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Early Atomic Theory Chapter 5
Larry Emme
Chemeketa Community College
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Early Thoughts
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• The earliest models of the atom were developed by
  the ancient Greek philosophers.

Leucippus of Miletus (490-??? B.C.). First to
introduce the idea of the atom, an indivisible
unit of matter. This idea was later extended by
his student, Democritus.

• Democritus (about 470-370 B.C.) thought that
  all forms of matter were made of tiny particles
  called atoms from the Greek atomos
  indivisible.

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According to Democritus atoms are

• Unchangeable and indivisible.
• Identical except for their size and shape.
• Always in motion.

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Democritus imagined that atoms of iron were
shaped like coils, making iron rigid, strong, and
malleable. Atoms of fire were sharp, lightweight,
and yellow.
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• Aristotle (384-322 B.C.) rejected the theory of
  Democritus and endorsed that of Empedocles that
  stated that matter was made of 4 elements air,
  earth, fire , and water.

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• Empedocles (492-432 B.C.) believed that these
  elements have always existed in fixed amounts,
  and that there two major forces which act upon
  these elements to both create and destroy Love
  and Strife. According to legend, he died by
  falling into a volcano's crater after failing to
  become a god as he predicted.

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• Aristotles influence dominated the thinking of
  scientists and philosophers until the beginning
  of the 17th century

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Alchemical Symbols
bismuth
antimony
arsenic
iron
gold
copper
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Alchemical Symbols
magnesium
mercury
phosphorus
potassium
silver
platinum
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Alchemical Symbols
sulfur
tin
zinc
lead
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Daltons Modelof the Atom
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2000 years after Aristotle, John Dalton, an
English schoolmaster, proposed his model of the
atomwhich was based on experimentation.
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Daltons Atomic Theory

1. Elements are composed of minute indivisible
   particles called atoms.

1. Atoms of the same element are alike in mass and
   size.
2. Atoms of different elements have different masses
   and sizes.
3. Chemical compounds are formed by the union of two
   or more atoms of different elements.

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Daltons Atomic Theory

1. Atoms combine to form compounds in simple
   numerical ratios, such as one to one, two to two,
   two to three, and so on.

1. Atoms of two elements may combine in different
   ratios to form more than one compound.

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Daltons atoms were individual particles.
Atoms of each element are alike in mass and size.
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Daltons atoms were individual particles.
Atoms of different elements are not alike in mass
and size.
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Daltons atoms combine in specific ratios to form
compounds.
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Composition of Compounds
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The Law of Definite Composition

• A compound always contains two or more elements
  combined in a definite proportion by mass.

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Composition of Water

• Water always contains the same two elements
  hydrogen and oxygen.

• The percent by mass of hydrogen in water is
  11.2.
• The percent by mass of oxygen in water is 88.8.
• Water always has these percentages. If the
  percentages were different the compound would not
  be water.

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Composition of Hydrogen Peroxide

• Hydrogen peroxide always contains the same two
  elements hydrogen and oxygen.

• The percent by mass of hydrogen in hydrogen
  peroxide is 5.9.
• The percent by mass of oxygen in hydrogen
  peroxide is 94.1.
• Hydrogen peroxide always has these percentages.
  If the percentages were different the compound
  would not be hydrogen peroxide.

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The Law of Multiple Proportions

• Atoms of two or more elements may combine in
  different ratios to produce more than one
  compound.

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Combining Masses of Hydrogen and Oxygen
Mass Hydrogen(g) Mass Oxygen(g)
Water 1.0 8.0
Hydrogen Peroxide 1.0 16.0
Hydrogen peroxide has twice as much oxygen (by
mass) as does water.
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Combining Ratios of Hydrogen and Oxygen

• Hydrogen peroxide has twice as many oxygens per
  hydrogen atom as does water.

• The formula for water is H2O.
• The formula for hydrogen peroxide is H2O2.

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The Nature of Electric Charge
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Properties of Electric Charge

• Charge may be of two types positive and
  negative.
• Unlike charges attract (positive attracts
  negative), and like charges repel (negative
  repels negative and positive repels positive).
• Charge may be transferred from one object to
  another, by contact or induction.
• The less the distance between two charges, the
  greater the force of attraction between unlike
  charges (or repulsion between identical charges).

