Concept of the Atom

Democritus

400 B.C.E., Greece

Democritus

Multiple Proportions

O/C

2.39

1.27

O/N

0.58

1.27

2.39

Oxides of Carbon %C %O

28 72

44 56

Carbonic Acid (1801)

Carbonous Acid (1789)

Oxides of Nitrogen %N %O

63.30 36.70

44.05 55.95

29.50 70.50

Nitrous Oxide (1810)

Nitrous Gas (1810)

Nitric Acid (1810)

%err

4

5

%err

2

11

5

(1)

(~2)

(~4)

(~2)

(1)

Rel.

Rel.

integral values consistent with simple atomic ratios

Experimental Evidence for the Existence of AtomsJohn Dalton (1766-1844 ), British

Put the pieces together to form the

first atomic theory!If elements combined in whole-number ratios,

there must be atoms!

(matter is made of discrete particles!)

• Was the first to create a table of atomic weights

• He was a quaker school teacher at age 12

• He was color-blind; never married

• More than 40 thousand people marched in his funeral procession.

Dalton

Dalton’s tableof elements and their combinations

Dalton’s Atomic Theory

1. Elements are composed of atoms solid, tiny, hard, unbreakable, spheres  

2. All atoms of a given element are identicalall carbon atoms have the same chemical and

physical properties  

3. Atoms of a given element are different from those of any other element

carbon atoms have different chemical and physical properties than sulfur atoms

 

Dalton’s Atomic Theory 

4. Atoms of one element combine with atoms of other elements to form compounds (in simple ratios).

Law of Constant Composition all samples of a compound contain the same proportions

(by mass) of the elements

 5. Atoms are indivisible in a chemical process. Reactions only cause a rearrangement of atoms.

Law of Conservation of Mass:all atoms present at beginning are present at the end atoms are not created or destroyed, just rearranged

atoms of one element cannot change into atoms of another element

cannot turn Lead into Gold by a chemical reaction

Discovery of the electron

“Could anything at first sight seem more impractical than a body

which is so small that its mass is an insignificant fraction of the

mass of an atom of hydrogen?"-- J.J. Thomson.

Joseph John Thomson (1856-1940), British

Used the cathode ray tube (modified by Crookes, 1850s) to do experiments that led to the idea of electrons.http://www.chem.uiuc.edu/clcwebsite/cathode.html

Electrons are negatively charged particles that exist in an atom

Thomson won the Nobel Prize in Physics, 1906 for evidence of electrons as particles.

Ironically, his son won the Nobel in 1937 for evidence of electrons as waves (it has been accepted since then that electrons have both wave and particle properties).

Plum Pudding Model of the AtomAtoms (matter) is electrically neutral.

If electrons are negatively charged, there mustbe positive charges to balance the electrons(later, protons were discovered by Goldstein).

Lord Kelvin(William Thomson, no relation to JJ)

JJ Thomson

Discovery of the nucleus

“All science is either physics or stamp collecting”

-Ernest Rutherford

Ernest Rutherford (1871-1937), British

His famous gold foil experiment proved theexistence of the nucleus in atoms and that 1) The nucleus was composed of positive

charge (protons). 2) Atoms are mostly empty space

Born and raised on a farm in New Zealand

Won scholarship to study at Cambridge Became a student of JJ Thomson

Performed extensive research in radioactivity

Nobel Prize in 1908 in Chemistry

Was a great mentor—several of his students also won Nobel Prizes

Gold Foil Experiment

Rutherford and JJ Thomson

(horsing around?)

Bohr’s Model

This model involves the idea of quantization (developed by Planck):

Energy levels (orbits) increase in quantized amounts of energy (discrete—not continuous—amounts of energy)

This means the light emitted from excited electrons are not continuous—only certain colors are seen (only QUANTA of energy can be absorbed and released)

"We are tracing the description of natural phenomena back to combinations of pure numbers which far transcends the boldest dreams of the Pythagoreans." -- Bohr

Wave-Particle Duality

Bohr’s model did not explain why electrons do not spiral into the nucleus. (It makes the assumption that this does not occur).

It was accepted by physicists that light seemed to behave as both waves and as particles (called photons). This idea is called “Wave-Particle Duality.”

Louis de Broglie asked “If light can be wave-like and particle-like, why can’t matter also be wave-like and particle-like?”

This led to the concept that

electrons can behave as waves and as particles.

Prince Louis de Brogliefirst got a history degree,then went into science.Won the Nobel Prizein physics (1929).His work helped shapequantum mechanics.

Electrons as waves

 Electrons behave like standing (stationary) waves (not traveling waves, like light)

Stationary waves are like the waves (vibrations) from plucking a guitar string.

There are only certain areas where the waves (electrons) are allowed. The 3-D space where electrons might be is described by orbitals.

This is onekind of orbitalcalled a p-orbital

Werner Heisenberg, 1901-1976 (Nobel Prize, 1932)

• The Uncertainty

Principle

Erwin Schrodinger (Quantum mechanical model of the atom)1887-1961 (Nobel Prize, Physics, 1933)

OrbitalsWave functions determine the sizes and shapes of orbitals

Four common orbital types are: s, p, d, f (theoretically, there are an infinite number of orbital types: g, h, I, j, k, etc…)

Let’s look at the difference between Bohr’s atom and Schrödinger’s:

http://www.colorado.edu/physics/2000/quantumzone/schroedinger.html

Light: Waves or Particles?

Joseph Louis Gay-Lussac

(1809)

Oxidation of Sugar, etc. with NaClO3

Cleans up Lavoisier's Mass Balance

1.9989 volumes of hydrogen per

1.0000 volumes of oxygen Water gives

3.08163 volumes of hydrogen per

1 volume of nitrogen Ammonia gives

Alternative to Dalton's Law of Greatest Simplicityhttp://www.bibliotheque.polytechnique.fr/associations/gaylussac/pages/AssofrGL.html

1804 - 7,016 m(1778-1850)