Transcript Electrons in Atoms

Electrons in Atoms
Chapter 5…the truth about electrons
Atomic Models
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1803
Dalton's model was that
the atoms were tiny,
indivisible, indestructible
particles and that each one
had a certain mass, size,
and chemical behavior
that was determined by
what kind of element they
were.
J.J. Thomson
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“plum pudding” model
Discovered the
“electron”
Electron embedded in a
sphere of positive
electrical charge
1897
Hantaro Nagaoka
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1904
Suggested that an atoms has a central nucleus
No evidence to back claim
Rutherford’s experiment
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Gold foil and alpha
particles 1911
Alpha particle (+) and Gold foil
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The above diagram shows
what we would expect the
result of Rutherford's
experiment to be if the
"plum pudding" model of
the atom is correct.
The above indicates the
actual result. Most of the
alpha particles are only
slightly deflected, as
expected, but occasionally
one is deflected back
towards the source.
Rutherford conclusions
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Atom mostly empty space (most of the alpha
particles passed right through)
Tiny positive nucleus
Nucleus has large mass (alpha particle did not
move nucleus, it bounced back so mass must
be big)
Electrons move around nucleus
Niels Bohr “the old quantum theory”
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Electrons found in specific
circular paths, planetary
model
Energy levels, energy
increases to valence
electrons
Quantum is the amount of
energy it takes to move and
energy level
1913
Quantum
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These ladder steps are
some what like energy
levels. (a) In an ordinary
ladder, the rungs are
equally spaced. (b) The
energy levels in atoms are
unequally spaced, like the
rungs in this ladder. The
higher energy levels are
closer together so the
amount of energy is not
always the same as you
move.
Ground State vs. Excited State
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Ground State= lowest energy configuration
“normal”
Excited State=higher energy configurations
than normal
Q: Why does hamburger have lower energy
than steak?
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A: Because it's in the ground state.
Why Bohr was not perfect 
The Bohr model gave
results in agreement
with experiment for the
hydrogen atom.
However, it still failed
in many ways to
explain the energies
absorbed and emitted
by atoms with more
than one electron.
De Broglie’s particle-wave dulaity
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In 1924 de Broglie's proposed that all moving
particles has a wavelength is inversely proportional
to momentum and that the frequency is directly
proportional to the particle's kinetic energy ; λ = h/p
This concept leads to one of the most important
ideas in 20th century science:
The small, light, fast moving electron also exhibits
wave-particle duality. It can be conceived of as a
particle or as a wave.
This development leads to atomic and molecular
orbitals.
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Edwin Schrödinger knew of de Broglie's
proposal that a electrons exhibited waveparticle duality. With this idea in mind, he
devised/constructed a differential equation for
a wavelike electron resonating in three
dimensions about a point positive charge.
Erwin Schrodinger
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In the 1920s, a whole new theory of physics, called
quantum mechanics, presented an even more radical
picture of the atom. The electrons cannot be
pinpointed but exist as a sort of cloud of probability
outside the nucleus.
The airplane propeller is somewhere in the blurry
region it produces in this picture, but the picture does
not tell you its exact position at any instant.
Similarly, the electron cloud of an atom represents
the locations where an electron is likely to be found.
Quantum Mechanical Model
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Determines the allowed energies an electron
can have and how likely it is (probability!) to
find the electron in various locations around
nucleus. Mathematical theory!
n= the Principle Quantum Number or
Principle energy level (energy levels the
electrons are found in)
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THE DISCOVERY OF THE ELECTRON
Atomic Orbitals
Region in space with a high probability to
find an electron
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Energy Sublevel
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Each energy sublevel corresponds to an
orbital to a different shape.
Different atomic orbitals are denoted by
letters.
S= spherical, P= dumbbell D & F more
complex
Summary of Principal energy levels,
sublevels, & orbital's
Principal energy level
# of sublevels
Type of sublevels
n=1
1
1s (1 orbital)
n=2
2
2s(1orbital),
2p(3orbitals)
n=3
3
3s(1 orbital),
3p(3orbitals),
3d(5orbitals)
n=4
4
4s(1 orbital),
4p(3orbitals),
4d(5orbitals),
4f(7orbitals)
James Chadwick – 1932 Nucleus
mystery solved
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Rutherford speculated
in 1920 that there
existed electrically
neutral particles with
the protons that make
up the missing mass
but no one accepted his
idea at the time.
Complete model!
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James Chadwick discovered a new type of
radiation that consisted of neutral particles. It
was discovered that these neutral atoms came
from the nuclei of the atom. This last
discovery completed the atomic model.
Quarks….where are we now?
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1964 Quarks are proposed by Murray GellMann and George Zweig (math based theory)
quarks are subatomic particles thought to be
elemental and indivisible. They are one of the
two kinds of spin-½ fermions (the other being
the leptons). Objects made up of quarks are
known as hadrons; well known examples are
protons and neutrons.