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Chapter 3
Section 3.1: The Atom: From
Philosophical Idea to Scientific Theory

Atoms: the smallest particle of an element
that retains the properties of that element.
 (Greek: atomos = indivisible)

Democritus (Greek teacher in the 4th
century BC)
 First suggested the idea that atoms
existed
The Atom
1700’s – chemists were able to relate
changes to individual atoms
 Average atom size:
 Mass = 1 x 10 –23 g
 Diameter = 1 x 10-8 cm
 How small is that?100,000,000 copper
atoms in a row would = 1 cm in length!


Law of Conservation of Mass
 Definition: mass cannot be created or destroyed
only transformed
Law of Definite Proportions
 Definition: a chemical compound contains the
same elements in exactly the same proportions
by mass regardless of the size of the sample or
source
 Example:
▪ Sodium chloride: NaCl always consists of exactly
39.34% sodium & 60.66%chlorine by mass
▪ Water: H2O always consists of exactly 11.18%
hydrogen & 88.82% oxygen by mass
Law of Multiple Proportions
 Definition: if two or more different compounds
are composed of the same two elements, then
the ratio of the masses of the second element
combined with a certain mass of the first
element is always a ratio of small whole
numbers
 Examples:
 CO & CO2 : 1:1 ratio & a 1:2 ratio
 H2O & H2O2: 2:1 ratio & a 2:2 ratio
John Dalton
 English school teacher
 Proposed an explanation for the 3 laws
 Established in 1808
Dalton’s Atomic Theory
1. All elements are composed of tiny
indivisible particles called atoms.
2. Atoms of the same element are
identical. The atoms of any one
element are different from those of
any other element.
3. Atoms of different elements can combine
with one another in simple whole number
ratios to form compounds.
 H2O
C12H22O11
NOT
H2.5O¾
4. Chemical reactions occur when atoms are
separated, joined or rearranged. Atoms of
one element are not changed into atoms of
another by chemical means!
5. Atoms can not be subdivided.
Section 3.2 – The Structure of the Atom
Most of Dalton’s Atomic Theory is accepted
 One major revision includes that idea that
atoms are indivisible….
There are 3 parts to an atom….
1. electrons
2. protons
3. neutrons
Discovery of the Electron
Negatively charged subatomic particles
 J.J. Thomson discovered in 1897
 Passed a electric current through gases at low pressures
called a “Cathode Ray Tube”
 Noticed the surface of the tube directly opposite the
cathode glowed.
 Why? Opposites attract and the electrons were attracted
to the positive ends and lights up!
 Cathode Ray Tube
Cathode Ray Tube
Cathode rays are identical regardless of the
element
 Therefore all elements must have electrons!
Other important findings:
 Atoms are electrically neutral, so they must
contain a positive charge to cancel it out
 Since electrons are so small, atoms must contain
other particles that account for their mass

Robert Millikan (1868-1953)
 Found quantity of charge in 1
electron (e-)
 Also determined the ratio of the
charge to the mass of 1 e Calculated the mass of 1 eElectrons weigh 9.109 x10-31 kg
J.J. Thomson – plum-pudding model
 e- are spread evenly though out the positive
charge of the rest of the atom
 Ms. Agostine’s “mint chocolate chip ice
cream model”
Ernest Rutherford (1911)
 nucleus of the atom is positively
charged
 Gold Foil Experiment

Most  particles go straight through

Positively charged  particles deflect off of the
positively charged nucleus(~1/8,000)

Gold Foil Experiment

“…it was as if you fired a 15-inch [artillery]
shell at a piece of tissue paper and it came
back and hit you.”

