Download Lesson 3.2 Defining the Atom

10/4/2015
Unit 3: Atomic Structure - The Nucleus
Lesson 3.2: Defining The Atom
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Lesson 3.2 Defining the Atom
• Define atom.
• Distinguish between the subatomic particles in
terms of relative charge and mass.
atom, including the
• Describe the structure of the atom
locations of the subatomic particles.
model: a visual, verbal, and/or mathematical explanation
of data collected from many experiments
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Lesson 3.2 Defining the Atom
•Atom
•Cathode ray
•Electron
•Nucleus
•Proton
•Neutron
An atom is made of a nucleus containing
protons and neutrons; electrons move
around the nucleus.
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Sizing up the Atom
• Elements are able to be subdivided into smaller
and smaller particles – these are the atoms, and
they still have properties of that element
• The smallest particle of an element that
retains the properties of the element is called
an atom.
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The Atom
• If you could line up 100,000,000 copper
atoms in a single file, they would be
approximately 1 cm long
• An instrument called the scanning tunneling
microscope (STM) allows individual atoms to
be seen.
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Iron Atoms Seen Through a Scanning Tunneling Microscope
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• Silicon
Atoms : As
“felt” by an
Atomic
Force
Microscope
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Not Really Indivisible
Atoms are composed of three major “subatomic”
particles (even they are divisible further—but we
won’t really deal with that):
In the nucleus: Nucleons
protons: mass of 1 amu (1.66E-24 g),
charge of +1
neutrons: mass of 1 amu, charge of 0
Outside the nucleus:
electrons: negligible mass (1/1836 amu),
charge of -1
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Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
particle: the electron
• Modern Cathode Ray Tubes
Television
Computer Monitor
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 Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
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The Electron
• J.J. Thomson measured the effects of both
magnetic and electric fields on the cathode ray
to determine the charge-to-mass ratio of a
charged particle, then compared it to known
values.
• The mass of the charged particle was much
less than a hydrogen atom, then the lightest
known atom.
• Thomson received the Nobel Prize in 1906 for
identifying the first subatomic particle—the
electron
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Conclusions from the Study of the
Electron:
a) Cathode rays have identical properties regardless
of the element used to produce them. All elements
must contain identically charged electrons.
b) Atoms are neutral
neutral, so there must be positive
particles in the atom to balance the negative
charge of the electrons
c) Electrons have so little mass that atoms must
contain other particles that account for most of the
mass
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Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like
plums embedded in a positively charged
“pudding,” thus it was called the “plum pudding”
model.
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Mass of the Electron
• In the early 1910s, Robert Millikan used
the oil-drop apparatus shown below to
determine the charge of an electron.
Mass of the
electron is
9.11 x 10-28 g
http://g.web.umkc.edu/gounevt
/Animations/Animations211/Mil
likanOilDropExp.swf
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The Electron (cont.)
• Charges change in discrete amounts —1.602  10–19
coulombs, the charge of one electron (now equated
to a single unit, 1–).
• With the electron’s charge and charge-to-mass
ratio known,
known Millikan calculated the mass of a
single electron.
the mass of a
hydrogen
atom
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The Nucleus
• In 1911, Ernest Rutherford studied how positively
charged alpha particles interacted with solid matter.
• By aiming the particles at a
thin sheet of gold foil
foil,
Rutherford expected the
paths of the alpha particles
to be only slightly altered by
a collision with an electron.
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Rutherford’s Gold Foil Experiment
• http://www.mhhe.com/physsci/chemistry/ess
entialchemistry/flash/ruther14.swf
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Ernest Rutherford’s Gold Foil
Experiment - 1911

Alpha particles are helium nuclei - The alpha
particles were fired at a thin sheet of gold foil
 Particles that hit on the detecting screen
(film) are recorded
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The Nucleus (cont.)
• Rutherford concluded that atoms are mostly
empty space.
• Almost all of the atom's positive charge and almost
all of its mass is contained in a dense and
positive region in the center of the atom called the
nucleus.
• Electrons are held within the atom by their
attraction to the positively charged nucleus.
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The Nucleus (cont.)
• The repulsive force between the positively
charged nucleus and positive alpha particles
caused the deflections.
• R
Rutherford’s
h f d’ model
d l
of the atom = the
nuclear model
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The Nucleus (cont.)
• Rutherford refined the model to include positively
charged particles in the nucleus called protons.
• James Chadwick received the
Nobel Prize in 1935 for discovering
the existence of neutrons, neutral
particles in the nucleus which
accounts for the remainder of an
atom’s mass.
James Chadwick
(1891 ‐ 1974)
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Protons
 Discovered by Ernest
Rutherford (2nd)
 Every element has a different
number of protons
 Atomic Number = an
element’s
element
s number of protons
 The periodic table lists
elements by Atomic Number
 Nuclear charge = # of protons
in the nucleus
 ALWAYS POSITIVE!!!
Ernest Rutherford
(1871 ‐ 1937)
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The Nucleus (cont.)
• All atoms are made of three
fundamental subatomic
particles: the electron, the
proton, and the neutron.
• Atoms are spherically
shaped.
• Atoms are mostly empty
space, and electrons travel
around the nucleus held by
an attraction to the positively
charged nucleus.
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The Nucleus (cont.)
• Scientists have determined that protons and neutrons
are composed of subatomic particles called quarks.
• Chemical behavior can be explained by considering
only an atom's electrons.
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