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Structure of an Atom
Suggested Reading:
Pages 72 - 76
Structure & Size of an Atom
Atomic Particles
 Proton – a positively charged particle
found in the nucleus (about same
mass as a neutron – slightly less)
 Neutron – a nuclear particle that has
no electrical charge (most massive)
 Electron – negatively charged
particle surrounding the nucleus
(least massive)
Subatomic Particles
 Subatomic particles include quarks,
leptons, bosons, gluons, mesons,
muons, and others.
J. J. Thomson (1897)
Concluded that
cathode rays
were made of
identical
negatively
charged particles,
which were
named electrons.
Cathode Ray Tube
Negative
electrode
is called
the
cathode.
Positive
electrode
is called
the anode.
Rutherford’s Gold Foil
Experiment
Nucleus
 A very small region located at
the center of an atom.
Atoms vs. ions
 In a atom, the number of
electrons has to equal the
number of protons. Thus
making the atom “electrically
neutral”.
 Ions have either gained or lost
electrons.
 If gained the ion is negative in
charge, if lost the ion is positive.
Atomic Number
 The number of protons in an
atom.
 The atomic number is used to
identify the element.
Mass Number
 mass # = protons + neutrons
 always a whole number
 NOT on the
Periodic Table!
Nuclear Symbols
 6 Protons, 6 neutrons, 6 electrons
Mass #
Atomic #
Hyphen notation: carbon-12
12
6
C
*Complete Nuclear Symbols
 Atomic # = 17
 Mass # = 37
 # of protons = 17
 # of electrons = 17
 # of neutrons = 20
Hyphen notation: chlorine-37
37
17
Cl
Cathode Ray Tube
Electron beam is bent as it is pulled toward a
magnet … you can see it because it
ionizes a gas in the tube that glows.
Phosphorescent rings glow due to electric
current.
Electrons are forced through a small hole to
form a beam of electrons
Electrons emitted by cathode
Rutherford’s Gold Foil Experiment