ELEMENTS AND ATOMS
• Evolution of the Element Theory
–
–
–
–
Monists
Four Elements and Alchemy
Phlogiston and the Study of Airs
Lavoisier and Chemistry
• Discovery of Elements
– Electrolysis
– Spectroscopy
– Periodic Laws
• The Electron
• Discovery of the Nucleus
• Models of the Atom
The Presocratic Monists
All material is made of one elementary substance:
– Thales of Miletus (624-546 BCE): all is water
– Anaximander of Miletus (610-546 BCE): all is apeiron
– Anaximenes of Miletus (585-528 BCE): all is air
– Heraclitus of Ephesus (535-475 BCE): all is fire
(=change)
– Pythagoras of Samos and Croton (570-495 BCE): all is
number
– Democritus of Abdera (460-370 BCE): all is atom
FOUR ELEMENT THEORIES
• Attributed to Empedocles of
Agrigentum (490-430 BCE)
and formalized by Aristotle of
Stagira (384-322 BCE).
• According to Aristotle in
his On Generation and
Corruption:
– Air is primarily wet and
secondarily hot.
– Fire is primarily hot and
secondarily dry.
– Earth is primarily dry and
secondarily cold.
– Water is primarily cold and
secondarily wet.
FOUR HUMOR THEORY
Humor (χυμός,
chymos = sap or juice)
Formalized by
Hippocrates of Kos
(460-370 BCE) who
tied a mechanistic
theory of disease to
the 4 elements and
the weather.
INFLUENCE OF ALCHEMY
• Egyptian material religion
[Khemeia (χημεία), meaning
black (for the hidden arts) or
black earth (an ancient
reference to Egypt)]. With
arabic prefix al-, it becomes
al-khemeia or alchemy.
• Purpose to improve material
• Used careful methods and
precise measurements
• Introduced to Greek Natural
Philosophy
• Continued and augmented
by alchemists in the Islamic
Empire
Paracelsus and Iatrochemistry
• Took name Paracelsus,
which means greater than
Celsus, author of a 1st
century medical
encyclopedia
• Rejection of Hippocrates’
theory of health and
Galen’s anatomy
• Merger of alchemy and
medicine
• Three principles: sulfur,
mercury, salt
• The poison is the cure
Philippus Aureolus Theophrastus
Bombastus von Hohenheim 1493-1541;
Switzerland and Austria
German Iatrochemical School
• Johann Joachim Becher,
devised theory of
combustion based on
phlogiston [from Greek
φλογιστόν- phlogistón
(burning up)]
• Georg Ernst Stahl,
provided experimental
support for phlogiston
Becher, 1635-1682,
Present-day Germany
and England
Stahl, 1659-1734,
Present-day Germany
Chemistry of Phlogiston
1728-1799
The Study of Airs
• Joseph Black: fixed
air (carbon dioxide)
• Henry Cavendish:
inflammable air
(hydrogen)
• Joseph Priestley:
dephlogisticated air
(oxygen)
1731-1810
1733-1804
Priestley, son of the Enlightenment
• Dissenting clergyman
• Theologian and founder
of Unitarianism
• Educator (English
Grammar & Chart of
History)
• Political theorist
• Inventor (soda water;
erasers)
• Natural Philosopher
– electricity
– gasses
Antoine-Laurent de Lavosier
• Minor nobility of France
• Degree in law but never
practiced
• Early work on Geology,
chemistry of minerals, and
meteorology
• Began the study of combustion
and noted that sulfur and
phosphorus increased in weight
after burning
• Thus, phlogiston would have to
have negative weight
• Met Priestley (1774)
• Began work on oxygen (1775)
and rejected phlogiston theory
Marie-Anne (1758-1836) and Antoine (1743-1794)
Elements of Chemistry
• Published in 1789
• Contained 55 substances
that could not be
decomposed into simpler
substances (elements)
• Statement of conservation
of mass
• First modern text that
defined the science of
chemistry
• Lavoisier had offended Marat and had
been a member of a tax collecting
commission
• Judgment of the court: The Republic
needs neither scientists nor chemists;
the course of justice cannot be
delayed.
• Despite important work on the
Gunpowder Commission and his
support of the revolution, he was
beheaded in May 1794 at age 50.
Humphry Davy
• Lavoisier's theory of
elements quickly took over
• Davy used Volta’s pile to
decompose substances by
electrolysis and discovered
magnesium, boron, and
barium
• Chemistry of chlorine and
iodine
• Mentor to Michael Faraday
1778-1829
Electrolysis of sodium chloride
John Dalton
• Interpreted the gas laws to
mean that elements must exist
as atoms
• Reactions are interactions of
atoms
• Atoms combine in whole
number ratios (Laws of simple
and multiple proportions)
• Atoms cannot change (Law of
constant composition)
• Atoms have a constant weight;
so the total mass of reactants
must equal the total mass of
product (Law of equivalent
weights)
1766-1844
• System of Dalton
• Worked to define
atomic weights based
on Hydrogen = 1
Jöns Jacob Berzelius
• Defined modern
notation
• Set weight of Oxygen =
100 and other
elements relative to
that
• Confirmed law of
simple and multiple
proportions and
supported Dalton’s
atomic theory
1779-1848, Sweden
Problem with Hydrogen Solved
• Amadeo Avagadro (17761856, Italy) proposed equal
volumes of all gasses at the
same temperature and
pressure contain the same
number of molecules
• Only possible if diatomics exist
• Berzelius adamant: no
diatomics possible
• Diatomics supported by
Stanislao Cannizzaro (18261910, Italy)
H
+
Cl
= HCl
Unless hydrogen and chlorine
exist as diatomics, this reaction
should produce only one
volume of HCl.
Spectroscopy
• Robert Wilhelm Bunsen
and Gustav Robert
Kirchhoff (Germany)
• Spectroscope (1859)
• Used spectral signatures
to search for new
elements
Kirchhoff (1824-1887); Bunsen (1811-1899)
Discovery of Helium
• August 18, 1868; unusual spectral line
detected in solar chromosphere during a solar
eclipse
Periodic Laws
• The elements seemed to have repeating or
periodic properties according to molecular
weights
• Thus, elements appeared to occur in families
like the halogens: Fluorine, Chlorine, Bromine,
Iodine, all of which have similar combining
properties
Development of Periodic Laws
Independently defined and
formalized into tabular form by
Dimitri Ivanovich Mendeleev (1869)
and Julius Lothar Meyer (1870)
Meyer, 1830-1895,
Germany
Mendeleev, 1834-1907,
Russian Empire
Discovery of the electron
Sir Joseph John (J. J.) Thomson (1856-1940)
Discovery of the Nucleus
Ernest Rutherford, 1st Baron Rutherford of Nelson
(1871-1937; New Zealand, Canada, and Britain)
The Bohr Atom
Niels Henrik David Bohr (1885-1962; Denmark)
Electron Shells
Glen Seaborg
Discoverer of plutonium
and 9 other
transuranium elements
Removed the rare earths
to make the table more
compact
Advisor to presidents
from Truman to Clinton
Former head of the AEC
1912-1999, USA