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Atomic Structure
Matter Consists of Particles
Aristotle
Democritus
John Dalton
1803
I am a
genius!
Atomic Theory of Matter based
on the following postulates:
1) Each element is composed of
particles called atoms.
2) All atoms of a given element are
identical.
3) Atoms are neither created nor
destroyed in any chemical reaction.
4) A given compound always has the
same relative numbers and kinds of
atoms.
What is an atom?
Smallest particle of an
element that retains
the properties of that
element
Gold Atom
Model
JJ Thompson (1897)
Used a cathode ray tube
and found negatively
charged particles
Discovered electrons (e)
Plum pudding model
Could not determine the
mass
Robert Millikan (1909)
Oil drop experiment
Measured charge of
electron

Charge = -1
Calculated mass of
electron


Mass = 9 x 10-28 grams
or….
1/2000 mass of H atom
Gold Foil Experiment
Ernest Rutherford (1909)
Discovered:
1. nucleus is positively charged, very dense, &
very small (1/10,000 of diameter)
 2. electrons are in space surrounding nucleus
 3. most of atom is empty space

What does this mean?
Thompson’s Plum Pudding model is now replaced with Rutherford’s
model.
Scientists Recap
Aristotle
•Believed universe made of 4 elements: earth, air,
fire, and water
Democritus
•Believed matter made of particles he called atoms
Dalton
•Atomic Theory of Matter
Thompson
•Discovered electrons using cathode ray tube
•Plum pudding model
Millikan
•Oil drop experiment
•Measured mass and charge of electron
Rutherford
•Gold foil experiment
•Nucleus positive and dense
•Electrons in space surrounding nucleus
Modern Atomic Theory
Electrons (e) occupy “cloud” outside of
nucleus
They DO NOT orbit around the nucleus like the
planets do around the sun…
 Impossible to know where an electron is at any
given time

Old
vs.
Orbit
New
Electron Cloud
Protons (P)
Proton
 +1 charge
 Mass: 1.673 x 10-23
g (almost mass of H
atom) or 1 amu
 Location: nucleus
Neutron (N)
No charge = 0
Mass = 1.675 x 10-23 g or 1
amu
Location: nucleus
Discovered by Chadwick
(1932)
Neutron is slightly heavier
than proton
(1.675 vs. 1.673 x 10-23 grams)
11
Atomic Number
Na
Atomic number will be the smallest number
shown!
22.99
Atomic # = number of protons in an atom
11 element have the same
All atoms of given
atomic number
Atoms are neutral therefore….
Na
Positive charge = negative charge OR
22.99
 Number of protons = number of electrons

Mass Number
Mass number = number of protons &
neutrons
Neutron
Electron
Nucleus
Mass number = 12
Atomic number = 6
Element = Carbon (C)
Proton
Isotopes
Atoms of the same element having different
number of neutrons
SAME: Element
DIFFERENT: # Neutrons
#P
#e
# Neutrons = mass number – atomic number
Masses
Note: Use the mass number on the periodic table, unless
I tell you otherwise.
Isotopes of Carbon
C-12
C-14
or
or
12
6
C
Nucleus
6P
6e
6N
14
6
C
6P
6e
8N
Nucleus
Isotopes of Hydrogen
Hydrogen – 1
(protium)
1P
1
H
1e
1
0N
Hydrogen – 2
(deuterium)
1P
2
H 1e
1
1N
Hydrogen – 3
(tritium)
1P
3
H 1e
1
2N
Radioactive
“Heavy Water”
Atomic Mass
Mass of atom relative to Carbon-12
(standard)
Unit = atomic mass unit or amu

1 amu = 1/12 mass of a C-12 atom
Let’s try
it.
Atomic mass on the Periodic Table is the
average atomic mass, based on abundances
of each isotope in nature.
What is the atomic mass of Li? 6.94 amu
What is the atomic mass of Cl? 35.45 amu
What is the atomic mass of As 74.92 amu
Why are they fractional?
Why is B 10.81 amu and not just 11?
There are two isotopes of B
10
11
B
B
5
5
Scientists take an average of the isotopes to
calculate the atomic mass
Which one is more abundant in nature?
*NOTE
Atomic masses are generally fractional
Mass numbers are rounded to the nearest
whole number.
For example:
Carbon’s atomic mass is 12.01
 Carbon’s mass number is 12
 What about Be, Na, B?

