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Chapter 2
Atoms & Elements
CHE 123: General Chemistry I
Dr. Jerome Williams, Ph.D.
Saint Leo University
Overview
• Historical Overview of Atom
• Fundamental Chemical Laws
• The Structure of Atoms
Historical Overview of Atom
• Some early philosophers believed matter had
an ultimate, tiny, indivisible particle (atomos)
– Leucippus and Democritus
• Other philosophers believed that matter was
infinitely divisible
– Plato and Aristotle
• Because there was no experimental way of
proving who was correct, the best debater was
the person assumed correct, i.e., Aristotle
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Scientific Revolution
• In the late 17th century, the scientific
approach to understanding nature became
established
– Robert Boyle (The Sceptical Chemist)
• For the next 150+ years, observations about
nature were made that could not easily be
explained by the infinitely divisible matter
concept
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Law of Conservation of Mass
• In a chemical reaction,
matter is neither created nor
destroyed
• Total mass of materials you
have before the reaction
must equal the total mass of
materials you have at the
end
– total mass of reactants =
total mass of products
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Antoine Lavoisier
1743-1794
Fundamental Chemical Laws
• Law of Definite Proportions
– Different samples of a pure chemical
substance always contain the same
proportion of elements by mass.
– Example is water (11.1% H by mass; 88.9% O
by mass)
– Proposed by Joseph Proust (1754 – 1826)
Proportions in Sodium Chloride
A 100.0 g sample of sodium
chloride contains 39.3 g of
sodium and 60.7 g of
chlorine
A 200.0 g sample of sodium
chloride contains 78.6 g of
sodium and 121.4 g of
chlorine
A 58.44 g sample of sodium
chloride contains 22.99 g of
sodium and 35.44 g of
chlorine
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Example 2.1: Show that two samples of carbon
dioxide obey the Law of Definite Proportions
Given:
Find:
Conceptual
Plan:
Relationships:
Sample 1: 25.6 g O and 9.60 g C
Sample 2: 21.6 g O and 8.10 g C
proportion O:C
g O 1, g C1
g O 2, g C2
O:C in each sample
all samples of a compound have the same
proportion of elements by mass
Solution:
Check: both samples have the same O:C ratio, so the result is
consistent with the Law of Definite Proportions
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Practice – If a 10.0 g sample of calcite
contains 4.0 g of calcium, how much
calcite contains 0.24 g of calcium?
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Practice – How much calcite contains 0.24 g
of calcium?
Given:
Find:
Conceptual
Plan:
Sample 1: 4.0 g Ca and 10.0 g calcite
Sample 2: 0.24 g Ca
mass calcite in Sample 2, g
g Ca1, g calcite1
g Ca2
g calcite2
because
Relationships:
Solution:
Sig Figs &
Round:
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0.6 g calcite = 0.60 g calcite
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Fundamental Chemical Laws
• Law of Multiple Proportions
– Elements can combine in different ways to
form different substances whose mass ratios
are small whole-number multiples of each
other.
– Proposed by John Dalton (1766-1844)
Example 2.2: Show that two oxides of
nitrogen are consistent with the Law of
Multiple
Proportions
Given: nitrogen
dioxide:
2.28 g O per 1 g N
dinitrogen monoxide: 0.570 g O per 1 g N
Find: O in nitrogen dioxide:O in dinitrogen monoxide
Conceptual
g O in nit. dioxide
O1:O2
Plan:
g O in dinit. monox.
Relationships:
samples of different compounds that have
the same elements show proportions by
mass that are small whole number ratios
Solution:
Check: because the compounds have O:O ratio that is
a small whole number, so the results are
consistent with the Law of Multiple Proportions.
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Dalton’s Atomic Theory
• Proposed theory of matter (four postulates)
1. Elements are made of tiny indestructible particles called
atoms.
2. Atoms of a given element have the same mass and other
properties that distinguish them from atoms of other
elements.
3. Chemical combinations of elements to make different
substances occur when atoms join together in small
whole-number ratios.
4. Chemical reactions only rearrange the way that atoms
are combined; the atoms themselves are unchanged.
Practice – Decide if each statement
is correct according to Dalton’s
model of the atom
• Copper atoms can combine with zinc
atoms to make gold atoms
• Water is composed of many identical
molecules that have one oxygen atom and
two hydrogen atoms
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Practice – Decide if each statement
is correct according to Dalton’s
model
of the
• Copper atoms
can combine
with atom
zinc atoms to make
gold atoms – incorrect; according to Dalton, atoms of
one element cannot turn into atoms of another
element by a chemical reaction. He knew this
because if atoms could change it would change the
total mass and violate the Law of Conservation of
Mass.
• Water is composed of many identical molecules that
have one oxygen atom and two hydrogen atoms –
correct; according to Dalton, atoms combine together
in compounds in small whole-number ratios, so that
you could describe a compound by describing the
number of atoms of each element in a molecule. He
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Practice – Decide if each statement
is correct according to Dalton’s
Model of the Atom
• Some carbon atoms weigh more than
other carbon atoms
• Because the mass ratio of Fe:O in wüsite
is 1.5 times larger than the Fe:O ratio in
hematite, there must be 1.5 Fe atoms in a
unit of wüsite and 1 Fe atom in a unit of
hematite
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Practice – Decide if each statement
is correct according to Dalton’s
model of the atom
• Some carbon atoms weigh more than other carbon
atoms – incorrect; according to Dalton, all atoms of
an element are identical.
• Because the mass ratio of Fe:O in wüsite is 1.5
times larger than the Fe:O ratio in hematite, there
must be 1.5 Fe atoms in a unit of wüsite and 1 Fe
atom in a unit of hematite – incorrect; according to
Dalton, atoms must combine in small whole-number
ratios. If you could combine fractions of atoms, that
would mean the atom is breakable and Dalton’s first
premise would be incorrect. You can get the Fe:Fe
mass ratio to be 1.5 if the formula for wüsite is FeO
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andA Molecular
the formula
for hematite
is Fe O .
Structure of Atoms
• The Players found in atom
– Electrons are negatively charged particles. Discovered by J.J.
Thompson, who also proposed model of atom (Plum Pudding
Model of Atom)
– Protons are positively charged particles. Much larger than
electron in mass (about 1800 times larger)
– Neutrons are neutral (no charge).
• Atoms have an overall neutral charge.
Structure of Atoms
• Two Main Structures found in atom
– Nucleus is small, positively charged central core where protons
and neutrons reside. Most of mass in atom found here.
Discovered by E. Rutherford who disproved Thompson Model of
Atom (Gold Foil Experiment)
– Electron Cloud is external to nucleus and is where electrons are
found. Electrons are negatively charged.