Download History of Atomic Structure

10/8/13
Atomic Structure
Matter Consists of Particles
Aristotle
John Dalton
1803
I am a
genius!
Democritus
What is an atom?
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.
JJ Thompson (1897)
!   Smallest particle of an
element that retains
the properties of that
element
Gold Atom
Model
Robert Millikan (1909)
!   Used a cathode ray tube
!   Oil drop experiment
and found negatively
charged particles
!   Discovered electrons (e)
!   Plum pudding model
!   Could not determine the
mass
!   Measured charge of
electron
n 
Charge = -1
!   Calculated mass of
electron
Mass = 9 x 10-28 grams
or….
n  1/2000 mass of H atom
n 
1
10/8/13
Gold Foil Experiment
Ernest Rutherford (1909)
!   Discovered:
n  1. nucleus is positively charged, very dense, &
very small (1/10,000 of diameter)
n  2. electrons are in space surrounding nucleus
n  3. most of atom is empty space
Scientists Recap
What does this mean?
Aristotle
Democritus
•Believed universe made of 4 elements: earth, air,
fire, and water
•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
Rutherford
•Gold foil experiment
•Nucleus positive and dense
•Measured mass and charge of electron
Thompson’s Plum Pudding model is now replaced with Rutherford’s
model.
Modern Atomic Theory
•Electrons in space surrounding nucleus
Old
vs.
New
!   Electrons (e) occupy “cloud” outside of
nucleus
n  They
DO NOT orbit around the nucleus like the
planets do around the sun…
n  Impossible to know where an electron is at any
given time
Orbit
Electron Cloud
2
10/8/13
Protons (P)
Neutron (N)
!   No charge = 0
!   Proton
!   Mass = 1.675 x 10-23 g or 1
n  +1
charge
n  Mass: 1.673 x 10-23
g (almost mass of H
atom) or 1 amu
n  Location: nucleus
amu
!   Location: nucleus
!   Discovered by Chadwick
(1932)
!   Neutron is slightly heavier
than proton
(1.675 vs. 1.673 x 10-23 grams)
Atomic Number
11
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
n  Positive
charge = negative charge OR
22.99
n  Number of protons = number of electrons
Mass Number
!   Mass number = number of protons &
neutrons
Neutron
Electron
Nucleus
Mass number = 12
Proton
Atomic number = 6
Element = Carbon (C)
Note: Use the mass number on the periodic table, unless
I tell you otherwise.
Isotopes
Isotopes of Carbon
!   Atoms of the same element having different
number of neutrons
SAME: Element
# Neutrons = mass number – atomic number
C-14
or
DIFFERENT: # Neutrons
#P
#e
C-12
Masses
12
6
C
Nucleus
or
6P
6e
6N
14
6
C
6P
6e
8N
Nucleus
3
10/8/13
Isotopes of Hydrogen
Hydrogen – 1
(protium)
1P
1
H
1e
1
0N
Hydrogen – 2
(deuterium)
1P
2
H 1e
1
1N
“Heavy Water”
Hydrogen – 3
(tritium)
1P
3
H 1e
1
2N
Radioactive
Atomic Mass
Let’s try
it.
!   Atomic mass on the Periodic Table is the
!   Mass of atom relative to Carbon-12
(standard)
!   Unit = atomic mass unit or amu
n  1
amu = 1/12 mass of a C-12 atom
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?
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
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
4
10/8/13
Put it all together
I need some volunteers!
Model of the atom 19
F
9
Draw the model on the board.
Electron
Cloud
9P
10 N
Nucleus
7
Li
3
39
K
19
16
O
8
23
Na
11
# Neutrons = atomic mass – atomic number
Nuclear Reactions
Stable Nuclei
!   The composition of the nucleus is changed.
!   Stable nuclei are NOT
radioactive
!   Stable nuclei are
elements #1-83
(#84àare radioactive)
!   Strong nuclear forces
= attraction between
particles in nucleus
that hold it together
n 
P
P
VERY STRONG!
