Chapter 3 Discovering the atom and subatomic particles (History of

Chapter 3 Discovering the
atom and subatomic particles
(History of modern chemistry)
3.1 Chemistry developed out
of interest in materials
The ability to control fire
allowed ancient people to
prepare copper (7000
years ago) and iron (3200
years ago) for tools and
384 - 322 B.C.
Aristotle described the composition and behavior of
matter in terms of four qualities: hot, cold, moist,
and dry. A material is possible to be transformed
to another material by changing the relative
proportion of these four qualities. Gold can be
made from other metals (Alchemy).
Democritus started thinking about “atom” (not cut
or that which is indivisible)
Father of modern chemistry
unfortunate association with a company organized to collect
taxes for King Louis
died at guillotine of no use to the New Republic of France
headless body buried in unmarked grave
Showed conservation of mass in chemical reactions
invented precise scale to weigh small amounts of matter
accurate to 0.0005 grams, about 1/100 of a drop of water
mass of all reactants = mass of all products
cannot create matter from nothing, can only change form
matter is not created from nothing, only changes form during
chemical reactions
Mass is conserved in a chemical reaction
16 X
8 X2Y
Law of mass conservation (质量守恒定律): There
is no detectable change in the total mass of
materials when they react chemically to form
new materials
Joseph Priestley found oxygen and Lavoisier
named it, which means “acid former”
Priestley, Joseph (1733-1804)
Philosopher, scientist and unorthodox
theologian , Joseph Priestley is perhaps best
known for his discovery of oxygen while
performing experiments with gases in 1774.
Ultimately, Priestley identified nine distinct
gases, including carbon dioxide and nitrous
oxide. He is credited with developing a sound
method for infusing carbon dioxide into water,
thus establishing himself as the father of
The Phlogiston Theory
Chemistry was so underdeveloped at the time Lavoisier gained
interest in it that it could hardly be called a science. The prevailing
view of combustion was the Phlogiston Theory which involved a
weightless or nearly weightless substance known as phlogiston.
Metals and fire were considered to be rich in phlogiston and earth
was considered oxygen poor.
• 普利斯特里(Joseph Priestly) ,英
• 舍勒(Carl Wilhelm Scheele),瑞典
• 拉瓦锡(Antoine Lavoisier),法国
2 H gO  
 2 H g  O2 
2 K N O3  
 2 KNO2  O2 
2 H g  O2  
 2 H gO
2 H gO  
 2 H g  O2 
Proust proposed the Law of Definite
Henry Cavendish isolated hydrogen and
Lavoisier named it meaning “water former”.
Lavoisier also realized that water is not an
element, but a compound.
Joseph Proust found that hydrogen and
oxygen always react in a particular mass
ratio and proposed the Law of Definite
Proportions: Elements combine in
definite mass proportions to form
3.3 Dalton deduced that matter is
made of atoms
John Dalton reintroduced the atomic
ideas of Democritus:
Each element consists of
called atoms. (not completely
Atoms can be neither created
nor destroyed in chemical
All atoms of a given element are
identical. (not completely correct)
Atoms chemically combine in
definite whole-number ratios to
form compounds
Atoms of different elements
have different masses.
Arguments on the Chemical
Structure of Water
Dalton’s suggestion for water: HO, oxygen is eight times as heavy
as hydrogen.
Joseph Gay-Lussac’s finding of water:
2 liters hydrogen gas + 1 liter oxygen gas → 2 liters water vapor
molecular formular for water should be H2O
Joseph Louis Gay-Lussac
Joseph Louis GayLussac and JeanBaptistse Biot in their
balloon on 24 August
1804 (7000 meters)
Amadeo Avogadro gave the right explanation to GayLussac’s experiment by introducing the idea of diatomic
particles. But his explanation was not accepted by
Dalton and the chemical community for 50 years.
Problem: How to measure the masses of
atoms of different elements?
At a conference in 1860, Stanislao Cannizzaro reintroduced Avogadro’s idea and also introduced a simple
method to measure the relative atomic masses by
weighing the gases of the same volume. This idea
gradually grew to be the periodic table.
1 liter Oxygen
1 liter Hydrogen
Mendeleev used known relative atomic masses
to create the Periodic Table
In 1860s, many scientists found that the properties of elements tended
to recur in cycles, exhibiting the periodicity.
In 1869, Dmitri Mendeleev produced a chart summarizing the
properties of known elements.
Main disadvantage of Mendeleev’s chart: ordered according to atomic
Today’s periodic table was ordered according to atomic numbers (原
3.4 The electron was the first subatomic
particle discovered
Electron was found
when Joseph John
studied the cathode
particles were much
indicating that the
atom was not the
smallest particle of
Now we know that
electrons determine
many of a material’s
properties, including
chemical reactivity,
color, etc.
Robert Andrews Millikan
(1868-1953) :excelled in his
experimental designs and
conclusions, in addition to
research, he also spent much
time preparing textbooks so
that his student didn’t have to
rely so much on lectures
Joseph John Thomson,
wan one of the first director of
the famous Cavendish
Laboratory of Cambridge
University in England, Where
almost all the discoveries
concerning subatomic particles
and their behavior were made
3.5 The mass of an atom is concentrated
in its nucleus
JJ proposed the “plumpudding model” for atoms.
Thomson proposed that the
atoms might be made of
thousands of
tiny ,negatively charged
particles swarming within
a cloud of positively
charge, much like plums
and raisins in an oldfashioned christmas plumpudding
Ernest Rutherford found the nucleus through
the “gold-foil” experiment
Rutherford’s Model of the Atom
3.6 The atomic nucleus is made of
protons and neutrons
Proton (质子) is a positively charged particle in a nucleus.
A proton is clearly 2000 times as heavy as an electron
but has the same (reverse) electric charge as an
The number of protons each atom of a given element
contains is called atomic number.
Neutron (中子) is another subatomic particle in nucleus,
having the similar mass as the proton but electrically
neutral. It has very important role in holding the atomic
nucleus together. The atom for a given element should
have a set number of proton, but the number of neutron
can be changed. Atoms of the same element that contain
different numbers of neutrons are isotopes (同位素).
Chemistry is the study of how atoms
combine to form materials. So chemistry
puts more attention to atoms (and
electrons in atoms) but not the nucleus.
The change of atoms through the
change of nucleus is not the center of
chemistry. It belongs to Nuclear Science
and Technology.
Chapter 4 (self study)

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