Isotopes, Percent Abundance, Atomic Mass | How to Pass Chemistry

In this video you will finally understand what an isotope is and how to calculate the

percent

age of

abundance

. Alright, let's do this. Hello Hello Melissa Maribel and I help students like you understand what they just learned in class so they stress less and graduate faster. Before we begin, let's take a deep breath. It will be much easier than you think. Starting with our first example, let's talk about the concept of

isotopes

. In 2009 he weighed one hundred and twenty pounds. Then in 2012 I weighed one hundred and thirty pounds and then in 2015 when I discovered my love for sushi I weighed one hundred and forty pounds.

So even though there are different weights or different versions of myself, I'm still the same person. Isotopes are different versions of the same element. So how does this relate to

chemistry

? Let's talk about gold. The chemical symbol for gold is Au. Take this gold bar, it weighs 196 amu. Our second gold bar weighs 197 amu and our third gold bar weighs 198 amu. All these gold bars have different weights but they are made of the same element which is gold. These are three different types of gold

isotopes

. If we looked at the periodic table and looked up where Au or Gold is, we would see that it has a lower number, right?

That

atomic

mass

. That

atomic

mass

is actually the average of all the different types of isotopes of that element. Isotopes have the same atomic number, the same protons and electrons. However, they do not have the same masses. So they have different masses and a different number of neutrons. Let's go to our first example talking about the

percent

age of

abundance

. A certain element X has four isotopes. 0.5600% of X has a mass of 83.91343 amu. The second isotope, 9.860% of X, has a mass of 85.90927 amu. our third isotope, 7.000% of X, has a mass of 86.90890 amu and our last isotope, 82.58% of Find the average atomic mass of element X.

You have four different isotopes, that's our data. That 0.5600% of our certain amount of mass. This is what we call percent abundance, where we have a certain amount of percent of our first isotope of a mass. As you see, it changes for each isotope; However, if we add all the percentages, it is always one hundred percent. In this example, you are finding your average atomic mass. The first step is to convert all percentages to decimals by dividing by 100. The first is 0.5600. We'll divide that by 100. And you'll get 0.005600. There is a trick to this. Instead, what we can do is move the decimal place twice to the left for each percentage to get our decimal form and we'll see that we continue doing this and it gives us our answer.

Moving on, we'll jump to our actual percentage abundance formula where our atomic mass is equal to the percentage converted to decimal, multiplied by the mass of that specific isotope and we'll continue adding each different type of isotope. Since we have four isotopes, I went ahead and added our first decimal form of that percentage multiplied by the mass of that first isotope and kept doing that for each isotope since we have four. Looking at this first part, we will multiply 0.005600 by the mass of 83.91343 and that gives us 0.469915. Then we would do this for each isotope and you would get this.

We will add them all, that will give us 87.616626 amu. The reason we're going to round to just four sig figures is because going back to our die, we actually only had four significant figures, 82.58 is four sig figures, same with 7,000 and you'll see that each one actually has four sig figures. figs. So we'll round up and our final atomic mass is 87.62 amu. In the second example, we are solving for percent composition or percent abundance, we are no longer solving for average atomic mass. Natural copper is made up of Cu at 63, this is just the isotopic notation, where our mass is equal to 62.9296 amu and our second isotope, copper at 65, whose mass is 64.9278 amu, quick note, they are just rounding here.

This is the mass number but they give us the exact amount of mass. Continuing with this question, with an average atomic mass of 63,546 amu. What is the percentage abundance of copper in terms of these two isotopes? You already have the two different types of isotopes, so we're really just focusing on our masses that are exact in this case. This is just the proper notation they normally have, but as mentioned, we're just focusing on these masses. They also give us the average atomic mass of 63.546 amu and ask us to find the percentage composition of copper. What they are really referring to here is how much of this isotope, what percentage, is within our atomic mass?

So we're really looking at two different percentages for our two different isotopes. Using our percentage abundance formula once again, our atomic mass is equal to that percentage, however, we do not know what the percentage of our first isotope is. So in that case, we'll just refer to this as X. I also went ahead and put in what we do know, what that atomic mass was. So referring to this percentage as X is just a way to be able to calculate what X is towards the end. Our mass of our first isotope was this. And then once again, we don't know what the second percent of our isotope is.

So what we're going to do is subtract it from 1. What 1 actually refers to is, it's outside of a hundred percent, so if we don't know what X is, then we're going to subtract it from 1 or a hundred percent and that would give us so our second percentage of our isotope. We will multiply this by our mass of our second isotope. Then, getting back to the math, let's go ahead and distribute this mass to both the one and that negative X. When we do this we end up getting this, and you want to group your X's, so we'll combine these two X. values.

We are actually subtracting since this is negative 64.9278. When we do that, we end up getting negative 1.9982x. From there we want to isolate our value of x. So we'll subtract that 64.9278 from both sides. These would then cancel out and subtracting these two would give us a negative 1.3818. And we want to isolate that These would then cancel out and our x value gives us a positive value of 0.6915. That was just for our first isotope, which was copper at 63. For our second isotope, what we're going to do is, taking that 1 minus . The next step is to multiply both decimals by 100.

The reason is that we want it to be a percentage, so multiplying it by 100 makes it our percentages. We will see that our first isotope, copper 63, has a percentage of 69.15%. Our second isotope, where copper 65, has a percentage of 30.85%. These are our percentage compositions of our isotopes. Now that we've worked through two possible test questions, hint, hint, it's your turn to see if you're ready for the exam. Master of music! So are you ready for your exam? As a tutor, I always tell my students to do these practice problems, to learn them, because I see them on exams over and over again.

Teachers like to reuse questions from their exams, so if you feel like you're not ready and need more help, check the description box below and you can find my available tutoring times. And be sure to LIKE, SUBSCRIBE and I'll see you next time.