Reading Atoms From Democritus to Dalton
• Empedocles – argued that all matter was composed of four elements:
fire, air, water, and earth.
• Democritus – infinitesimally small pieces of matter atomos, meaning
"indivisible." He suggested that atomos were eternal and could not
be destroyed.
• Torricelli & Bernoulli – showed that air had weight and was capable of
pushing down on a column of liquid mercury, pictured air and other
gases as loose collections of tiny billiard-ball-like particles that are
continuously moving around and bouncing off one another.
• Priestley – substances could combine together or break apart to form
new substances with different properties.
• Lavoisier – Law of Conservation of Mass
Goal of reading: Understand key hypotheses that allow us to get out of
the particle model.
Dalton's atomic theory had four main concepts:
1. All matter is composed of
indivisible particles called atoms.
2. All atoms of a given element are
identical; atoms of different
elements have different
properties.
3. Chemical reactions involve the
combination of atoms, not the
destruction of atoms.
4. When elements react to form
compounds, they react in
defined, whole-number ratios.
Dalton’s atomic theory vs.
Atomic Theory
Atomic Theory – states that
matter is composed of discrete
units called atoms
In the decades after their work, other
scientists would show that atoms are not
solid billiard balls, but complex systems of
particles
Worksheet #2
Hydrogen
Hydrogen
Oxygen
Chlorine
Water
Product
Worksheet #2
Nitrogen
Hydrogen
Ammonia
Reactions of Gases
• Gay-Lussac noted that gases appear
to react in simple integer ratios
– Example: Two volumes of hydrogen
reacted with one volume of oxygen to
produce two volumes of water
• These findings appeared to
contradict the idea that equal
volumes of gases have equal
numbers of particles
Avogadro’s Key Contribution:
The molecules of some gaseous elements must
contain two atoms.
Hydrogen
Chlorine
Hyrdrogen Chloride
Avogadro’s Hypothesis assumed:
1. Equal volumes of gases have
equal numbers of molecules.
2. These particle can be split into
half-particle during chemical
reactions (particles of some
gaseous elements must
contain two atoms.)
3. That particles of elemental
gases could contain more than
a single atom.
Worksheet #2
Hydrogen
Oxygen
Water
Avogadro’s Hypothesis
• Two volumes of hydrogen react with one
volume of oxygen to produce two volumes of
water when hydrogen and oxygen can be split
into half-molecules!
• A diatomic is made of two of the same
element bonded together (7 + 1).
Is everything a simple particle?
• One of the greatest challenges early chemists
faced was trying to find a way to connect the
mass of a substance to the number of
particles in the sample.
Avogadro’s Hypothesis
• If we accept Avogadro’s Hypothesis, we can
compare the mass of various gases and
deduce the relative mass of the molecules.
• To do this, we pick a weighable amount of the
lightest element (how about 1.0) then use
mass ratios to assign atomic masses to the
other elements.
Relative Mass
• To assign relative masses to elements it is
necessary to know that the samples being
compared have the same number of
particles
– If particles are all the same size, the same size
sample would have the same number of
particles
– If particles are different sizes, the same size
sample would have different numbers of
particles
Relative Mass
• At the time, chemists did not know which was
true and tended to think the second one was
more likely
– Example: A bucket of baseballs has fewer balls
than an identical bucket of golf balls
– If this is true in the macroscopic world, why
wouldn’t it be true in the microscopic one?
Relative Mass
• Consider earlier this year when we studied
density: was iron more dense than the stopper
because iron had more particles per given volume
than the stopper or because iron’s individual
particles were more massive than the stopper’s?
Could it be some combination of both?
Relative Mass
• We did not know earlier
this year, neither did
Dalton.
Percent Composition
• Law of conservation of mass
– 100 gram of carbon that react with 100
grams of oxygen.
– After the compound is made there is left
43g O and 58g C.
– How much does compound weight?
– How many grams of oxygen and carbon are
in the products?
Law of Definite Proportions
Law of Definite Proportions – A
compound is always composed of the
same elements in the same proportion
by mass
Use percent mass to calculate
proportion of each element.
Percent Composition
Mass of element * 100 = % element
mass of compound
A 20.00 g sample of table sugar was
analyzed by mass. Carbon was 8.44g,
hydrogen was 1.30g, and the rest was
oxygen. Determine its percent
composition of each element.
Percent Composition
Since the Law of Definite Proportions
states that a compound is always
composed of the same elements in the
same proportion by mass:
This table sugar compound is always
42.20% carbon, 6.50 % hydrogen, and
51.30% oxygen, regardless of the sample
size.
Law of Multiple Proportions
The masses of one element which
combine with a fixed mass of the
second element are in a ratio of simple
whole numbers.
Dalton’s Atomic Theory
• Dalton's Atomic Theory had four main
concepts:
– All matter is composed of indivisible particles
called atoms.
– All atoms of a given element are identical;
atoms of different elements have different
properties.
– Chemical reactions involve the combination of
Dalton’s
Law of ofMultiple
Proportions
atoms, Law:
not the destruction
atoms.
– When elements react to form compounds,
they react in defined, whole-number ratios.
ANSWER
• The best way is…..
– If it is a gas (same number of particle per
unit volume)
– If it is a liquid or gas (different number of
particle per unit volume)