Isotopes

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ISOTOPES
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Isotopes are

• atoms that have the SAME number of PROTONS, but a
  DIFFERENT number of NEUTRONS ? isotopes have the
  SAME atomic number, but DIFFERENT mass numbers
• For example
• H-1 vs H-2 vs H-3
• Cl-35 vs Cl-37

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Note that

• The CHEMICAL properties of isotopes are
  INDISTINGUISHABLE
• Chemical properties are determined by the
  arrangement of an atoms electrons
• There is NO difference in the SIZE of the atoms
  of isotopes of an element
• The size of an atom is determined by the volume
  in which the electrons move

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• The PHYSICAL properties of isotopes are DIFFERENT

• An ice cube of heavy ice sinks in a beaker of
  ordinary water because it is DENSER than ordinary
  water (ie the hydrogen atoms of the ice have more
  neutrons thus each nucleus has more mass than
  normal water)

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Isotopic Abundances

• Most elements have more than one NATURALLY
  occurring isotope
• e.g., hydrogen has three isotopes

Hydrogen-1 (Protium)
Hydrogen-2 (Deuterium)
Hydrogen-3 (Tritium)

• The isotopes of hydrogen are the ONLY isotopes
  with special names
• The identity of the isotope is indicated by
  writing the name of the element followed by the
  mass number

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• Each isotope of an element is usually NOT present
  in EQUAL amounts

Hydrogen-3 (Tritium)
Hydrogen-1 (Protium)
Hydrogen-2 (Deuterium)
99.985
0.015
0
The relative amount in which each isotope of an
element in nature is referred to as its ISOTOPIC
ABUNDANCE which is expressed as either a percent
or a decimal fraction
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• Because an element will NATURALLY occur as a
  mixture of its different isotopes average atomic
  masses are recorded on the Periodic Table

The atomic mass of an element as listed on the
periodic table is an AVERAGE based on all of its
isotopes and their abundances.
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Example 1 -- Calculating the Average Atomic Mass

• Magnesium has three naturally occurring isotopes.
  The mass and percent abundance of each isotope
  are listed below
• Isotopic Mass Natural Abundance
• 23.9850 amu 78.99
• 24.9858 amu 10.00
• 25.9826 amu 11.01
• Calculate the atomic mass of magnesium.

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Example 2 -- Calculating the Abundance of an
Isotope

• The average atomic mass of boron, B, is 10.81
  amu. Boron has only two stable isotopes,
  boron-10 and boron-11 with masses of 10.00 amu
  and 11.00 amu, respectively. What is the
  percentage abundance of boron-11?

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Radioisotopes are

• UNSTABLE isotopes that decay, releasing energy
  and subatomic particles
• The decay process is referred to as RADIOACTIVITY

All isotopes beyond bismuth are unstable and
radioactive because repulsive forces between the
protons in the nucleus cannot be moderated.
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• Extensive experimentation has revealed
  radioisotopes give off three types of radiation
  upon decay
• Alpha particles (?) which
• are composed of 2 protons and 2 neutron but NO
  electrons ? is equivalent to a helium nucleus
• i.e.

• have variable yet relatively slow speed so upon
  decay they penetrate the air only a few
  centimeters so a sheet of paper is an effective
  barrier

i.e. Uranium 238 will undergo alpha emission as
it starts to decay yielding thorium -234
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• Beta particles (ß) which
• are high energy electrons thus carry a 1 charge
• i.e.

• have variable yet relatively fast speed so upon
  decay they penetrate the air a few meters so a
  piece of metal 1-2 mm thick is an effective
  barrier

i.e. tritium decay will undergo beta emission as
it starts to decay yielding helium-3
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• Gamma rays (?) which
• are high energy electromagnetic waves

• have variable fast speeds so upon decay they have
  an unlimited penetration of the air so a piece of
  lead or concrete 1m thick is needed as an
  effective barrier

i.e. plutonium-240 decay will undergo gamma
emission as it starts to decay
Note there is no change to the mass number or
atomic number of the isotope with gamma emissions
but other radioactive changes do occur to
eventually change the nucleus
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• It should be noted that when radioisotopes decay
  they form an element below it in the periodic
  table that may or may not be stable itself. As a
  result, a radioactive decay series (multiple
  alpha and beta emissions) can result.

The time reference for each is its half-life.
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Half-Life
Every radioisotope has a characteristic property
called its half-life.

• is the time required for half the sample of a
  radioactive isotope to decay
• i.e. time needed for ½ the original radioactive
  atoms to decay
• Varies from 5 x 1015 years for Cs-142 to 0.16 s
  for Po-216
• indicates the relative stability of a radioactive
  isotope
• Ie. the larger the half-life the more stable the
  isotope
• is dependent upon the neutron to proton ratio

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Element isotopes having a neutron to proton ratio
of approx 1 are generally stable isotopes.
Ratios exceeding 1 generally result in
instability and radioactivity. Atoms having high
proton counts also have high neutron count.
Their resultant ratios in many cases is
significantly greater than 1 which leads to
instability and radioactive decay for high mass
atoms and their isotopes!
Homework page 32 questions 9 to 14