Atomic mass
You will fly into table salt crystal and find out that chlorine ions are not the same. There are two naturally abundant isotopes. You will find out that atomic mass is counted as average of masses of naturally abundant isotopes.
This lesson is a part of MEL VR Science Simulations. Learn more →
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Transcript
In the periodic table, the atomic mass of elements generally increases with the atomic number, with a few exceptions.
The main contribution to the mass of any atom is made by heavy protons and neutrons, which the nucleus of an atom consists of. Electrons do not count, they are too small.
Since protons and neutrons have the same mass, approximately equal to 1 atomic mass unit or a.m.u., the atomic mass of an element should be close to a whole number.
The mass of this chlorine atom is close to 35 a.m.u.
But look, in the periodic table, the atomic mass of chlorine element is nearly 35 and a half! Why? Let's find out.
Today in our lab we have table salt. Sodium Chloride. Look inside.
Do you see the sodium and chlorine? Сlick on one of the chlorine particles.
Its mass has appeared in the table and is approximately 35.
Now click on another chlorine particle. Look, its mass is close to 37. It is another isotope of chlorine.
We invite you to play, now. Try to select the different chlorine particles. For each click, their masses will appear in the table.
When you click on 30 different chlorine particles, we calculate the atomic mass of the chlorine element. Let's go!
Your work is done, now look at the arithmetic average of atomic masses.
It is approximately the same as the atomic mass of the chlorine element in the Periodic Table. The average atomic mass would be more accurate if you took into account more atoms.
Let's go to the Laboratory.
Chlorine has two stable isotopes: chlorine-35 and chlorine-37. In nature, the abundance of chlorine-35 is close to 75%, and chlorine-37 - to 25%.
So, we have nearly 35 and a half.
Bromine has two stable isotopes, bromine-79 and bromine-81. The natural abundance of both isotopes is close to 50%.
What is the atomic mass of the element bromine? Yes, the atomic mass of the bromine element is close to 80.
Teacher's notes
Keywords
element, atom, atomic number, nucleus, protons, electrons
Students will
- Learn that the mass of an atom is very close to the mass of its nucleus, as electrons are very light.
- Find out that the mass of a proton and the mass of a neutron are very close to 1 a.m.u., and thus the mass of an atom should be somewhat close to a whole number.
- Find out that some elements in the Periodic Table have a mass which is not close to a whole number (for example, chlorine).
- Learning the properties of table salt crystals. Learning that chlorine found in nature has two stable isotopes with masses 35 and 37.
- Learn that the atomic mass of an element is calculated as an average mass of naturally abundant isotopes.
- Be asked to calculate the atomic mass of bromine with naturally abundant isotopes 79Br and 81Br and abundance close to 50/50.
Common misconceptions
- Confusion between atomic mass and atomic number
- Confusion between an atom and an element
Hands-on activities
After VR
The aim is to provide students with an example of the average mass of real-life objects. Ask students to weigh a variety of objects (for example, acrylic and glass beads of the same size, or metal and plastic bolts and nuts of the same size) and calculate the average mass of an object. Then weigh them separately and compare results with the average.
History and sources of knowledge
- Early attempts to establish the unit for atomic mass
- Mass spectrometry as a method of ‘weighing’ atoms.
Topics to discuss
- Two sources providing numbers for atomic mass that are not whole numbers: isotopic abundance and for heavy elements, the difference between proton mass and neutron mass.
- Real life examples, when the average contradicts common sense: the average number of people in a household (2.5), average population density (fractions of people per square km), etc.
Fun facts and quotes
- More than 75% of all elements on Earth are a mixture of their isotopes.
- Elements in the Periodic Table were originally in order of their atomic weight because subatomic particles were not known at the time.
Questions
- Why the atomic mass might not be close to a whole number?
- Why we don’t take into account the mass of electrons when calculating atomic mass?
Calculating
Calculate the average mass of Cu, if copper has two naturally abundant isotopes 63Cu (69.17%) and 65Cu (30.83%). Compare the result with the copper atomic mass in the Periodic Table.
