Isotopes and Atomic Masses
Big Idea
In the years 1802-1805, John Daltlon, an English science and math teacher developed the first concepts of matter as a network or collection of atoms (what we call "compound") - which has become to be known as "Dalton's Atomic Theory" (sometimes abbreviated as DAT). The basis of Dalton’s Atomic Theory was the notion that an atom of any particular element has a representative and distinct mass. Since Dalton’s time we have discovered that most naturally occurring elements are composed of a mixture of isotopes, each isotope having its own characteristic mass. Chemists have also discovered that the percentages of the different isotopes in most naturally occurring elements have not changed over time and are usually independent of the source of the element – whether the elemental sample is from the Earth, the Moon, meteorites, or even from Mars!
So, what does this mean?
It means that Dalton actually determined the average atomic masses (actually average relative masses – as you will learn). Since we encounter naturally occurring mixtures of elements, it is these average atomic masses that we need to use in all of our calculations involving elements, mixtures, or compounds.
In the years 1802-1805, John Daltlon, an English science and math teacher developed the first concepts of matter as a network or collection of atoms (what we call "compound") - which has become to be known as "Dalton's Atomic Theory" (sometimes abbreviated as DAT). The basis of Dalton’s Atomic Theory was the notion that an atom of any particular element has a representative and distinct mass. Since Dalton’s time we have discovered that most naturally occurring elements are composed of a mixture of isotopes, each isotope having its own characteristic mass. Chemists have also discovered that the percentages of the different isotopes in most naturally occurring elements have not changed over time and are usually independent of the source of the element – whether the elemental sample is from the Earth, the Moon, meteorites, or even from Mars!
So, what does this mean?
It means that Dalton actually determined the average atomic masses (actually average relative masses – as you will learn). Since we encounter naturally occurring mixtures of elements, it is these average atomic masses that we need to use in all of our calculations involving elements, mixtures, or compounds.
Supplemental Readings
CK-12 Chemistry: Isotopes and Atomic Mass
Videos
What is a Weighted Average? by Tyler DeWitt [12:49]
What is a weighted average? How do you calculate a weighed average? It is a type of average based on the abundance of what you are measuring. Weighted averages are used for atomic mass and atomic weight, based on the mass numbers of various isotopes.
Atomic Mass: Introduction by Tyler DeWitt [8:56]
What is atomic mass? It is a weighed average of the different isotopes of an element. It is sometimes referred to as atomic weight, relative atomic mass, or average atomic mass. We look at how to calculate and determine the weighed average of elements using atomic mass units.
What's the Difference between Mass Number and Atomic Mass? by Tyler DeWitt [8:56]
What's the difference between mass number and atomic mass? Mass number is the number of protons and neutrons in an atom, and it tells us about the mass of the atom in amu, or atomic mass units. Atomic mass is the average mass of all the isotopes of a certain type. It is a weighted average that takes into account the abundances of all of the different isotopes.
How to Calculate Atomic Mass Practice Problems by Tyler DeWitt [6:10]
How do you calculate atomic mass? You have to multiply the atomic weight of an atom (in amu, or atomic mass units) by the percent abundance expressed as a decimal. Then you add these together for all the isotopes of an atom.
Atomic Mass Calculations: Extra Help and Explanation by Tyler DeWitt [2:12]
Check out this video if you need more help with how to determine and calculate isotope abundance and atomic mass. There is extra help and a good analogy.
Atomic Mass: How to Calculate Isotope Abundance by Tyler DeWitt [11:48]
How do you determine and calculate isotope abundance when you know the relative atomic mass (also known as atomic weight), as measured in amu or atomic mass numbers? Here we will go through the algebra and reasoning to figure out the amount of abundances of the isotopes, in percentages and in decimals.
Discovery of the Nucleus: Rutherford's Gold Foil Experiment by Tyler DeWitt [15:58]
In 1911, Ernest Rutherford and his colleagues discovered the nucleus of the atom using their famous gold foil experiment. They shot alpha particles at a sheet of gold foil, and noticed that most went through, but some bounced back. This showed that atoms have a nucleus, and it disproved Thompson's plum pudding model of the atom.
Additional Resources
Advanced Video
Mass Spectrometry by BozemanScience [8:19]
Animations/Simulations
Calculation of Atomic Weight from Isotopic Composition by Wisc-Online
Practice
Average Atomic Mass Tutorial by J. Fredericks (online practice)
Average Atomic Mass Lesson w/ Problems & Answers by J. Fredericks
Chemistry: Average Atomic Mass by AlgebraLAB
Calculate the average atomic weight when given isotopic weights and abundances by ChemTeam
ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCE -- Quiz by Ohio State
PhET Interactive Simulation - requires JAVA to run
CK-12 Chemistry: Isotopes and Atomic Mass
Videos
What is a Weighted Average? by Tyler DeWitt [12:49]
What is a weighted average? How do you calculate a weighed average? It is a type of average based on the abundance of what you are measuring. Weighted averages are used for atomic mass and atomic weight, based on the mass numbers of various isotopes.
Atomic Mass: Introduction by Tyler DeWitt [8:56]
What is atomic mass? It is a weighed average of the different isotopes of an element. It is sometimes referred to as atomic weight, relative atomic mass, or average atomic mass. We look at how to calculate and determine the weighed average of elements using atomic mass units.
What's the Difference between Mass Number and Atomic Mass? by Tyler DeWitt [8:56]
What's the difference between mass number and atomic mass? Mass number is the number of protons and neutrons in an atom, and it tells us about the mass of the atom in amu, or atomic mass units. Atomic mass is the average mass of all the isotopes of a certain type. It is a weighted average that takes into account the abundances of all of the different isotopes.
How to Calculate Atomic Mass Practice Problems by Tyler DeWitt [6:10]
How do you calculate atomic mass? You have to multiply the atomic weight of an atom (in amu, or atomic mass units) by the percent abundance expressed as a decimal. Then you add these together for all the isotopes of an atom.
Atomic Mass Calculations: Extra Help and Explanation by Tyler DeWitt [2:12]
Check out this video if you need more help with how to determine and calculate isotope abundance and atomic mass. There is extra help and a good analogy.
Atomic Mass: How to Calculate Isotope Abundance by Tyler DeWitt [11:48]
How do you determine and calculate isotope abundance when you know the relative atomic mass (also known as atomic weight), as measured in amu or atomic mass numbers? Here we will go through the algebra and reasoning to figure out the amount of abundances of the isotopes, in percentages and in decimals.
Discovery of the Nucleus: Rutherford's Gold Foil Experiment by Tyler DeWitt [15:58]
In 1911, Ernest Rutherford and his colleagues discovered the nucleus of the atom using their famous gold foil experiment. They shot alpha particles at a sheet of gold foil, and noticed that most went through, but some bounced back. This showed that atoms have a nucleus, and it disproved Thompson's plum pudding model of the atom.
Additional Resources
Advanced Video
Mass Spectrometry by BozemanScience [8:19]
Animations/Simulations
Calculation of Atomic Weight from Isotopic Composition by Wisc-Online
Practice
Average Atomic Mass Tutorial by J. Fredericks (online practice)
Average Atomic Mass Lesson w/ Problems & Answers by J. Fredericks
Chemistry: Average Atomic Mass by AlgebraLAB
Calculate the average atomic weight when given isotopic weights and abundances by ChemTeam
ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCE -- Quiz by Ohio State
PhET Interactive Simulation - requires JAVA to run