Development of Atomic Theory
(and various people along the way)
The Greek Philosophers
who
thought
but did not
experiment
LeucippusTeacher of Democritus
First proposed that if matter was divided and divided over and over and over, it would reach a
smallest particle
he called an
ATOM.
MATTER IS PARTICULATE!
Why don’t you hear about Leucippus?
His writings didn’t survive.
We know about him from his student
Democritus
who wrote a lot more about the idea of
atoms.
Democritus (470 - 380 BC)
Proposed that all matter is
made up of
tiny, invisible particles
he called
ATOMS.(this was from Leucippus)
Also proposed that atoms had different
SHAPESthat gave them different
properties.(Democritus’ own ideas)
Aristotle ( 384 – 322 BC )
Aristotle believed…
If you divided matter over and over and over, you would still
be able to divide it.
Matter is continuous.
Disagreed with Democritus on the basis that there was nothing to
hold atoms together. Also, what’s in between the
atoms?
Aristotle believed everything was made up of :
•Water
•Earth
•Air
•Fire
Alchemy
• Based on Aristotle’s beliefs that all different substances had different proportions of earth, air, fire, and water, although later other elements were added.
• By changing proportions, alchemists could achieve two goals:– Change other metals to gold– Discover substance promoting eternal youth
Alchemy
Problem with alchemy:
IT
DOESN’T
WORK!!
These beliefs lasted hundreds of years –until CHEMISTRY
replaced ALCHEMY
WHY NOT CHANGE IDEAS ABOUT MATTER?
• The Roman Catholic Church supported Aristotle’s views because he also proposed, along with Ptolemy, the idea of a geocentric universe: everything revolves around the Earth. This fit Catholic theology.
• The Catholic church provided most education, so what people learned was Aristotle.
(so nothing changed for a long, long time)
Why did things change?
• Roger Bacon (13th century): first stated scientific method
• Copernicus (14th century): proposed heliocentric universe: planets orbited sun.
• Martin Luther (15th century): started the Protestant reformation (break with Catholic Church)
• Henry VIII (15th century): like Luther (but independently) broke with Catholic Church and started educational system not run by Catholic Church.
Antoine Lavoisier (1743 – 1794)
Law of Conservation
of Mass:During a chemical reaction,
the total mass of all reactants = total mass of all products
1754-1836
Law of Constant Composition
A compound always contains the same
elements in the same proportion by mass.
Atomic Theory of Matter
John Dalton (1766–1844)
• Elements are made up of small indivisible particles (atoms)
• All atoms of the same element have identical properties.
• Atoms cannot be created, destroyed or transformed during a chemical reaction.
(law of conservation of mass)
• Compounds are formed when atoms of different elements combine with each other in simple numerical ratios.
• In a compound, relative numbers and kinds of atoms are constant.
(law of constant composition)
Dalton’s Law of Multiple Proportions• When two elements form a series of compounds,
the ratios of the masses of the second element combine with a fixed mass of the first element can always be expressed as the ratio of small whole numbers.
• When two elements can combine to form more than one compound and the same amount of the first element is used in each, then the ratio of the amounts of the other element will be a whole number ratio.
Dalton’s Atoms
Solid spheres
Different elements have
different masses
Benjamin Franklin (1706–1790)
Concluded from experimentation that there were 2 types of electric charge. +,-
Michael Faraday (1791-1867)
Suggested that the structure of the atom is related to
electricity
Cathode Ray Tubes (mid 1800’s)
Cathode (-) Anode (+) Electric plates (one +, one -)
partially evacuated glass tube that produces a beam between
the cathode and anode
When filled with a gas that fluoresced in the presence of electric charge, a stream
of radiation(the cathode ray) was seen
from cathode to anode.
J. J. Thomson (1856 – 1940)
Investigated cathode rays.
Thomson and Cathode Rays
• If electrically charged metal plates were placed above and below the ray, the ray bent. It ALWAYS bent toward the positive plate.
• If a different metal was used to make the cathode, he got the same results.
• If a different gas filled the tube, he got the same results.
The Atom had a Substructure
• Cathodes made of different metals gave off identical particles. Meaning: particles are in all atoms!
• Particles were attracted to positive plate and repelled by negative plate. Meaning: particles have negative charge!
• Because original atoms were neutral, same amount of positive charge remained in the atom as left with the electrons.
What are the particles?
electrons
Robert Millikan (1868 – 1953)
Calculated the mass of an electron (9.11 x 10-28 g) and its charge (-1 fundamental
charge units)
THE
OIL DROP
EXPERIMENT
Oil Drop Experiment
Henri Becquerel (1852 – 1908)
• Discovered radioactivity in 1896
Marie Curie (1867-1934)• Discovered
that radiation accompanies changes in identity of the radioactive atom
Others also investigating radioactivity:Pierre CurieOtto Hahn
Lise Meitner
Ernest Rutherford (1871 – 1938)
Discovered:
Alpha particles
and
Beta particles
The Gold Foil Experiment
• Alpha particles have a very small size and mass and a positive charge.
• Rutherford shot alpha particles at a very thin sheet of gold foil.
• Rutherford expected alpha particles to “blast” a hole in the gold foil and go straight through because he thought the positive charge was evenly spread through the foil.
