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Wize University Chemistry Textbook > Atoms, Ions, & Isotopes

Isotopes and Atomic Weight

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Isotopes and Atomic Mass

Isotopes

  • When two atoms have the same number of protons and electrons, but a different number of neutrons, we call these isotopes.
  • Isotopes have the same atomic number, but a different mass number.


How is the Atomic Mass in the Periodic Table Calculated?

  • The atomic mass is written as 12amu for C for example, but this does not mean that each C atom weighs exactly 12amu!
  • The atomic mass # is actually a weighted average based on the relative abundance of isotopes
  • Isotopes have similar reactivity to one another, that's why we can form C bonds with either C-12 or 13 etc

To determine the average mass of an element, use this equation:

A.W.=∑i=1n(mass of each isotopei)(abundance of each isotopei)\boxed{{A.W.=\displaystyle\sum_{i=1}^{n}(mass\space of\space each\space isotope_i)(abundance\space of\space each\space isotope _i)}}


Watch Out!
Plug in the mass of each isotope in amu and plug in the relative abundance of each isotope in the form of a decimal not percentage!
For example, above we are told the relative abundance of C-12 is 98%. You would want to plug in 0.98 for this isotope's relative abundance.


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Example: Solving for the Atomic Weight


Chlorine can be found in nature as 35Cl^{35} \text{Cl} (mass 34.969u, 75.78% abundance) and 37Cl^{37} \text{Cl} (mass 36.966u, 24.22% abundance). What is the average atomic mass of Cl\text{Cl}?


A.W.=(mass 35Cl)(abundance 35Cl)+(mass 37Cl)(abundance 37Cl)A.W.=\left(mass\ ^{35}Cl\right)\left(abundance\ ^{35}Cl\right)+\left(mass\ ^{37}Cl\right)\left(abundance\ ^{37}Cl\right)

A.W.=(34.969u)(0.7578)+(36.966u)(0.2422)A.W.=\left(34.969u\right)\left(0.7578\right)+\left(36.966u\right)\left(0.2422\right)

A.W.=35.45 gmolA.W.= 35.45\ \frac{g}{mol}

Practice: Solving for the Weight of an Isotope


Naturally occurring potassium contains two stable isotopes. The lighter isotope, 39K ( 38.9637 amu) is the more abundant isotope, accounting for 93.26% of the nuclei. What is the weight of the heavier isotope, 41K?
Bromine is a noxious fuming red liquid in its elemental form. Naturally occurring bromine has two stable isotopes 79Br (78.918u) and 81Br (80.916u). What are the relative abundances of these two isotopes?