Calculating the average atomic mass
Cl: 75.5% is Cl-35 (atomic mass = 34.97 amu)
24.5% is Cl-37 (atomic mass = 36.97 amu)
What is the atomic mass on the Periodic Table?
.755(34.97) + .245(36.97) = 35.46 = 35.5 amu
Put it all together
Model of the atom 19
F
9
9P
10 N
Electron
Cloud
Nucleus
# Neutrons = atomic mass – atomic number
I need some volunteers!
Draw the model on the board.
7
Li
3
39
K
19
16
O
8
23
Na
11
Nuclear Reactions
The composition of the nucleus is changed.
Stable Nuclei
Stable nuclei are NOT
radioactive
Stable nuclei are
elements #1-83
(#84are radioactive)
Strong nuclear forces
= attraction between
particles in nucleus
that hold it together

VERY STRONG!
P
P
• #1-20 equal number
of protons & neutrons
for stable nuclei
• #21-83 nuclei need more
& more neutrons to be
stable
• #84 radioactive (all
isotopes
Types of Radioactive Decay
1. Alpha particles
2. Beta particles
3. Gamma particles
=
α
2
4
Alpha Particles
Consists of 2 protons
& 2 neutrons
Has a +2 charge
Identical to a He-4
nucleus
Stopped by paper
226
88
Ra  α + Rn
4
2
222
86
4
2
He
Alpha decay problems
Write the nuclear equation for the alpha
231
decay of
Pa
91
Pa

91
231
4
2
α
227
89
+ Ac
Write the nuclear equation for the alpha
decay of 244 Pu
94
244
Pu
94
 α
4
2
+
240
92
U
Beta particles
0
-1
0
-1
β= e
High speed electron is
emitted out from atom
-1 charge
Stopped by heavy
clothing
Neutron changes into
a proton & an electron
I  e + Xe
131
53
0
-1
131
54
Beta Decay problems
Write a nuclear equation for the beta decay of
223
Fr
87
223
87
Fr 
0
-1
223
e + Ra
88
Write a nuclear equation for the beta decay of
50
22
Ti
50
22
Ti 
0
-1
50
23
e+ V
0
Gamma Radiation
High energy
Radiant energy
0 charge, 0 mass
Most penetrating

Stopped by lead or
concrete
0
Recap
Other Nuclear Reactions
Fission is splitting of the nucleus
Fusion is joining of nuclei
Fission
Chain Reaction
Fusion
The Mole
The Mole
1 mole contains 6.02 x 1023 atoms
The atomic mass of an element expressed in
grams is:

