Download “Plum Pudding” model

Atomic Structure
Syllabus Ref: 7.1—
7.1—The atom
7.1.1: Describe a model of the atom that
features a small nucleus surrounded by
electrons.
7.1.2: Outline the evidence that supports a
nuclear model of the atom
7.1.3: Outline one limitation of the simple model
of the nuclear atom.
7.1.4: Outline evidence for the existence of
atomic energy levels. (emission/absorption
spectra)
“Plum Pudding” model
Electron discovery in
1890’s (J.J. Thomson)
 Atomic structure
First visualized as a
homogenous sphere
of positive charge w/
negative charges
within. Like a plum
pudding.
Rutherford, Geiger, Marsden
19111911-Shot a beam of α-particles at a
thin gold foil.
Most passed through unaffected or
deflected at small angles
– Atom must be mostly empty space
Some deflected at very large angles!
– Must be a small, dense, positively
charged “nucleus”
nucleus”
Rutherford model
Further electron scattering
showed the nucleus to have a
very small radius
– r ~ (A1/3) 1.2 x 10-15 m
– Compare to 10-10 m for atom
“Planetary”
Planetary” or “Nuclear”
Nuclear” model
Positive nucleus containing 99.9% of the
mass of the atom
Surrounded at a great distance by orbiting
electrons
– 666,000 times the scale of the solar system
Not a perfect model
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Problems with planetary model
Orbiting electrons are accelerating
and should give off EM radiation
– Should decrease energy and spiral into
the nucleus (in a few ns).
– Frequency should increase also
Should observe (but don’
don’t):
– Continuous spectrum of radiation
Actually find discrete spectra
– Unstable atoms
Many are stable
Atomic Spectra
Spectral Lines
Emission and
absorption
spectra
Atomic Spectra
Balmer and Rydberg discovered
mathematical relationship
Niels Henrick David Bohr model
Planetary model is
good, but should
incorporate the
recent quantum
theories of Planck
and Einstein, and
should explain
atomic spectra.
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Bohr Model Postulates:
Perhaps electrons
only move in
quantum fashion
from one “orbital”
orbital”
to another and they
give discrete
amounts of energy
when they do this
Bohr Model:
Complies with observed spectra of
Hydrogen and made predictions of
lines later to be discovered
Assumptions:
– Fixed electrons do not emit radiation
– Angular momentum is quantized
– Can’
Can’t explain how electrons move from
one energy state to the next
Won Nobel Prize in 1922 (37 yrs old!)
Limitations of Bohr’s
Model
Doesn’t work with atoms with
multiple electrons
Didn’t explain relative brightness
of lines
Didn’t explain bonding
No experimental evidence for the
postulates
Does the nucleus have
structure?
Complicated, still not entirely
understood
30,000 carbon nuclei would stretch
across a single carbon atom
1930’s, understood to be made of
both protons and neutrons
Protons and Neutrons
Charge and Mass
~2000 times more massive than
electrons.
Essentially mass of the nucleus =
mass of the atom
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Vocabulary:
Nucleons
Nuclear Symbols
– constituents of the nucleus (protons and
neutrons)
Nuclides
– the different types of nuclei collectively
Atomic Number (Z)
– number of protons in the nucleus
Atomic Mass Number (A)
– total number of nucleons in the nucleus
Isotope
– Nuclei that contain the same number of
protons, but different amounts of neutrons
Isotopes and mass
Discovered w/ mass
spectrometers
Isotopes are atoms with different
numbers of neutrons
– Same chemical properties, different
mass and nuclear properties
Atomic Mass Unit (u)
Atomic Structure
Syllabus Ref: 7.1—
7.1—The atom
7.1.1: Describe a model of the atom that
features a small nucleus surrounded by
electrons.
7.1.2: Outline the evidence that supports a
nuclear model of the atom
7.1.3: Outline one limitation of the simple model
of the nuclear atom.
7.1.4: Outline evidence for the existence of
atomic energy levels. (emission/absorption
spectra)
– 1/12 of a neutral 12C atom
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