Transcript Thompson’s “Plum Pudding” Model

Atomic Physics
What is the ATOM???
MATTER = ATOM
• All matter is composed of atoms.
• Atoms are the smallest part of an element
that keeps that element’s properties.
• If an atom were the size of a football field, the
nucleus would be the size of a marble and
nearly all of the mass of the atom is in the
nucleus.
• The space around the nucleus and the
electron have very, very little mass. This
means that matter is mostly empty!
The Atom and the nucleus
The nucleus contains most of the mass of the
atom (protons + neutrons). The rest is
mostly empty space!
Democritus
The Greek Understanding
• Democritus (BC 400) and the
“Atomists” debate the “four”
elements (fire, water, earth, and air).
• Democritus concluded that matter
could not be divided into smaller
and smaller pieces forever.
Eventually, the smallest piece of
matter would be found.
Democritus (400 B.C.)
• He used the word
“atomos” to describe the
smallest possible piece of
matter.
• Not based on experimental
data
Alchemy (next 2000 years)
• Mixture of science and mysticism.
• Lab procedures were developed, but alchemists did not
perform controlled experiments like true scientists.
John Dalton
• John Dalton (1766-1844)
proposed an atomic theory
• British Schoolteacher
• based his theory on others’
experimental data
• Billiard Ball Model
• atom is a
uniform,
solid sphere
Dalton’s Four Postulates
1. All elements are composed of
indivisible particles called atoms.
2. Atoms of the same element are
exactly alike (isotopes not known
yet).
3. Atoms of different elements are
different.
4. Compounds are formed by joining
atoms of two or more elements.
1. In a chemical reaction, atoms are
rearranged, but not changed.
Dalton’s Atomic Theory
While this theory was not completely correct, it
revolutionized how chemists looked at matter and
brought about chemistry as we know it today instead
of alchemy
Thus, it’s an important landmark in the history of
science.
Law of definity proportions
law of multiple proportions
Problems with Dalton’s Atomic Theory?
1. matter is composed, indivisible particles
Atoms Can Be Divided, but only in a nuclear reaction
2. all atoms of a particular element are identical
Does Not Account for Isotopes (atoms of the same
element but a different mass due to a different number
of neutrons)!
3. different elements have different atoms
YES!
4. atoms combine in certain whole-number ratios
YES! Called the Law of Definite Proportions
5. In a chemical reaction, atoms are merely rearranged to
form new compounds; they are not created, destroyed,
or changed into atoms of any other elements.
Yes, except for nuclear reactions that can change
atoms of one element to a different element
Henri Becquerel (1896)
• Discovered radioactivity
• spontaneous emission of
radiation from the nucleus
• Three types:
• alpha () - positive
• beta () - negative
• gamma () - neutral
Thompson’s “Plum Pudding” Model
• Discovered negatively charged
particles coming off a gas known to
be neutral.
• Called these “early” electrons,
corpuscles.
• Predicted that there must be positive
particles (protons) present in equal
number to balance negative charges.
• Proposed atoms were made of
pudding like positive charges, with
negative electrons inside scattered
like plums.
J. J. Thomson (1903)
• Cathode Ray Tube
Experiments
• beam of negative particles
• Discovered Electrons
• negative particles within the
atom
• Plum-pudding Model
J. J. Thomson (1903)
Plum-pudding Model
• positive sphere (pudding)
with negative electrons
(plums) dispersed
throughout
Ernest Rutherford (1911)
• Gold Foil Experiment
• Discovered the nucleus
• dense, positive charge in the
center of the atom
• Nuclear Model
• 99.9% of total mass is in
nucleus
Rutherford’s Model
• Fired tiny positive particles at gold
•
•
•
•
foil…most went through without change
of course. What did this prove?
Some particles bounced back as if they
hit something solid.
Reasoned that they were repelled by
similar charged particles.
Discovered and proved the existence of
the proton.
Reasoned that atoms are mostly “empty”
space, but with a small dense center with
positive charges.
Rutherford’s experiment.
Rutherford’s Gold Foil
Experiment
Results of
foil
experiment if
Plum
Pudding
model had
been correct.
What Actually Happened
Ernest Rutherford (1911)
• Nuclear Model
• dense, positive nucleus surrounded by
negative electrons
• These deflections were not consistent with Thomson's
model. Rutherford was forced to discard the Plum
Pudding model.
• He reasoned that the only way the alpha particles
(positively charged) could be deflected backwards was
if most of the mass in an atom was concentrated in a
nucleus.
Robert A. Millikan
• Experiments showed that the mass of
an electron is smaller than the
simplest type of hydrogen atom.
• Mass of electron =
9.109 x 10-31kg
• Electron has a negative charge.
Based on what was learned about
electrons,
• Two other inferences were made
about atomic structure:
• 1.
Because atoms are electrically neutral,
they must contain a positive charge to
balance the negative electrons.
• 2. Because electrons have so much less
mass than atoms, atoms must contain other
particles that account for most of their mass.
Mosely
• Determined
electron
neutron
proton
the number of
PROTONS in
elements
The Bohr Model of the Atom
• Neils Bohr suggested that only certain electron
orbits are allowed.
• Orbits are given number n=1,2,3,4,5,…infinity.
• n=1 is the ground state (ground orbit) and has
the lowest energy. No lower orbit is permitted.
• The excited states (higher orbits) have more
energy.
• The orbits get closer together as n increases.
• To change orbits, the electron must either gain
or loose energy.
• Loss of energy appears as light
– What we see as light is actually the release of
a photon
• Each type of atom has a unique set of energy
levels.
Niels Bohr (1913)
• Bright-Line Spectrum
• tried to explain presence of
specific colors in hydrogen’s
spectrum
• Energy Levels
• electrons can only exist in
specific energy states
• Planetary Model
• Based on Hydrogen
Niels Bohr (1913)
Bright-line spectrum
• Planetary Model
• electrons move in circular
orbits within specific energy
levels
Bohr Model
65
4
• Energy of photon
3
2
1
depends on the
difference in energy
levels
• Bohr’s calculated
energies matched the
IR, visible, and UV
lines for the H atom
Neils Bohr’s Model
Erwin Schrodinger (1926)
• Quantum mechanics
• electrons can only exist in
specified energy states
• Electron cloud model
• orbital: region around the
nucleus where e- are likely to
be found
Erwin Schrodinger’s Model
• Developed wave Quantum Mechanical
Model.
• Treats atoms as a 3 dimensional system
of waves.
• Contains ideas of Bohr’s and
incorporates others.
Quantum Mechanical Model
• Quantum numbers
specify the electrons
probable location and
energy level
• Because electrons are
always moving their
exact whereabouts are
unknown
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)
• dots represent probability of finding an enot actual electrons
Erwin Schrodinger’s Model
• Developed wave Quantum Mechanical
Model.
• Treats atoms as a 3 dimensional system
of waves.
• Contains ideas of Bohr’s and
incorporates others.
James Chadwick (1932)
• Discovered neutrons
• neutral particles in the
nucleus of an atom
• Joliot-Curie Experiments
• based his theory on their
experimental evidence
James Chadwick (1932)
Neutron Model
• revision of Rutherford’s Nuclear Model