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Isotopes
You have learned that the number of protons in an atom determines the identity of that atom. For example, all hydrogen atoms have one proton, all helium atoms have two protons, and all carbon atoms have six protons. You have also learned that not all atoms of an element have the same number of neutrons. Hydrogen atoms, for example, can have no neutron, one neutron, or two neutrons. These different hydrogen atoms are called isotopes of hydrogen. Isotopes are atoms of the same element that have different numbers of neutrons. The three isotopes of hydrogen are modeled in Figure 11. Note how the isotopes’ structures are similar and how they differ. Keep in mind that all atoms of the same element are isotopes. Isotopes are not a special kind of element or atom.
You can use atomic symbols to distinguish between isotopes, as shown in Table 2.
When you compare these symbols, you can see that isotopes differ in mass number but not in atomic number. The atomic number is the same for all three isotopes because the isotopes are all hydrogen atoms with the same proton number. Isotopes only differ from each other in the number of neutrons. The more neutrons an isotope has, the higher the mass number of that isotope is. This means that as the mass number goes up, the mass of the isotope increases.
Fill in the blank.
Atomic Mass
Chemists must be aware of isotopes when analyzing an element. This is because no element exists in nature in just one isotopic form. A sample of carbon taken from decaying plant matter, for example, contains 98.99% carbon-12 and 1.11% carbon-13. Other isotopes of carbon also exist in nature, but their abundance is less than 0.01%.
This distribution of isotopes in nature affects the way chemists measure the mass of one atom of an element. For each element, chemists use an average value called the atomic mass. The atomic mass of an element is an average mass based on the relative amounts and masses of the naturally occurring isotopes of that element.
For carbon, the atomic mass is 12.01 amu. This number comes from considering that both carbon-12 and carbon-13 isotopes are present in a carbon sample. A carbon-12 atom has a mass of 12.00 amu and represents 98.89% of the carbon sample. A carbon-13 atom has one more neutron than a carbon-12 atom, with a mass of 13.00 amu. Carbon-13 atoms represent 1.11% of the carbon sample. The average mass of the sample is determined according to the abundance of each isotope as shown below:
Average mass =
Therefore, the atomic mass of carbon is 12.01 amu.
For a particular element, you will often see the atomic mass listed with other information in a form like the example shown below. Be aware that mass number is a whole number showing the number of the nucleons of a particular isotope of an element, whereas the atomic mass is an average based on the natural abundance of isotopes for that element.
A student added the masses of one proton and one electron to find the atomic mass of hydrogen.
To find the atomic mass of hydrogen, the student has to consider
how abundant each hydrogen isotope in nature is.
the number of electrons of each hydrogen isotope.
the mass of each hydrogen isotope.
all of the isotopes of hydrogen found in nature.
Fill in the blank.
The
for an element is an average mass based on the relative amounts and masses of the naturally occurring isotopes of that element.
End of Topic