1 mole of that substance OR its gram-atomic
mass (GAM)
What is a mole really?
Lets try it.
16.00 g of O = 1 mol of O atoms and contains
6.02 x 1023 atoms
32.06 g of S = 1 mol of S atoms and contains
_____
6.02 x 1023atoms.
_________
_____
24.31 g of Mg = 1mol of Mg atoms and contains
6.02 x 1023 atoms.
_________
Put it together
1 mole = GAM (from P.T.) and
1 mole = 6.02 x 1023 particles
Avogadro’s Number
Mole conversions
What is the mass in grams of 4.00 moles of
krypton (Kr) atoms?
4.00 moles Kr x 83.80 g Kr = 335.2 = 335 g Kr
1 mole Kr
83.80 g Kr = 1 mole Kr
From Periodic Table
Practice
What is the mass of 0.30 moles of sulfur (S)?
0.30 mol S x 32.06 g S
= 9.618 = 9.6 g of S
1 mol S
32.06 g S = 1 mole S
More practice
How many moles of boron (B) are present in
22 grams?
22 g B x 1 mol B
= 2.035 = 2.0 mol of B
10.81 g B
10.81 g B = 1 mole B
Again
How many moles are there in 9.3 g of
phosphorous (P)?
9.3 g P
x
1 mole P = .30029= .30 mol of P
30.97 g P
1 mole P = 30.97 g P
Last time
Find the number of moles in 22.5 g of
beryllium (Be).
22.5 g Be x 1 mol Be = 2.4972 = 2.50 mol of Be
9.01 g Be
1 mole Be = 9.01 g Be
The mole map
6.02 x 1023 atoms
1 mol
MOLE
1 mol
6.02 x 1023 atoms
GAM
1 mol
1 mol
GAM
g
# particles
(atoms)
STOP!
MOLE
A mole (mol) is defined as the number of
atoms in exactly 12 grams of Carbon-12.
The mole is the SI unit for the amount of a
substance.
The Mole Start of Chem
Calculations:
The Carbon-12 based Atomic Mass ScaleBy definition, an atom of this isotope is defined as
having the mass of exactly 12.000 amu (atomic mass
units) . In other words, an amu is defined as 1/12th of
the mass of one atom of Carbon-12.
Why Carbon-12?
Carbon is a very common element, available to any
scientist and by choosing the amu to be this size, the
atomic masses of nearly all the other elements are
almost whole numbers, with the lightest atom having
a mass of ~1.
Hydrogen-1= 1.007825amu when Carbon is assigned a
mass of exactly 12.
Mol
Molar Mass [g/mol]= mass of one mole in grams
to two decimal places off the periodic table.
Grams
MOL
Molecules
1 mol=1GAM=6.02 x1023molecules
6.02 x 1023molecules - Avogadro’s #
Avogadro’s number: is a defined mass of an
element (it’s atomic weight) there is a precise
number of atoms.
CONVERSIONS-dimensional
Analysis (again!)
Mole: amount of substance in grams
PRACTICE:
COMPLETE FORMULA WS
COMPLETE PROBLEM WS
Determine which of Dalton’s postulates
explains each observation?
1. Matter can never really be thrown away. That is one reason
that recycling is important
2. The formula for ethanol is C2H6O, and the formula for
acetic acid in vinegar is C2H4O2
3. There is no difference between Cu found in an ancient
Mayan necklace and Cu wire freshly made from Cu ore.
4. Zn is a metal that is softer that Fe and it reacts more readily
with acid than Fe does.
5. When Methane, CH4, Burns, it combines with O2, in the air
to form molecules of H2O and CO2.
I. Law of Conservation of Matter:
Discovered by:?
II. Law of Constant Composition:
Joseph Proust
A given compound always contains the same
elements in the same proportions by mass
Water= 11% H2 , 89%O2
Iron Oxide= FeO= 25% O2, 75%Fe
JJ Thompson (1897)
CATHODE RAY TUBE
•JJ Thompson conducted a series of systemic studies on cathode rays
•Watched the deflection of charges in a CRT and put forth the idea that
Atoms were composed of (+) and (-) charges.
•Negative charges = electrons
•Electron mass ratio= 1.76 x 108 C/g COULD NOT DETERMINE
MASS!
Electrical current: moving stream of electrical charge
Cathode: negative electrode
Anode: positive electrode
“Could anything at first sight
seem more impractical than a
body which is so small that its
mass is an insignificant fraction
of the mass of an atom of
hydrogen?"
-- J.J. Thomson.
JJ Thompson
•Cathode Ray Tube (CRT)
•Discovered the electron
Michael Faraday
Suggested that the structure of atom
was somewhat related to electricity.
Atoms contain particles that have
electrical charge.
Story of electricity and the atom
includes a certain American whose
name you should find very familiar?
Benjamin Franklin
Made distinction between
2 kinds of electrical charge,
Positive (+) and negative (-)
Opposite charges: attract.
Like Charges: repel
Where do + and – charges
come from?
What are their physical properties?
Led to his discovery of another
Electrical device called a battery.
Question:
How did Rutherford use the results of his
alpha scattering experiment experiment to
challenge the plum pudding model of the
atom?
How does Rutherford’s model of the atom
differ from the plum pudding model?
Radioactivity
Henri Becquerel (1896)
• Discovered that U exhibits radioactivity
(spontaneous emission of radiation from an
element)
• Uranium one of many naturally radioactive
elements
Radioactivity
Radioactivity: spontaneous release of energetic particles or
photons from the nuclei. Original nucleus decomposes,
decays to form a new nucleus, releasing radiation in the
process.
There are 3 types of radioactive decay.
1.Alpha
2.Beta
3.Gamma
Radioactive decay- when an atom emits ,, radiation.
Radioactivity
Marie and Pierre Curie
- Isolated 2 other
radioactive elements:
Polonium and Radium
Ernest Rutherford
Student of JJ Thompson
Gold Foil Experiment (α scattering expt)
Began in depth study of radioactivityFound that U emits two forms of radiation
and developed his nuclear model of atom.
Alpha decay
Alpha decay: a nucleus emits an alpha particle consisting
of 2 neutrons and 2 protons, which is a helium nucleus.
Alpha and beta decay practice
problems:
NUCLEAR EQUATION- to keep track of
the reaction components
Alpha decay Example:
226
222
4
Ra
Rn +
He
88
86
2
Nucleus rejects a helium nucleus or alpha particle and becomes
A smaller nucleus with less positive charge
Beta decay
Beta decay: a neutron in a nucleus spontaneously decays into a proton, an
electron, and a neutrino, thus creating a different element.
Carbon-14 > nitrogen-14. Due to a weak force.
Stream of high speed electrons- not electrons in motion around nucleus
Beta radiation comes from changes in nucleus: nuetrons changes into a
proton and a electron, proton remains in nucleus, electron ( particle)
propelled out of nucleus at high speeds.
Mass # is zero
100x more penetrating that alpha radiation
Able to pass through clothing and damage skin
Beta Decay Problems
Beta Decay Example:
131
131
I
Xe
53
54
+
0
-1