Nucleus was very small

If a nucleus were a marble
 the atom would be a football field




Protons (p+)
Positively charged
particles
Mass = 1.673 x 10-27
kg
1,836 times heavier
then an electron
Neutrons (no)
Subatomic particles
with no charge
 Discovered by Sir
James Chadwick
 Mass is nearly the
same as a proton
 Mass = 1.675x10-27 kg


Particle Symbol Relative
Charge
Electron
e-
1-
Relative
Mass
(amu)
1/1836
Actual
Mass (kg)
Proton
p+
1+
1
1.67x10-27
neutron
no
0
1
1.68x10-27
9.11x10-31
Atomic Number : the number of protons in the
nucleus of an atom of an element
Atoms are electrically neutral
 Tells how many electrons there are also!
Periodic Table
#1 – Hydrogen: has 1 p+ and 1 e#6 – Carbon: has 6 p+ and 6 e-
Mass Number – total number of protons and
neutrons in a nucleus
# of neutrons = mass #
- atomic #
= (# p+ + # no) - (# p+)
Ex) Beryllium – 9
 Hyphen notation: The number “9” is the
mass number
# of p+?
# of no?
# of e-?
Definition – atoms that have the same number
of protons but different numbers of neutrons
 Different types of the same element
 Ex) Carbon – has 3 isotopes
 Carbon – 12
 Carbon – 13
 Carbon – 14
 Differ by # of no
All have the same # of p+
 If not, it would be a different element
 All have 6 protons
Carbon – 12
 Has 6 neutrons
Carbon – 13
 Has 7 neutrons
Carbon – 14
 Has 8 neutrons
Hydrogen-1: 1 p+ and 0 no
 Relative abundance = 99.985 %
 Commonly called normal “hydrogen”
Hydrogen-2: 1 p+ and 1 no
 Relative abundance = 0.015%
 Commonly called heavy hydrogen or “deuterium”
Hydrogen-3: 1 p+ and 2 no
 Relative abundance = ~0.00%
 Commonly called “tritium”
Definition – weighted average mass of the
atoms in a naturally occurring sample of the
element
Carbon-12 = 98.89 % abundant
Carbon-13 = 1.11% abundant
Carbon-14 = ~0.0000001% abundant
Formula:
Atomic = relative • mass # + relative • mass #
mass
abund.
#
abund.
+
Repeats for as many isotopes as exist for that element….
Units: atomic mass unit (amu): defined as
exactly 1/12 the mass of a carbon-12 atom
 1 amu = approximately the mass of 1 proton
 amu’s are used so you don’t have to use scientific notation
when talking about such small masses
Sample Problem:
 Chlorine has 2 isotopes:
chlorine-35 which is 75.77%
abundant and chlorine-37
which is 24.33% abundant.
What is the atomic mass of
chlorine?
35 Cl
= 75.77% abundant = 0.7577 rel. abund.
37 Cl = 24.33% abundant = 0.2433 rel. abund.
Atomic mass =
= (35 amu x 0.7577) + (37 amu x 0.2433)
= (26.5195 amu)
+ (9.0021 amu)
= 35.5 amu
Compare to value on Periodic Table = 35.45 amu
which rounds to 35.5 amu
We must have a way to relate masses in grams
to numbers of atoms…
How many definitions
did you come up with?
Any of various small
insectivorous mammals,
living chiefly
underground.
2. A spy who becomes part
of and works from within
the ranks of an enemy
governmental staff or
intelligence agency.
1.
3. Machinery. A large, powerful
machine for boring through
earth or rock, used in the
construction of tunnels.
4. A small, congenital spot or
blemish on the human skin,
usually of a dark color,
slightly elevated, and
sometimes hairy.
5. A massive structure, esp.
of stone, set up in the
water, as for a
breakwater or a pier.
6. An anchorage or harbor
protected by such a
structure.
7. A fleshy mass in the
uterus formed by a
hemorrhagic dead ovum.
(ew)
8. A spicy sauce flavored with
chocolate, usually served with
turkey or chicken.
9. The amount of a substance
that contains as many atoms,
molecules, ions, or other
elementary units as the
number of atoms in 12.01 g C.
The number is 6.02 × 1023, or
Avogadro's number.
Named in honor of
Amedeo Avogadro
di Quaregna
(1776-1856)
Proposed:
 Equal volumes of different gases at
the same temperature and pressure,
contain the same number of
molecules.
 How do you order donuts?
 How do you order cans of soda?
 How do you order computer paper?
 How do you order jelly beans?
 How do you order lunchmeat?
 You use a counting/measuring
amount!
1
1
1
1
1
dozen donuts = 12 donuts
case of soda = 24 cans
ream of paper = 500 sheets
pound of jelly beans = ~350 j.b.
pound of turkey = ~25 slices
 How big is that number really?!?!
 602,000,000,000,000,000,000,000
 How long of a paper clip chain?
 How big of a marshmallow
mountain?
 How big of a marble mountain?
 How tall would a stack of pennies
be?
How big is that number really?!?!
602,000,000,000,000,000,000,000
 1 mole of paperclips: goes around earth 4
trillion times
 1 mole of large marshmallows: cover the
continental USA 650 miles deep
 1 mole of marbles: 116 times the size of
Mt. Everest


