanium | | | |

|Kidney Beanium | | | |

|Pinto Beanium | | | |

Average Mass of your Sample: _________

Application to real isotope problems:

1. Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?

2. Uranium is used in nuclear reactors and is a rare element on earth. Uranium has three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of uranium?

3. Naturally occurring chlorine that is put in pools is 75.53 percent 35Cl (mass = 34.969 amu) and 24.47 percent 37Cl (mass = 36.966 amu). Calculate the average atomic mass. (always use the amu mass if it is given)

4) Copper used in electric wires comes in two flavors (isotopes): 63Cu and ?Cu. 63Cu has an atomic mass of 62.9298 amu and an abundance of 69.09%. The other isotope, ?Cu, has an abundance of 30.91%. The average atomic mass between these two isotopes is 63.546 amu. Calculate the actual atomic mass of ?Cu and write it’s nuclear symbol.

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