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Discovery of Ions
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• Michael Faraday discovered that certain
  substances, when dissolved in water, conducted an
  electric current.

• He found that atoms of some elements moved to the
  cathode (negative electrode) and some moved to
  the anode (positive electrode).
• He concluded they were electrically charged and
  called them ions (Greek wanderer).

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Michael Faraday
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• Svante Arrhenius reasoned that an ion is an atom
  (or a group of atoms) carrying a positive or
  negative electric charge.

• Arrhenius accounted for the electrical conduction
  of molten sodium chloride (NaCl) by proposing
  that melted NaCl dissociated into the charged
  ions Na and Cl-.

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NaCl ? Na Cl-

• In the melt the positive Na ions moved to the
  cathode (negative electrode). Thus positive ions
  are called cations.
• In the melt the negative Cl- ions moved to the
  anode (positive electrode). Thus negative ions
  are called anions.

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Svante Arrhenius
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Subatomic Partsof the Atom
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An atom is very Small
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The diameter of an atom is 0.1 to 0.5 nm.
This is 1 to 5 ten billionths of a meter.
If the diameter of this dot is 1 mm then 10
million hydrogen atoms would form a line across
the dot.
Even smaller particles than atoms exist. These
are called subatomic particles.
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Subatomic Particles
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Electron
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In 1875 Sir William Crookes invented the Crookes
tube.
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• Crookes tubes experiments led the way to an
  understanding of the subatomic structure of the
  atom.

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• Crookes tube emissions are called cathode rays.
• Below are Crookes cathode-ray tubes. The
  cathode-rays (streams of electrons) can be
  clearly seen.

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"Maltese Cross" Crookes Tube Demonstrates that
radiant matter is blocked by metal objects
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Other Interesting Crookes Tubes May Be Found At
the Sites Below

• http//www.sparkmuseum.com/GLASS.HTM
• http//www.oneillselectronicmuseum.com/page9.html

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In 1897 Sir Joseph Thomson demonstrated that
cathode rays

• travel in straight lines.
• are negative in charge.
• are deflected by electric and magnetic fields.
• produce sharp shadows
• are capable of moving a small paddle wheel.

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Paddle Wheel
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Thomsons Apparatus
batteries
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Thomsons Lab
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J.J. Thomson determined and is given credit for
finding

• The charge to mass (e/m) ratio of the cathode
  ray.
• The cathode ray was re-named the electron.
• Thomson discovered the electron.

http//www.aip.org/history/mod/fission/fission1/01
.html
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Can atoms be split apart? Does each atom have
inner workings? Parts which can be separated?
Parts which can perhaps be put to some use? These
questions had already come to mind in 1898, when
J. J. Thomson isolated the electron. That was the
first solid proof that atoms are indeed built of
much tinier pieces. Thomson speaks of the
electron in this recorded passage...
Could anything at first sight seem more
impractical than a body which is so small that
its mass is an insignificant fraction of the mass
of an atom of hydrogen, which itself is so small
that a crowd of these atoms equal in number to
the population of the whole world would be too
small to have been detected by any means then
known to science.
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Robert Millikan

• Determined the charge of the electron.
• Experiment called the Oil Drop Experiment.

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Oil Drop Apparatus
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Apparatus Used by Millikan
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Modern Apparatus
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Proton
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• Eugen Goldstein, a German physicist, first
  observed protons in 1886

• Thomson determined the protons characteristics.
• Thomson showed that atoms contained both positive
  and negative charges.
• This disproved the Dalton model of the atom which
  held that atoms were indivisible.

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Thomsons Plum-Pudding Model of the Atom
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Neutron
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• James Chadwick discovered the neutron in 1932.

• Its actual mass is slightly greater than the mass
  of a proton.

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Ions
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• Positive ions were explained by assuming that a
  neutral atom loses electrons.

• Negative ions were explained by assuming that
  extra electrons can be added to atoms.

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When one or more electrons are lost from an atom,
a cation is formed.
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When one or more electrons are added to a neutral
atom, an anion is formed.
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The Nuclear Atom
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X-rays were discovered by Wilhelm Röentgen in 1895
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• Röentgen observed that a vacuum discharge tube
  enclosed in a thin, black cardboard box had
  caused a nearby piece of paper coated with the
  salt barium platinocyanide to phosphorescence.
• From this and other experiments he concluded that
  certain rays, which he called X-rays, were
  emitted from the discharge tube, penetrated the
  box, and caused the salt to glow.

