UNIT # 3 Introduction Objectives 3.1 Structure of the Atom 3.2 3.3 Arrangement of Electrons 3.4 3.5 Atomic Mass Problems 1 | 2 | 3

# ATOMIC STRUCTURE 3.5 - Atomic Mass

3.5.2 - Calculate the Atomic Mass of Carbon

Carbon, as we find it in our universe, does not exist as solely as 12C. There is another naturally occurring isotope of carbon, 13C. 13C also contributes to the atomic mass of carbon.

What about carbon-14?
Are you wondering why we don't include the contribution of 14C to the atomic mass of carbon?

Let's go through the steps to calculate the atomic mass of carbon. It's a 4-step process.

Step 1

We need to address the following questions:

1. "How many naturally occurring isotopes does carbon have?"
2. "What is the abundance of each of the isotopes?"

Step 2

Go and find the information needed for step 1.

##### Ordinarily, you would take a stroll to your library to dig out the information. Since you are on-line already, let's go to the World Wide Web (WWW). We will tap into the WebElements database at the University of Sheffield in England. So, depending on the time of day, Internet traffic may be heavy. Be patient, it might take a moment.

Step 3

• If you are unable to follow the above link, try again later. I've summarized the isotopic data that you should have retrieved from the site.
• If you are able to follow the above link, check that your finding is similar with what I summarized below. Depending on the source, some isotopic measurements are more accurate and will have more significant figures.
 Naturally occurring isotope Fractional abundance Mass (amu) 11C 0 11.011430 12C 0.989 12.000000 13C 0.011 13.003355 14C 0 14.003241

Points to note about the isotopic data:
1. There are two naturally occurring isotopes of carbon:
• 12C
• 13C
So these are the masses that will contribute to the atomic mass of carbon.
2. From the fraction of abundance column, note that there are much more 12C isotopes than 13C isotopes. (i.e. 98.9% for 12C and 1.1% for 13C)
3. The sum of all the fractions of abundance of each naturally occurring isotopes (ie - add up column 2) should equal 1.0000 or 100%. (i.e. 98.9 % + 1.1 % = 100%)
4. Look at the mass of 12C. This is defined to be 12.000000 amu. Now look at 13C, it is,as expected, heavier than 12C. All the masses reported here have 8 significant figures!!

Step 4

Let's calculate the atomic mass of carbon using this data.

 atomic mass of carbon = (0.989)(12.000000 amu) + (0.011)(13.003355 amu) = 11.8680 amu + 0.1430369 amu = 12.011 amu ( the atomic mass of Carbon as found on the periodic table )

Now you can try calculating the atomic mass for other elements by clicking on the buttons to get isotopic data.
Of course, you can check your answers by looking up the atomic masses of Li, O, and Mg in the periodic table.

Click on the at the top of the page and find periodic table icon, .

Section 4.5
Isotopes ..p86

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