Question

average atomic mass
the bottom number in an element’s box in the periodic table represents its atomic
mass. we know that the atomic mass is measured in atomic mass units (amu),
and represents the total mass of the atom (mostly protons and neutrons).
the atomic mass of oxygen is 15.9994 amu why isn’t it just 16, or some other
whole number? this number is actually the average atomic mass of all of the
known isotopes of oxygen that are found in nature.
oxygen actually has three naturally - occurring isotopes. we can calculate the
average atomic mass of oxygen by taking the individual mass of each isotope and
multiplying it by the natural abundance. the natural abundance is the percent of all known atoms that
match that isotope.

isotope	atomic mass (amu)	natural abundance
oxygen - 17	16.99	0.038%
oxygen - 18	17.99	0.205%

15.995 × 0.9975 =
16.999 × 0.00038 =
17.999 × 0.00205 =
add together →
average atomic mass of
oxygen
practice
calculate the average atomic mass for each set of isotopes.

1. carbon is the backbone of all organic molecules, including sugars and proteins. 98.93% of carbon

occurs as carbon - 12, and 1.07% occurs as carbon - 13.

1. there is more hydrogen present in the universe than any other element. 99.99% of all hydrogen is

the hydrogen - 1 isotope. the other 0.01% is hydrogen - 2, also called deuterium.

Explanation:

Step1: Calculate for Carbon-12

The atomic mass of Carbon - 12 is 12 AMU (standard value, since Carbon - 12 is defined as having a mass of 12 AMU for reference) and its natural abundance is 98.93% or 0.9893 in decimal. So we calculate the contribution of Carbon - 12 to the average atomic mass: \(12\times0.9893 = 11.8716\)

Step2: Calculate for Carbon-13

The atomic mass of Carbon - 13 is 13 AMU (approximate value, as the mass of Carbon - 13 is close to 13 AMU) and its natural abundance is 1.07% or 0.0107 in decimal. The contribution of Carbon - 13 is: \(13\times0.0107=0.1391\)

Step3: Sum the contributions

To find the average atomic mass of carbon, we add the contributions from Carbon - 12 and Carbon - 13: \(11.8716 + 0.1391=12.0107\)

Step4: Calculate for Hydrogen - 1

The atomic mass of Hydrogen - 1 is 1 AMU and its natural abundance is 99.99% or 0.9999 in decimal. The contribution of Hydrogen - 1 is: \(1\times0.9999 = 0.9999\)

Step5: Calculate for Hydrogen - 2

The atomic mass of Hydrogen - 2 (deuterium) is 2 AMU and its natural abundance is 0.01% or 0.0001 in decimal. The contribution of Hydrogen - 2 is: \(2\times0.0001=0.0002\)

Step6: Sum the contributions for Hydrogen

The average atomic mass of hydrogen is the sum of the contributions from Hydrogen - 1 and Hydrogen - 2: \(0.9999+0.0002 = 1.0001\)

Answer:

1. Average atomic mass of carbon: \(\boldsymbol{12.0107}\) AMU
2. Average atomic mass of hydrogen: \(\boldsymbol{1.0001}\) AMU