1. Concept of the Atom Leucippus and Democritus, circa 400 B.C.E., Greece
Chem 101
Concept of the Atom
Democritus
Leucippus and Democritus,
circa 400 B.C.E., Greece
Quotes attributed to Democritus:
I would rather discover one scientific fact than become King of Persia. Quoted in D MacHale, Comic Sections (Dublin 1993)
Everything existing in the Universe is the fruit of chance and necessity.
Nothing is so easy as to deceive one's self; for what we wish, that we readily believe.
Nothing exists except atoms and empty space; everything else is opinion. Diogenes Laertius
The Development of Atomic Theory

2. Experimental Evidence for the Existence of Atoms
Antoine Lavoisier ( ), French
Law of Conservation of Matter:
Matter (mass) is neither gained nor lost in a chemical reaction.
Like most scientists, he came from a wealthy family—science was not a real profession yet.
Wrote the first chemistry textbook
Is often called the “Father of Chemistry”
Was executed by guillotine during the French Revolution
Lavoisier
Law of conservation of Matter:
Mass of reactants = Mass of products
(no mass is lost from a reaction)

3. Experimental Evidence for the Existence of Atoms
Joseph Louis Proust ( ), French
Law of Constant Composition
In a pure compound, the elements combine in definite proportions to each other.
Also known as the
Law of Definite Proportions
all samples of a given chemical compound have the same elemental composition.
Example for both:
Pure water is always 8/9 oxygen and 1/9 hydrogen
Proust

4. Experimental Evidence for the Existence of Atoms
Berzelius
Jöns Jacob Berzelius (1779 – 1848), Swedish
Also arrived at the Law of Constant Composition
Parents died before he reached 12 years old
He supported himself in school with tutoring
Became a physician and later a professor of chemistry
Gave elements abbreviations (called chemical symbols) that we still use today! He also wrote chemical formulas.
Berzelius discovered the elements cerium, selenium, and thorium. He isolated silicon, zirconium, and titanium
Like Lavoisier and Dalton, also known as the Father of Chemistry!

5. Experimental Evidence for the Existence of Atoms
Dalton’s table
of elements and
their combinations
John Dalton ( ), 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

6. Dalton’s Atomic Theory
1. Elements are composed of atoms
solid, tiny, hard, unbreakable, spheres  
2. All atoms of a given element are identical
all 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

7. 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

8. Discovery of the electron
Joseph John Thomson ( ), British
Used the cathode ray tube (modified by Crookes, 1850s) to do experiments that led to the idea of electrons.
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).
“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.

9. Plum Pudding Model of the Atom
Atoms (matter) is electrically neutral.
If electrons are negatively charged, there must
be positive charges to balance the electrons
(later, protons were discovered by Goldstein).
Lord Kelvin
(William Thomson,
no relation to JJ)
JJ Thomson

10. Discovery of the nucleus
Ernest Rutherford ( ), British
His famous gold foil experiment proved the
existence of the nucleus in atoms and that
The nucleus was composed of positive charge (protons).
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
“All science is either physics or stamp collecting” -Ernest Rutherford

11. Gold Foil Experiment
Rutherford and
JJ Thomson
(horsing around?)

12. Rutherford’s Atom Atoms are not hard spheres!
Due to the gold foil experiment:
1. Atoms are mostly empty space
(the alpha particles went straight through the gold foil most of the time)
Atoms contain a dense nucleus composed of positive charge
(later, neutrons were discovered and believed to stabilize the protons in the nucleus)