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Atomic TheoryWhat is it?Who figured it out?When did they do it?How did they do it?Why do we believe it?
What is an ATOM?
An atom is the smallest particle of an element that retains the identity of the element.
ZZZZZZZZZ
The Ancient Greeks
DemocritusLived 450 B.C.Proposed that all
the stuff in the world is “atomos”
Tiny, indivisible particles
AristotleQuestioned theory
of DemocritusRejected it for lack
of proof
ALCHEMISTS
Tried to turn base metals into precious metals.
Developed knowledge and techniques
Not true scientists
Copper into Gold Laboratory
Alchemists tried to turn normal copper into gold coins!
If they succeeded, they would never need to work!
Prove that the coins are GOLD
How is copper different than gold?How can you test it?What techniques are you using…that
Democritus did not use?
Lavoisier
LAW OF CONSERVATION OF MATTERMatter, like energy, is neither created nor
destroyed in a chemical reaction.
This concept established modern chemistry.
What was different?
Experimentation He used a balance to
study the role of oxygen in rusting and burning.
Other people did it, too!
Priestly did similar experiments.
He believed a false theory of Phlogiston.
Kept his mind closed to new idea.
PROUST
1799 Law of Constant
Composition A given compound
always contains the same elements in the same proportion by mass.
WATER11% Hydrogen
WATER89% Oxygen
DALTON’S ATOMIC THEORY OF MATTER
1. All matter is composed of submicroscopic (extremely small) indivisible particles called ATOMS.
2. All atoms of a given element are identical. The atoms of any one element are different from those of any other element.
DALTON’S ATOMIC THEORY OF MATTER
3. Atoms are neither created nor destroyed in a chemical reaction. Chemical reactions occur when atoms are separated, joined, or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction.
DALTON’S ATOMIC THEORY OF MATTER
4. Atoms of different elements can mix physically or can combine chemically with one another in simple whole number ratios to form compounds.
Scientists had been doing many experiments with electricity since Ben Franklin flew his kite.
Faraday suggested that electricity might explain the atom
English physicist, J.J. Thomson 1856-1940
Discovered electrons in 1897
Thomson experimented with a “cathode ray tube”
Thomson’s Cathode Ray Tube Experiment
Flow of electric current through gases. Sealed gas in glass tube with metal plates at the end. Connected plates to high voltage source: Anode + and Cathode –
This ray could be deflected toward a positive charge…
It has a negative charge.
This ray could move things…
It was made of particles of matter.
This ray acted the same no matter what materials were used…
It was not atoms…it must be part of all atoms
Cathode Ray Tube
Cathode ray is composed of very small negatively charged particles that are part of
atoms
ELECTRONS
Atomic Theory
Millikan devised experiments to determine the mass and charge of the electron.
Protons : discovered in 1886
Positively charged particles
Also discovered with cathode ray tube…these particles went the other direction.
The Nuclear Atom
RutherfordIdentified three types of radiation
Alpha…positive particleBeta…negative particleGamma…high energy
Devised the GOLD FOIL EXPERIMENT
Now…where were we….?
Rutherford’s Nuclear Atom
Most of the mass of an atom is in the center…the nucleus…with electrons moving around it.
Nuclear atom
By 1932, neutron was discovered, too.The nucleus is the central core of the
atom, composed of protons and neutrons. Because, protons and neutrons have much greater mass than electrons, almost all of the mass of the atom is concentrated in a tiny nucleus…a dime in a football stadium!
As the wavelength increases, the frequency decreases.
The greater the frequency the greater the energy.
Light acts like Particles
Electromagnetic radiation also has the properties of particles.
Bohr suggested that energy is emitted and absorbed in discrete quantities called
Quanta or Quantumpackets or pieces of energy
“Jumps”
Dual Nature of Light is the next tool for understanding the atom
Energy is directly proportional to frequency…wave nature.
Einstein proposed that light consists of quanta of energy that behave like particles of light…he called these photons.
Example: photoelectric effect
This is the DUAL NATURE OF LIGHT
What does this have to do with ATOMS?
Atoms give off energy as photons of light.Line spectra of elements reveal that
electrons exist in quantized (discrete) energy levels.Rainbow…continuous spectrumLine…discontinuous spectrum
The color allows us to calculate the energy higher frequency = higher energy… purple higher than red
Emission Spectra
Line Spectra of elements show discrete or quantized energy levels in the atom.
These energy levels are different for each element.
The color or wavelength of light shows the energy level because energy can be calculated from frequency of light emitted.
Bohr’s Model of the Atom Bohr applied Quantum
Theory to the structure of the hydrogen atom.
Quantum theory means that electrons jump from level to level.
Bohr pictured the electrons spinning around set orbits, like planets around the sun.
Planetary Model
Quantum Leaps!
Electrons exist at low energy…ground state
Add energy to go to higher energy…excited state• Electron drops back down to ground
state as soon as it can • It releases the exact energy it needed to
jump up as a photon of a frequency or color.
Schroedinger’s Quantum Mechanical
(Today’s) ModelWe still have energy levels, like BohrHeisenberg Uncertainty Principle
It is impossible to know the velocity and position of a particle at the same time
We can not know the exact location of an electron.
Any effort to do so, will change the position.We can only figure the probability of finding it
in a certain region.
Orbitals
Area where electrons are most likely to be found.
Example: If your mom wants you to do some work, she can find you in your room most of the time, or the house, or the yard…but you could be at the mall.And sometimes her efforts to find you, make you move!
Today’s Atom
Dense, small,nucleus (with protons and neutrons) is surrounded by a fuzzy cloud shapes where electrons (that act more like waves) are most likely to be found… orbitals.
Links
Go to Holt On-Line for a nice review of all this. my.hrw.com/
Go to this site to see some great atomic atrwork!www.chemsoc.org/.../orbital/images/elev3.jpg