Animation of
the Gold Foil Experiment
Rutherford.exe
What he Sawwas
NOTwhat heexpected
“It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of
tissue paper and it came back and hit you.”
WHAT HE SAW AND WHAT IT MEANT: part 1
Data: Most alpha particles went straight through the foil as if it was not there.
Conclusion: there was nothing where the atoms passed through.
Meaning: most of gold atom is empty space.
WHAT HE SAW AND WHAT IT MEANT: part 2
Data: Some alpha particles bounced straight back from the foil toward the alpha particle source.
Conclusion: the alpha particle hit something much more massive and dense than itself.
Meaning: There is a very small part of the atom that contains most of the atom’s mass.
WHAT HE SAW AND WHAT IT MEANT: part 3
Data: Some alpha particles deflected: bent away without bouncing.
Conclusion: the positive alpha particle was repelled by something else that was positive.
Meaning: The part of the atom containing the mass has a positive charge.
Rutherford’s Theory
The atom contains a center which is• Very small• Positively charged• Contains almost all the atom’s mass.
He called it the
NUCLEUSHis model was the
NUCLEAR MODEL
Moseley (1887-1915)
Graduate student of Rutherford.
Used X-ray scattering to determine that positive charge in nucleus occurred in increments.
The charge was
QUANTIZED
Mosley, Rutherford, and Protons
• Moseley suggested that the increments of positive charge matched positive particles called protons.
• He was killed before he could prove it.• Rutherford finished Moseley’s experiments.• Rutherford’s data supported Moseley’s
hypothesis.
Atomic number:NUMBER OF PROTONS
Moseley’s Periodic Law
The properties of the atoms vary in a periodic fashion according to the amount of positive
charge in their nuclei.
The positive charge in the nuclei:
Atomic number:NUMBER OF PROTONS
The number of protons in a nucleus determines the identity of an element.
The number of protons in an
atom is is is is is isthe atomic number
James Chadwick (1891-1974)
Discovered the existence of
neutronsby bombarding
elements Be to K with alpha particles.
Max Planck and the beginning of the WEIRD
STUFFIn some cases, the energy of vibrating objects is
QUANTIZED:There are some amounts of energy that are
“allowed”…
And some that are “not allowed.”
What’s the deal with quantization (quantized things)?
Compare to a ramp vs. steps:
Different places along the ramp have different heights above the ground. Any height is possible. Situations like this are called
CONTINUOUS.
Steps have specific heights above the ground but cannot have “in-between” heights.
Situations like this are called QUANTIZED.
Things believed about light• Light is a wave, which is continuous.• Light can be measured with wave measurements:
wavelength (), frequency (), speed (c), and amplitude.
• The speed of light is always the same.c=
• The energy of light depends on how bright it is (amplitude).
• Color of light is determined by wavelength or frequency.
Big Al and the photoelectric effect
The photoelectric effect: when light shines on a metal, the metal gives off electrons.
Brighter lights have no effect on the number of electrons given off.
The color of light (which depends on the frequency) has a HUGE effect on how many electrons are given off.
Big Al and the photoelectric effect
Conclusion:Light occurs in quantized bundles of
energy called PHOTONS.Photon energy is determined by the light
frequency.Electrons can absorb a whole photon or
none of it (not part of a photon).If the electron absorbs a photon with the
right energy, it jumps out of the atom.
This was a very strange idea about light.
Light (like a rainbow) was believed to be
CONTINUOUSbut Einstein said it was
PARTICULATE.
WAVE-PARTICLEDUALITY
Light is a wave (continuous).
Light is a particle (quantized).
Niels Bohr (1885- 1962)
Developed atomic model where
electrons move in quantized energy
levels.
Bohr’s ideas came from absorption and emission spectra.
When an element is heated up, it gives off light of specific colors:
emission spectrum
When light goes through an element, it absorbs light of specific colors:
absorption spectrum
Bohr’s model• Electrons can be described mathematically (but not
physically) as orbiting the nucleus in circular orbits like planets around the sun.
• The orbits are called energy levels.• The orbits have quantized energy.• Electrons absorb energy (as photons – light of specific
energy) to “jump up” to a higher energy level.• Electrons emit energy (as photons – light of specific
energy) to “drop down” to a lower energy level.
Bohr’s model was called the
Planetary ModelIt had one problem.
It only worked for hydrogen.
No other elements.
DeBroglie and matter waves• Einstein said light (which had always be
perceived as a wave) behaved like a particle.
• DeBroglie said matter (which had always been perceived as particles) behaved like waves.
• Why don’t we notice matter waves? They’re so small they are only noticeable with things that are very small and moving very fast, like electrons.
Heisenberg, who was uncertainThe Heisenberg Uncertainty Principle:
The more you know about where an electron is, the less you know about where it’s going.
You’ll never be able to describe an orbit for an
electron.
Schrodinger solves the case• Electrons behave like waves (deBroglie).
• Formula for the wave describes a region where there is a high probability (Heisenberg) of finding an electron with a specific, quantized energy (Bohr).
• Region of high probability: ORBITAL
• Each orbital is described by QUANTUM NUMBERS which identify size, shape, arrangement in space, and number of electrons in that orbital.
Schrodinger’s Model
Wave model
Wave model
Quantum Mechanical model
(This is what we use today.)