1 mole of pennies stacked up:
distance to the moon 1.55x1012 times
= 1,550,000,000,000 times!
How much is a mole of water molecules?
Is equal to 18 mL
…that’s how small
molecules really are!
 Mole of iron = 55.85 g
 Mole of sulfur = 32.07 g
 It’s not individual grains of
iron or
sulfur!
 Where are these numbers coming
from?
…Periodic table!!!
 Definition:
the mass of one mole
of a substance (Units: g/mol)
 AKA the atomic mass on the
periodic table
 Round all elements masses to two
decimal places
 Exception: Hydrogen gets 3 decimal
places
12.0 grams of Carbon and 1.0 gram of
Hydrogen contain the same number of
atoms
 6.022 x 1023 atoms
 The gram atomic mass of any two
elements must contain the same
number of atoms
 Carbon
= 12.0107 g/mol
 Rounds to 12.01 g/mol
 Oxygen = 15.9994 g/mol
 Rounds to 16.00 g/mol
 Hydrogen = 1.00794 g/mol
 Rounds to 1.008 g/mol
What if you have
a compound?
 1 mole NaCl =
58.44 g/mol
 How did you
get that mass?
NaCl
1 Na = 22.99 g/mol
+ 1 Cl = 35.45 g/mol
58.44 g/mol
What is the molar mass of sugar?
C12H22O11?
C12H22O11
12-C x 12.01g/mol = 144.12 g/mol
22-H x 1.008 g/mol = 22.176 g/mol
+ 11-O x 16.00 g/mol = 176.0 g/mol
342.30 g/mol
What is the molar mass of aluminum
sulfate?
Al2(SO4)3
Count the atoms!
Al = ____, S=____, O=_____
What is the molar mass of aluminum
sulfate?
Al2(SO4)3
Count atoms - (distribute the 3)
Al = 2, S=3, O=12
Al2(SO4)3
2-Al x 26.98 g/mol = 53.96 g/mol
3-S x 32.07 g/mol = 96.21 g/mol
+ 12-O x 16.00 g/mol = 192.0 g/mol
342.17 g/mol
1. Gram Atomic Mass (gam)
2. Gram Molecular Mass (gmm)
3. Gram Formula Mass (gfm)
GAM: molar mass of an atom or
element
Ex) Fe, Cu, P, S…
GMM: molar mass of a molecule
(nonmetal compound)
Ex) H2O, Cl2, O2, CO2, P2O5
GFM: molar mass of a formula
unit (ionic compounds)
Metal-nonmetal compounds
Ex) NaCl, FeBr3, Zn3(PO4)2
If one mole of water is 18.02 g, how much
would two moles weigh?
 36.04 g
How much would 3 moles weigh?
 54.06 g
If one mole of water is 18.02 g, how many
moles are in 90.1 g?
 5 moles
Three types of Mole Conversions
 Mole
 Mole
– mass (g)
– representative particles
(molecules, atoms, formula units)
 Mole – volume (L)
What is the mass of 4 moles of NaCl?
Given:
Unknown:
Conversion Factor:
Solve:
Given: 4 moles NaCl
Unknown: g NaCl
Conversion Factor: molar mass of NaCl
1 mole NaCl = 58.44 g NaCl
Solve:
4 mol NaCl x 58.44 g NaCl = 233.76 g NaCl
1 mol NaCl
How many moles is 321.42 g NaCl?
Given:
Unknown:
Conversion Factor:
Solve:
Given: 321.42 g NaCl
Unknown: mole NaCl
Conversion Factor: molar mass of NaCl
1 mole NaCl = 58.44 g NaCl
Solve:
321.42 g NaCl x 1 mol NaCl = 5.5 mol NaCl
58.44 g NaCl
