1.4: Isotopes
Isotopes
• Atoms of the same element that have a different
number of neutrons (protons and electrons remain
unchanged)
• Example: Carbon-12 and Carbon-14 are two
isotopes of carbon.
• They both have 6 protons and 6 electrons.
• Carbon-12 has 6 neutrons,
• Carbon-14 has 8 neutrons.
Isotopes
• Isotopic Abundance – The percentage of a given isotope in
a sample of an element
• For example, natural magnesium is a mixture of three
isotopes:
• Magnesium-24 (78.7% abundance),
• Magnesium-25 (10.1%), and
• Magnesium-26 (11.2%).
• Every single atom of Mg does not have the exact same
mass!!
• The atomic mass of an element on the periodic table is
actually an average!
Isotopes
• Mass Spectrometer – an instrument used to determine the
mass and abundance of Isotopes
• Atomic Mass Unit (amu) – approximately the mass of one
proton or neutron, which is 1/12 the mass of a carbon-12
atom.
• Unit symbol = µ
Isotopes
Application: Carbon-14 Dating
• C-14 loses those extra neutrons and becomes C-12. This is
called radioactive decay. It happens regularly, like a clock.
For carbon, this decay is relatively short (a few thousand
years).
• Some elements take longer and others have a decay that
happens over a period of minutes.
Isotopes
Calculations
Determining the Atomic Mass of Elements
Atomic mass =
% abundance of isotope 1 (mass of isotope 1) +
% abundance of isotope 2 (mass of isotope 2) …
** convert percentages into decimals!!
Isotopes
Calculations
Example: Calculate the atomic mass of magnesium. The
isotopic abundances are:
Magnesium-24 78.7%
Magnesium-25 10.1%
Magnesium-26 11.2%
It was found that an element was composed of isotopes with
atomic masses 29.0μ, 30.0μ and 31.0μ. Isotope-29 has a
15% abundance and isotope-31 has a 58% abundance.
What is the average atomic mass of the element?
An element is composed of 30% of an isotope with an atomic
mass of 57.0μ. If the other isotope contains 3 more neutrons
per atom, what is the average atomic mass of the element?
Two isotopes have masses of 120μ and 124μ. If the relative
abundance of the more massive isotope is 5 times that of the
less massive, determine the average atomic mass of the
element.
Read pages 23 – 29 and answer
questions1-9 on page 29.