Enter your answer as a percentage (ex. If the answer is 16.26%, enter 16.26 as the answer)
Extra Practice
Practice: Isotopes
An atom with 6 neutrons and a mass number of 13 is an isotope of which element?
Elements: Isotopes
Bromine is a noxious fuming red liquid in its elemental form. Naturally occurring bromine has two stable isotopes 79Br (78.918u) and 81Br (80.916u). What are the relative abundances of these two isotopes?
Enter your answer as a percentage (ex. If the answer is 16.26%, enter 16.26 as the answer)
The natural abundance of Bromine is 79.904 g/mol
Elements: Isotopes
Bromine is a noxious fuming red liquid in its elemental form. Naturally occurring bromine has two stable isotopes 79Br (78.918u) and 81Br (80.916u). What are the relative abundances of these two isotopes?
Enter your answer as a percentage (ex. If the answer is 16.26%, enter 16.26 as the answer)
Elements: Isotopes
Bromine is a noxious fuming red liquid in its elemental form. Naturally occurring bromine has two stable isotopes 79Br (78.918u) and 81Br (80.916u). What are the relative abundances of these two isotopes?
Enter your answer as a percentage (ex. If the answer is 16.26%, enter 16.26 as the answer)
Elements: Isotopic Mass
Fe has three isotopes in relatively high abundance.
54Fe 53.94 amu 5.8 % rel. abundance
56Fe 55.94 amu 91.8 % rel. abundance
Practice: Isotopes
An atom with 6 neutrons and a mass number of 13 is an isotope of which element?
Mg has three isotopes. 24Mg is the most abundant (78.70%). Calculate the percentages of the other two isotopes. The exact masses are given below:
24Mg = 23.98504 u
25Mg = 24.98584 u
The Atom
2. Calculate the average mass of chlorine to two decimal places given its two isotopes: chlorine-35 (34.97 amu, 76% abundance) and chlorine-37 (36.97 amu, 24% abundance).
The Atom
Calculate the average mass of chlorine to three decimal places given its two isotopes: bromine-79 (78.918 amu, 51% abundance) and bromine-81 (80.916 amu, 49% abundance).
Elements: Isotopes and Atomic Mass
Hydrogen exists as two naturally occurring isotopes 1H (1.00782504u) (99.9885%) and deuterium, 2H. What is the atomic mass of deuterium? The atomic mass of hydrogen is 1.00794u.
Elements: Isotopes
Bromine is a noxious fuming red liquid in its elemental form. Naturally occurring bromine has two stable isotopes 79Br (78.918u) and 81Br (80.916u). What are the relative abundances of these two isotopes?
Enter your answer as a percentage (ex. If the answer is 16.26%, enter 16.26 as the answer)
Elements: Isotopes
Nitrogen has two naturally occurring isotopes, nitrogen-14 (14.003074008u) and nitrogen-15 (15.0001088982). What is the percent abundance of each isotope if the standard atomic weight of nitrogen is 14.0067?
Elements: Isotopes
The transition metal rhenium has two stable isotopes. The atomic mass of rhenium is 186.2 g/mol. Given that 37.1% of natural rhenium is Re-185, what is the other stable isotope?
If the abundance of 6Li (6.015121 amu) is 7.500% and the abundance of 7Li (7.016003 amu) is 92.500%, what is the average atomic mass?
Boron has an average atomic mass of 10.81 amu. It has 2 isotopes: 11B (relative abundance 80 %) and 10B (relative abundance 20 %). If 11B has an atomic mass of 11.01 amu, how much does 10B weigh in amu?
Chlorine has two isotopes of interest. One has an atomic mass of 34.97 amu (relative abundance 75 %), while the other make up the remaining 25 % of this element. If Cl has an average atomic mass of 35.47 amu, what is the mass of the second isotope in amu?
B has two isotopes that are available naturally in meaningful abundance. 10B (10.01 g/mol) has a relative abundance of 20 %, while 11B (11.01 g/mol) has a relative abundance of 80 %. If you are given a 12 g sample, how many moles of 10B are in the sample (answer in mol)?
Elements: Isotopic Mass
Copper exists as two naturally occurring isotopes 63Cu (62.92960) (69.17%) and 65Cu. What is the atomic mass of 65Cu?
Elements: Isotopic Mass
Fe has three isotopes in relatively high abundance.
54Fe 53.94 amu 5.8 % rel. abundance
56Fe 55.94 amu 91.8 % rel. abundance
How would you determine the number of neutrons in an atom? Let's say the element is F.
Atom: Isotopes
Selenium has a number of isotopes associated with it.
76Se = 75.92 amu rel. abundance = 0.094
77Se = 76.92 amu rel. abundance = 0.076
Atom: Determining the mass
What is the total mass of a C atom?
Given the following information:
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Atom: Isotopes and Atomic Weight
An ion with a -2 charge has a mass number of 52 and contains 26 electrons. Which element is this ion? Specify the isotope as well.
Carbon is made up of two stable isotopes, 12C and 13C. 12C is the most abundant form (98.93%) with 13C making up the remaining 1.07%. The 12C nuclei is defined as exactly 12 amu, what is the weight of the 13C nuclei? For this problem use the precise atomic weight of carbon, 12.0107
Elements: Calculating Isotopic Abundances
There are two naturally occurring and stable isotopes of lithium, 6Li (6.0151 amu) and 7Li (7.0160 amu). What is the abundance of 7Li in naturally occurring Lithium?
Atom: Calculating the Mass of One Isotope
Naturally occurring potassium contains two stable isotopes. The lighter isotope, 39K ( 38.9637 amu) is the more abundant isotope, accounting for 93.26% of the nuclei. What is the weight of the heavier isotope, 41K?
Boron (atomic weight 10.81) is made up of two stable isotopes, 11B and 10B. The boron-11 nuclei weighs 11.0093 amu and the boron-10 nuclei weighs 10.0129 amu. The percentage of 11B and 10B in naturally occurring boron respectively is,
How many protons does 13C have?
An atom with 6 neutrons and a mass number of 13 is an isotope of which element?
How many electrons are there in 2759CO3+?^{59}_{27}CO^{3+}?
Elements: Isotopes
Which of the following species has an isotope with a mass number of 42, 18 electrons, and an overall charge of 2+?