How many moles is 2.107x1024 mo of
O2?
Given:
Unknown:
Conversion Factor:
Solve:
Given: 2.107x1024 mo O2
Unknown: mol O2 ?
Conversion Factor: Avogadro’s #
1 mole O2 = 6.02x1023 mo O2
Solve:
2.107x1024 mo O2 x 1 mol NaCl = 3.5 mol O2
6.02x1023 mo O2
How many atoms are in 12.5 moles of
Copper?
Given:
Unknown:
Conversion Factor: ?
Solve:
Given: 12.5 mol Cu
Unknown: at Cu?
Conversion Factor: Avogadro’s #
1 mole Cu = 6.02x1023 at Cu
Solve:
12.5 mol Cu x 6.02x1023 at Cu = 7.53x1024 at Cu
1 mol Cu
If one mole of Helium weighs 4.00
g/mol, how much does 2 moles weigh?
Can you weigh out something less dense
then air?
You CANNOT weigh a gas less dense
then air!
Gravity does not pull it down on the
scale!
The volume of mole of a gas is much more
predictable than that of a liquid or solid
 That is, under the same physical conditions
(STP)
 Standard temperature and pressure
▪ Standard Temperature is 00C
▪ Standard Pressure is 1 atm
At STP, 1 mole of any gas will
occupy a volume of 22.4 L
 22.4 L is known as the molar volume of a gas
What does it mean?
 It means that 22.4 L of any gas at STP contains
6.02 X 1023 representative particles of that gas.
Standard Temperature and Pressure (STP):
specific conditions that can be reached in the lab
 Standard Pressure: 1 atmosphere (atm)
 Standard Temperature: 0oC or 273 K
Temperature = a measurement of the
average kinetic energy of the particles of an
object
Scales
 Fahrenheit (oF)
 Celsius (oC)
 Kelvin (K)
Boiling
Point
Room
Temp
Freezing
Point
Absolute Zero – coldest possible
temperature where all movement stops,
theoretical value
0 Kelvin = -273.15 oC = -459 oF
Equations:
K = oC +273
oC = K – 273
Example:
What is 98 K = _-175 oC?
What is 159 oC = _432 K?
How many moles is 145.6 L of O2 gas at
STP?
Given:
Unknown:
Conversion Factor:
Solve:
Given: 145.6 L O2
Unknown: mol O2 ?
Conversion Factor: molar volume
1 mole O2 = 22.4 L O2
Solve:
145.6 L O2 x 1 mol O2 = 6.500 mol O2
22.4 L O2
How many liters is 11 moles of O2 gas at
STP?
Given:
Unknown:
Conversion Factor:
Solve:
Given: 11 mol O2
Unknown: L O2
Conversion Factor: molar volume
1 mole O2 = 22.4 L O2
Solve:
11 mol O2 x 22.4 L O2
1 mol O2
= 246.4 L O2
How many liters of gas are 3.612x1024
atoms of Neon gas at STP?
Given:
Unknown:
Conversion Factor:
Solve:
Given: 3.612x1024 atoms Neon
Unknown: L Ne
Conversion Factor: needs TWO!
Avogadro’s number
1 mole Ne = 6.02x1023 at Ne
molar volume
1 mole O2 = 22.4 L O2
THERE IS NO DIRECT ROAD FROM ATOMS  L
Solve:
3.612x1024 at Ne x __1 mol Ne_ x 22.4 L Ne =
6.02x1023 at Ne 1 mol Ne
In your calculator it looks like this:
3.612e24 x 1 x 22.4 ÷ 6.02e23 ÷ 1
= 134.4 L Ne