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• Radioactivity was discovered by Henri Becquerel
  in 1896.

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• Shortly after Röentgens discovery, Antoine Henri
  Becquerel attempted to show a relationship
  between X-rays and the phosphorescence of uranium
  salts.

• Becquerel wrapped a photographic plate in black
  paper, sprinkled a sample of a uranium salt on
  it, and exposed it to sunlight.

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• When Becquerel attempted to repeat the experiment
  the sunlight was intermittent.

• He took the photographic plate wrapped in black
  paper with the uranium sample on it, and placed
  the whole setup in a drawer.

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• Several days later he developed the film and was
  amazed to find an intense image of the uranium
  salt on the plate.

• He repeated the experiment in total darkness with
  the same result.
• This proved that the uranium salt emitted rays
  that affected the photographic plate, and that
  these rays were not a result of phosphorescence
  due to exposure to sunlight.

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• Two years later, in 1898, Marie Curie coined the
  name radioactivity.

Radioactivity is the spontaneous emission of
particles and/or rays from the nucleus of an atom.
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Marie Curie, in a classic experiment, proved that
alpha and beta particles are oppositely charged.
radiation passes between the poles of an
electromagnet
Gamma rays are not deflected by the magnet.
Alpha rays are less strongly deflected to the
negative pole.
a radioactive source was placed inside a lead
block
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The Rutherford Experiment
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Ernest Rutherford
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• In 1899 Rutherford began to investigate the
  nature of the rays emitted by uranium.

• He found two particles in the rays. He called
  them alpha and beta particles.

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• Rutherford in 1911 performed experiments that
  shot a stream of alpha particles at a gold foil.

• Most of the alpha particles passed through the
  foil with little or no deflection.
• He found that a few were deflected at large
  angles and some alpha particles even bounced back.

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Rutherfords alpha particle scattering experiment.
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• An electron with a mass of 1/1837 amu could not
  have deflected an alpha particle with a mass of 4
  amu.

• Rutherford knew that like charges repel.
• Rutherford concluded that each gold atom
  contained a positively charged mass that occupied
  a tiny volume. He called this mass the nucleus.

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• If a positive alpha particle approached close
  enough to the positive mass it was deflected.

• Most of the alpha particles passed through the
  gold foil. This led Rutherford to conclude that
  a gold atom was mostly empty space.

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• Because alpha particles have relatively high
  masses, the extent of the reflections led
  Rutherford to conclude that the nucleus was very
  heavy and dense.

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Deflection and scattering of alpha particles by
positive gold nuclei.
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Ideas about the atom were refined by one of
Thomson's students, Ernest Rutherford. He showed
that the mass in an atom is not smeared out
uniformly throughout the atom, but is
concentrated in a tiny, inner kernel the
nucleus. Rutherford wanted to understand the
nucleus, not for any practical purpose, but
because he was attracted to the beauty of its
simplicity. Fundamental things should be simple
not complex. Here is how he explains himself in
1931...
The bother is that a nucleus, as you know, is a
very small thing, and we know very little about
it. Now, I had the opinion for a long time,
that's a personal conviction, that if we knew
more about the nucleus, we'd find it was a much
simpler thing than we suppose, that these
fundamental things I think have got to be fairly
simple. But it's the non-fundamental things that
are very complex usually. I am always a believer
in simplicity being a simple person myself.
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• The gamma ray, a third type of emission from
  radioactive material, was discovered by Paul
  Villard in 1900.

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Alpha, Beta, and Gamma Radiation
Name Nuclide Symbol Particle Symbol Mass (amu) Charge
Alpha ? 4 2
Beta ? 1
Gamma Ray 0 0
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General Arrangement of Subatomic Particles
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• Rutherfords experiment showed that an atom had a
  dense, positively charged nucleus.

• Chadwicks work in 1932 demonstrated the atom
  contains neutrons.
• Rutherford also noted that light, negatively
  charged electrons were present in an atom and
  offset the positive nuclear charge.

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• Rutherford put forward a model of the atom in
  which a dense, positively charged nucleus is
  located at the atoms center.

• The negative electrons surround the nucleus.
• The nucleus contains protons and neutrons

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Atomic Numbers of the Elements
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• The atomic number of an element is equal to the
  number of protons in the nucleus of that element.

