Ultra-slow, stopped, and compressed light in Bose-Einstein condensates
You too can be as smart as Einstein (almost). The elements Earth – dry, heavy Water – wet,...
-
Upload
frank-garrison -
Category
Documents
-
view
215 -
download
1
Transcript of You too can be as smart as Einstein (almost). The elements Earth – dry, heavy Water – wet,...
History - Greeks
The elementsEarth – dry, heavyWater – wet, heavyAir – cool, lightFire – warm, light
The composition of a substance could be estimated from its properties.
History - Greeks
These ideas were based on observation, logic and reason, but not experimentation.
Democritus (460 B.C. - 370 B.C.)
History - Greeks
Matter is made of small, hard indivisible particles called atoms, which exist in the void.
These atoms differ in size and shape, but not in any other way.Quantitative differences (how much) vs. Qualitative differences
(what kind)
History - French
Antoine Lavoisier (1743-1794)
Discoverer of Oxygen (disputed)
His work refuted the phlogiston theory
Responsible for the law of conservation of matter.
History - French
Berthollet – “compounds do not have a fixed composition”.
Cu + S CuxSy
Every time he tried the experiment
he got a different result.
History - French
Proust - compounds have a fixed composition.
2H2 + O2 2H2O
He always got the same result. Proust’s argument is called The Law
of Definite Proportions. He was proved to be right.
History - English
John Dalton (ca. 1804)
The father of modern atomic theory
Schoolteacher Colorblind –
studied colorblindness
Dalton’s Atomic Theory
The points of Dalton’s theory All matter is made of atoms Atoms are indivisible and indestructible All atoms of one element are exactly
alike, and atoms of different elements are different.
Atoms combine in small whole number ratios to form compounds.
Dalton’s Atomic Theory
The Law of Multiple Proportions:If two elements combine to make two
different compounds, the ratios of the elements involved are small
whole numbers. Examples: CO and CO2
CuS and Cu2S
H2O and H2O2
The Electron
Thomson discovered the electron - he called it a “corpuscle”.
He used an instrument called a Crookes tube.
Cathode (-) Evacuated tube Anode (+)
The Electron
He noticed a stream of charged particles coming from the cathode, called cathode rays.
Thomson proposed the "plum pudding" atomic model - negatively charged corpuscles swarm inside a cloud of massless positive charge.
Gold Foil Experiment
Most of the alpha particles went straight through, and a few were bounced straight back.
Rutherford’s interpretation: The atom has a small, hard, dense and positively charged nucleus. The electrons are outside the nucleus.
The Proton and the Neutron
Discovery of the proton: Henry Moseley (1913) Moseley bombarded metals with x-rays Each successive element had one more
positive charge – called “atomic number”
Rutherford proved that the nucleus of nitrogen contains hydrogen nuclei – a “proton” (1918-19)
Discovery of the neutron – James Chadwick (1932)
Parts of the Atom
Name Charge Mass (amu)
Location Discoverer
Electron -1 1/2000 outside nucleus
Thomson
Proton +1 1 nucleus Moseley/Rutherford
Neutron 0 1 nucleus Chadwick
Isotopes
Atomic number = number of protons in the nucleus
Atomic number determines the identity of the element
Mass number = protons + neutrons Number of electrons = number of protons Isotopes: two atoms of the same
element with different numbers of neutrons
C-12 and C-13 are isotopes of carbon
Average atomic mass
Average mass of all the isotopes of an element
Average is weighted Example: Boron has two isotopes, B-10
and B-11
B-10: 19.9%B-11: 80.1%
Average atomic mass of boron:10x0.199 = 1.9911x0.801 = 8.811
Average atomic mass = 1.99 + 8.811 = 10.8amu
Rutherford-Bohr Model of the Atom (1911-1913)
Rutherford suggested that electrons orbit around the nucleus like planets around the sun.
This did not explain emission spectra, which gave sharp lines.
He theorized that electrons could only travel in certain sized orbits, and not anywhere in between.
Bohr Model of the Atom
The orbits were called energy levels. Each orbit has a specific energy.
Electrons can jump from one level to another; as they do, they absorb or emit energy.
Quantum Mechanics
Schrödinger’s work showed that electrons do not move in actual “orbits”.
Electrons move randomly and form “probability clouds”. The shape of these clouds is similar to the shape of Bohr’s orbits.
The position and momentum of an electron cannot be determined simultaneously (Heisenberg Uncertainty Principle)
Electron Energy Level Populations
Bohr suggested that electrons inhabit energy levels around the nucleus.
Each level has a specific energy associated with it.
The outermost (highest energy) level is called the “valence shell”.
The electrons in the valence shell are called the “valence electrons”.
The valence electrons are the most important electrons in the chemistry of the atom.
Electron Energy Level Populations
The number of levels depends on the number of electrons.
The first level (K) holds two electrons. The second level holds eight electrons. The third level holds 18, and the fourth
32. No atom can have more than eight
electrons in its valence shell. When the valence shell reaches eight
electrons, the next two electrons are put in a higher level. Then the lower level can be filled.
Lewis Electron Dot Structures
Lewis dot structures show how many electrons are in the valence shell of an atom.
Lewis dot structure for sodium The first electron always goes to the right
of the symbol. The second is paired with the first.
Lewis Dot Structures
Lewis dot structure of magnesium The third goes on top.
Lewis dot structure of aluminum
Lewis Dot Structures
The fourth goes on the left, and is not paired. The fifth goes on the bottom, and successive electrons are paired until a total of eight is reached.
Lewis dot structure of silicon
Atomic Spectra
Bohr’s model based on atomic spectra
Obtaining emission atomic spectra Energy is applied to a gas or liquid sample.
Flame test (for samples in solution) Gas discharge tube
The energy makes an electron or two jump to a higher energy level.
The electrons fall back down to a lower level, and give off energy in the form of light – bright lines against a dark background.
Atomic Spectra
Absorption spectra – light is passed through a sample and analyzed – looks like a rainbow with dark lines
Interpreting atomic spectra The light given off is viewed through a
spectroscope. The spectroscope has either a prism or
a grating, which splits the light into its component colors.
Atomic Spectra
Only a few sharp lines appear in the spectrum.
Each line corresponds to a specific electron transition.
Transition = jump from one energy level to another
Light Energy
Color depends on frequency. High frequency = violet end of
spectrum Low frequency = red end of spectrum
Energy also depends on frequency, so each color has its own energy. Blue or violet is higher energy than red or green.
When a specific color line is seen in a spectrum, the energy of the electron transition responsible can be calculated.