Turns a neutron in the nucleus into a proton, ejecting a beta
Particle (-) or electron in the process.
In Radioactive Decay- sums of mass # and
Atomic # are the same before and after the rxn.
Gamma decay
Gamma radiation: nuclear particles shift energy levels, like
electrons undergoing a quantum leap, and X-rays or
gamma rays are emitted.
Very dangerous
does not consist of particles
Penetrates solid material including body tissues
Stopped by lead or concrete
Radiation is dangerous because it strips away the electrons from
atoms in cells, causing them to malfunction.
Radiation Experiment
(+)
(-)
Half Life
Contents [hide]
Half-life: the amount of time it takes half of a batch of radioactive
material to decay. Ranges from less than a second to billions of years,
depending upon the isotope.
Radiometric dating: The technique of measuring quantities of isotopes
in order to determine time scales. e.g.: the ratio of radioactive carbon14 to carbon-13 in an organism at the time of its death is the same as
the general environment. After death, no more carbon is taken in, so
the % of C-14 goes down as it decays into N-14. Half decays every
5700 years, so time of death can be calculated.
Eg the Shroud of Turin and the Dead Sea Scrolls were dated at the
University of Arizona with this technique.
EXTRA CREDIT ON THE TEST: LOOK UP HOW RADIOACTIVE
DATING WAS USED FOR THE KENNEWICK MAN AND THE
Haraldskｾr Woman
Half Life-