Isotopes and Average Atomic Mass
Topics covered: How to read the element box, isotopes, calculating average atomic mass
Take a look at the picture above. The arrows indicate different parts of the element box of hydrogen. Quite self-explanatory, right? The symbol shows the chemical symbol of an element, the name shows the name of an element, the atomic mass shows the mass, and the atomic number shows the number... But wait, do you REALLY understand what they mean? What exactly is an atomic number? How do you calculate an atomic mass?
Let’s start by answering the first question: what is an atomic number? Atomic number is the number of protons that an atom has. Remember that in the last post I said the protons determine the identity of an atom? This is why! The number of protons is like the name tag of an atom; while the number of neutrons and electrons can change, the number of protons never changes. Changing the number of protons is like changing the name tag of an atom, so if you change the number, you would create a completely different element with new properties!
Now let’s move on to the mass number (The atomic mass and the mass number AREN’T THE SAME THING!). The mass number is the sum of the number of protons and neutrons that an atom has*.
For example, if an element has 11 protons and 12 neutrons, that element would have an atomic mass of 23 amu (11 + 12). You might be wondering, “what’s the difference between atomic mass and mass number?” To answer that question, we first have to learn about isotopes.
Isotopes are the atoms that have the same number of protons but different number of neutrons. Think of isotopes as different flavors of Oreos. There are different types of Oreos, including chocolate creme, mint, and so on. However, even though they come in different flavors, they all fall under the “Oreo” category, no matter what their flavors are. The same goes for isotopes. The type of Oreo flavors is equivalent to the number of neutrons that an atom has. Depending on its number of neutrons, an atom can have many isotopes. However, no matter how many neutrons they have, these isotopes still fall under one atom, because their number of protons always stays the same.
Now, instead of Oreos, I’ll give you a real life example using carbon isotopes. Every carbon atom has 6 protons, which determine carbon’s identity. However, the number of neutrons that a carbon has can vary a lot; some carbon atoms have 7 neutrons, some have 6 and some have 5! These are all called carbon isotopes because they have different numbers of neutrons (7, 6, and 5) but the same number of protons (6).
Because they have different numbers of neutrons, isotopes have different masses. In an example above, carbon that has 7 neutrons would have the mass of 13 amu (6 protons + 7 neutrons), while a carbon that has 6 neutrons would have the mass of 12 amu (6 protons + 6 neutrons).
There are two ways to name an isotope, and I call them the “dash way” and the “symbol way”. A dash way uses a dash between the element name and the element’s mass number. A symbol way writes the symbol of an element and puts the mass number at the upper left corner and the atomic number (= the number of protons) at the lower left corner.
This “dash way” tells us that we’re looking at a chlorine atom that has the mass of 35.
This “symbol way” tells us the same thing, but shows the atomic number at the lower left corner. Now, if you want to calculate the number of neutrons that this isotope has, simply subtract the atomic number from the mass number (Since Mass = Protons + Neutrons, Neutrons would equal to Mass - Protons). If you do that, you would know that this is a chlorine isotope that has 17 protons and 18 neutrons (35 - 17 = 18).
Finally, let’s talk about the atomic mass. The atomic mass (or “average” atomic mass) is an average mass for all natural isotopes of an element. There are many isotopes of a certain atom, but not all of them are present in nature in an equal amount. The percent abundance represents the natural abundance of an atom in percentage.** To calculate atomic mass, you multiply each isotope’s mass by its percent abundance (don’t forget to convert the percentage to decimal!) and add them all together, depending on how many isotopes that atom has.
If you are confused about mass number and atomic mass, remember this: mass number is simply proton + neutron, while the atomic mass is an average (and plus, calculating the atomic mass is more complicated!).
Do you want practice? Here’s the example problem of finding the average atomic mass of an unknown element. Don’t be intimidated if you don’t know the identity of the element! Convert the percent to decimals, multiply them with respective mass numbers, and add them all together. After you calculate the average atomic mass of an unknown element, your periodic table will tell you the identity of the element!
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*As explained in the last post, the nucleus of an atom takes up most of the atom’s mass, so anything that’s located outside of the nucleus (eg. electrons) wouldn’t affect the mass of an atom. That’s why you only combine the number of protons and neutrons to calculate the mass number!
** If you have a hard time understanding the concept of the percent abundance, I’ll give you an example. If chlorine-37 has 24% of percent abundance, that means that out of 100 randomly chosen chlorine atoms, 24% of them would have a mass of 37!
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