• The atomic number of an atom determines which
  element the atom is.

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Every atom with an atomic number of 1 is a
hydrogen atom.Every hydrogen atom contains 1
proton in its nucleus.
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Every atom with an atomic number of 6 is a carbon
atom.Every carbon atom contains 6 protons in
its nucleus.
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1H
Every atom with an atomic number of 1 is a
hydrogen atom.
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6C
Every atom with an atomic number of 6 is a carbon
atom.
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92U
Every atom with an atomic number of 92 is a
uranium atom.
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Isotopes of the Elements
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• Atoms of the same element can have different
  masses.

• They always have the same number of protons, but
  they can have different numbers of neutrons in
  their nuclei.
• The difference in the number of neutrons accounts
  for the difference in mass.
• These are isotopes of the same element.

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• Isotopes of the Same Element Have
• Equal numbers of protons
• Different numbers of neutrons

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Isotopic Notation
Mass number is also the number of nucleons in the
nucleus. Nucleons protons and/or neutrons
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Relationship Between Mass Number and Atomic
Number
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The mass number minus the atomic number equals
the number of neutrons in the nucleus.
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Isotopic Notation
6 protons 6 neutrons
12
C
6
6 protons
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Isotopic Notation
6 protons 8 neutrons
14
C
6
6 protons
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Isotopic Notation
8 protons 8 neutrons
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O
8
8 protons
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Isotopic Notation
8 protons 9 neutrons
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O
8
8 protons
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Isotopic Notation
8 protons 10 neutrons
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O
8
8 protons
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Hydrogen has three isotopes
1 proton 0 neutrons
1 proton 1 neutron
1 proton 2 neutrons
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• Examples of Isotopes
• Element Protons Electrons Neutrons Symbol
• Hydrogen 1 1 0
• Hydrogen 1 1 1
• Hydrogen 1 1 2
• Uranium 92 92 143
• Uranium 92 92 146
• Chlorine 17 17 18
• Chlorine 17 17 20

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Atomic Weight
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• The mass of a single atom is too small to measure
  on a balance.

• Using a mass spectrometer, the mass of the
  hydrogen atom was determined.

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A Modern Mass Spectrometer
From the intensity and positions of the lines on
the mass spectrogram, the different isotopes and
their relative amounts can be determined.
A mass spectrogram is recorded.
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A typical reading from a mass spectrometer. The
two principal isotopes of copper are shown with
the abundance () given.
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• Using a mass spectrometer, the mass of one
  hydrogen atom was determined to be 1.673 x 10-24 g

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This number is very small.
small
small
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small
small
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small
small
small
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small
small
small
small
small
small
small
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The mass of a hydrogen atom is very small.
Numbers of this size are too small for practical
use.
To overcome this problem a system of relative
atomic weights using atomic mass units was
devised to express the masses of elements using
simple numbers.
1.673 x 10-24 g
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The standard to which the masses of all other
atoms are compared to was chosen to be the most
abundant isotope of carbon.
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• A mass of exactly 12 atomic mass units (amu) was
  assigned to

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• 1 amu is defined as exactly equal to the
  mass of a carbon-12 atom

1 amu 1.6606 x 10-24 g
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• Average atomic weight 1.00797 amu.

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• Average atomic weight 39.098 amu.

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• Average atomic weight 248.029 amu.

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Average RelativeAtomic Weight
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• Most elements occur as mixtures of isotopes.

• Isotopes of the same element have different
  masses.
• The listed atomic mass of an element is the
  average relative mass of the isotopes of that
  element compared to the mass of carbon-12
  (exactly 12.0000amu)

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• To calculate the atomic mass multiply the atomic
  mass of each isotope by its percent abundance and
  add the results.

Isotope Isotopic mass (amu) Abundance () Average atomic mass (amu)
62.9298 69.09
64.9278 30.91
63.55
(62.9298 amu)
0.6909
43.48 amu
(64.9278 amu)
0.3091
20.07 amu
63.55 amu
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Isotope Practice(Fill-in the Blanks)
symbol atomic no mass no e n p
8 16 10
Pt 117
30P3
53 74
48 36
34 45
40Ca2
8
8
78
195
78
78
15
18
15
15
30
127
53
53
36
36
84
34
79
34
20
40
18
20
20
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The End