t,
(= 3 Transparency Master
Types of Radioactive Decay
130
120
•  #1-20 equal number
of protons & neutrons
for stable nuclei
•  #21-83 nuclei need more
& more neutrons to be
stable
•  #84à radioactive (all
isotopes
110
100
90
{\--
a'
c
BO
f
70
o
L
o
c
o
L
o
_o
60
E
z-,
50
40
30
!   1. Alpha particles
!   2. Beta particles
!   3. Gamma particles
20
10
0
10
20
30 40
50
60
70
80
Number of protons
@ Prentice-Hall, lnc
Chapter
3
7
5
10/8/13
Alpha Particles
4
=
α
2 He
2
4
!   Consists of 2 protons
Write the nuclear equation for the alpha
231
decay of
Pa
& 2 neutrons
!   Has a +2 charge
!   Identical to a He-4
nucleus
!   Stopped by paper
226
88
Alpha decay problems
91
231
227
Pa à 42 α + 89 Ac
91
Write the nuclear equation for the alpha
decay of 244 Pu
4
222
244
2
86
94
Ra à α + Rn
Beta particles
0
-1
0
β =-1 e
94
Pu
240
4
à α + U
92
2
Beta Decay problems
Write a nuclear equation for the beta decay of
223
!   High speed electron is
emitted out from atom
!   -1 charge
!   Stopped by heavy
clothing
!   Neutron changes into a
proton & an electron
131
53
Fr
87
223
87
Fr à
0
-1
e + Ra
223
88
Write a nuclear equation for the beta decay of
50
22
Ti
0
131
50
-1
54
22
I à e + Xe
Gamma Radiation
0
0
Ti à
0
-1
50
e+ V
23
Recap
!   High energy
!   Radiant energy
!   0 charge, 0 mass
!   Most penetrating
n  Stopped by lead or
concrete
6
10/8/13
Other Nuclear Reactions
!   Fission is splitting of the nucleus
Fission
Chain Reaction
!   Fusion is joining of nuclei
Fusion
The Mole
The Mole
What is a mole really?
!   1 mole contains 6.02 x 1023 atoms
!   The atomic mass of an element expressed in
grams is:
n  1
mole of that substance OR its gram-atomic
mass (GAM)
7
10/8/13
Lets try it.
Put it together
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.
_________
1 mole = GAM (from P.T.) and
1 mole = 6.02 x 1023 particles
_____
24.31 g of Mg = 1mol of Mg atoms and contains
6.02 x 1023 atoms.
_________
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
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
83.80 g Kr = 1 mole Kr
32.06 g S = 1 mole S
From Periodic Table
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
8
10/8/13
The mole map
6.02 x 1023 atoms
1 mol
Find the number of moles in 22.5 g of
beryllium (Be).
1 mol
# particles
(atoms)
6.02 x 1023 atoms
GAM
1 mol
22.5 g Be x 1 mol Be = 2.4972 = 2.50 mol of Be
9.01 g Be
MOLE
GAM
1 mol
Last time
1 mole Be = 9.01 g Be
g
MOLE
!   A mole (mol) is defined as the number of
STOP!
The Mole Start of Chem
Calculations:
atoms in exactly 12 grams of Carbon-12.
!   The mole is the SI unit for the amount of a
substance.
Mol
!   The Carbon-12 based Atomic Mass Scale-
By 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.
!   Molar Mass [g/mol]= mass of one mole in grams
to two decimal places off the periodic table.
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.
!   Grams
9
10/8/13
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.
JJ Thompson (1897)
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
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
10
10/8/13
JJ Thompson
“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.
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?
• Cathode Ray Tube (CRT)
• Discovered the electron
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:
Radioactivity
!   How did Rutherford use the results of his
!   Henri Becquerel (1896)
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?
•  Discovered that U exhibits radioactivity
(spontaneous emission of radiation from an
element)
•  Uranium one of many naturally radioactive
elements
11
10/8/13
Radioactivity
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 α,β,γ radiation.
!
Ernest Rutherford
!
!   Marie and Pierre Curie
- Isolated 2 other
radioactive elements:
Polonium and Radium
Alpha decay
Student of JJ Thompson
!   Alpha decay: a nucleus emits an alpha particle consisting
!   Gold Foil Experiment (α scattering expt)
of 2 neutrons and 2 protons, which is a helium nucleus.
!   Began in depth study of radioactivity!   Found that U emits two forms of radiation
and developed his nuclear model of atom.
Alpha and beta decay practice
problems:
!   NUCLEAR EQUATION- to keep track of
the reaction components
Alpha decay Example:
226
222
Ra
Rn +
88
86
4
He
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 (β 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
12
10/8/13
Beta Decay Problems
Gamma decay
!   Gamma radiation: nuclear particles shift energy levels, like
!   Beta Decay Example:
131
I
53
131
54
Xe +
0
-1
β
! 
! 
Turns a neutron in the nucleus into a proton, ejecting a beta
Particle (-) or electron in the process.
! 
! 
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
In Radioactive Decay- sums of mass # and
Atomic # are the same before and after the rxn.
Radiation is dangerous because it strips away the electrons from
atoms in cells, causing them to malfunction.
Radiation Experiment
! 
(+)
(-)
Half Life-
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
carbon-14 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
13