Early Atomic Theorists
Democritus (460-370 BC) John Dalton (1766-1844)
All matter is composed of atoms that are indestructible and indivisible
ResearchNo Research
Dalton’s Atomic Theory
All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
Atoms cannot be created, divided into smaller particles or destroyed.
Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds.
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
The Atom
The smallest particle of an element that retains the property of that element
Relative Size
World Population 6000000000
Atoms in a penny 29000000000000000000000
Discovering the Electron
Experiments with electricity, using cathode ray tubes led to the discovery cathode rays.oCathode rays were a stream of charged particlesoThe particles carried a negative charge
Cathode (-)Anode (+)
JJ Thompson
Disproved Daltons theory that atoms were indivisible because he determined that the mass of a cathode particle was far less than that of a hydrogen atom.
Determined the Charge to Mass ratio of cathode particles (~1/1840 the mass of a hydrogen atom) and discovered the electron
Proposed the “Plum Pudding” model of the atom
+ +
+ +
+ +
+
-
-
-
-
- -
-
-
Ernest RutherfordDiscovered the nucleus of the atom with the “Gold Foil” experiment
Nuclear Model of the Atom
o Most of the atom consist of electrons moving rapidly through empty space.
o Electrons are held in place in the atom by their attraction to a positively charged nucleus
Does this picture accurately represent Rutherford’s Nuclear Model?
James Chadwick
Rutherford model, consisting of electrons and protons could not account for the total mass of the atom, which led to the discovery of the neutron.
Discovered the Neutron
Properties of Subatomic Particles
Particle Symbol LocationRelativeElectricalCharge
Relative Mass
ActualMass (g)
Electron e- Space surrounding the nucleus
1- 1_ 1840
9.11 x 10-28
Protonp+
Nucleus1+ 1 1.637 x 10-24
Neutronn0
Nucleus0 1 1.675 x 10-24
Review
6
C12.011
Atomic Number
Atomic Number = Mass Number = protons + neutrons
The weighted average mass of all the isotopes of an element
= # protons
= # electrons
Change in the number of protons results in a new atom
Change in the number of electrons results in an ion (+ or – charge)
Change in the number of neutrons results in an isotope
Isotopes
C146
Nuclear or Nuclide notation
(used in nuclear equations)
C-14Mass Number
Atomic Mass
Atoms of the same element that contain the same number of protons and different numbers of neutrons.
Isotope Notation
Mass Number
Atomic Mass Problems
Boron has two naturally occurring isotopes: boron-10 (19.8%, 10.013 amu) and boron 11 (80.2%, 11.009 amu). What is the atomic mass of boron?
Find the Abundance x Mass for each isotope and then add products together.
0.198 x 10.013 = 1.98
0.802 x 11.009 = 8.83
1.000 10.81 amu
Unstable Nuclei and Radioactive Decay
o Nuclear Reactions involve a change in the nucleus of an atom
o Some substances spontaneously emit radiation in a process called radioactivity due to nuclear instability (stability is determined by neutron to proton ratios)
o Rays and particles emitted by radioactive materials are called radiation
o Unstable nuclei decay until they form stable non-radioactive nuclei
NC 147
01
146
Types of Radiation
NC 147
01
146 RnHeRa 222
8642
22688
Alpha Beta Gamma< <
00
42
23490
23892 HeThU
Pre-class Activity 11/11/08
What type of nuclear decay has neptunium-237 undergone in the following reaction?
___23391
23793 PaNp
Alpha, or He42
Pre-Class Activity 11/12/08
Which subatomic particles are involved in chemical reactions?
Which subatomic particles are involved in nuclear reaction?
Electrons
Protons, Neutrons and Electrons
Characteristics of Chemical and Nuclear Reactions
Chemical Reactions Nuclear ReactionsOccur when bonds are broken and formed
Occur when nuclei emit particles and/or rays
Atoms remain unchanged, although they may be rearranged
Atoms are converted into atoms of another element
Only Valence (outermost) electrons are involved
May involve protons, neutrons, and electrons.
Associated with small energy changes
Associated with large energy changes
Reaction rates are influenced by temperature, pressure, concentration, and catalysts.
Reaction rates are not influenced by temperature, pressure, concentration, and catalysts.
Relative Strength and Mass of Radioactive Particles
Increasing Mass
Increasing Strength
(Blocked By)
Alpha Beta Gamma
Metal Foil
Not completely blocked by lead or cement
6.64 x 10-24 9.11 x 10-28 0
Paper
Nuclear Stability(Electrostatic Force vs. Nuclear Force)
Electrostatic force arises from the interaction between two protons (repulsive force)
Nuclear force arises between protons and neutrons due to their close proximity to one another
Neutron to Proton Ration and the Band of Stability
As the atomic numberincreases, more and more neutrons are needed to create a strong nuclearforce to oppose and increasing electrostatic force
1:1
1.5:1
Radioactive Decay and Stability
1:1
1.5:1 Alpha Decay Reduces the number of neutrons and the number of protons in the nucleus.
Alpha Decay often occurs in elements with an atomic number of 83 or higher.
A
B
In what region of the graph would this type of decay be most effective? C
C
Radioactive Decay and Stability
1:1
1.5:1
Beta Decay occurs in atoms that has too many neutrons relative to its number of protons
0111
10 pn
A
B
In what region of the graph would this type of decay be most effective? A
Radioactive Decay and Stability
1:1
1.5:1Positron emission occurs in atoms where the number of protons is high relative to its number of neutrons
0110
11 np
A
B
In what region of the graph would this type of decay be most effective? B
Radioactive Decay and Stability
1:1
1.5:1Electron Capture occurs in atoms where the number of protons is high relative to its number of neutrons
nep 10
01
11
A
B
In what region of the graph would this type of decay be most effective? B
Writing Nuclear Equations
http://www.sciencegeek.net/Chemistry/taters/Unit1NuclearEquations.htm
Band of Stability Practice Questions
http://www.algebralab.com/practice/practice.aspx?file=Reading_TheBandOfStability.xml
Pre-Class Activity
Complete the following nuclear equation, state the type of decay and explain why this nuclide decays in this way.
NdPm 14260
14261 __ e01
Electron Capture, the neutron to proton ration of Pm-142 falls below the band of stability
Chapter 25 Homework Quiz – Friday 11/20
Chapter 25 Test – Tuesday 11/24
Transmutation
All nuclear reactions are:
transmutation reactionso Some transmutation reactions are inducedo All transuranium elements
(atomic #93 and greater) have
been produced through induced
transmutation.
Conversion of an element into anatom of another
element
Writing Induced Transmutation Reactions
Write the balanced nuclear equation for the induced transmutation of aluminum-27 into sodium-24 by neutron bombardment. An alpha particle is released in the reaction.
HeNanAl 42
2411
10
2713
Write the balanced nuclear equation for the alpha particle bombardment of Pu-239. One of the reaction products is a neutron.
nCmHePu 10
24296
42
23994
Radioactive Decay
Radioactive decay rates are measured in half-lives (amount of time it takes for half of a sample of radioactive nuclei to decay)
n
initial
final
2
1
Equation Possible Variables
Initial amount of isotope
Final amount of isotope
Time elapsed
Number of half-lives
Half-life
Sample Problem
A sample of radioactive iridium has a half life of 12 years. In 60 years, how much iridium would remain from a 50g sample?
32
1
50
Final
Initial=
Final=
Time Elapsed=
# of half lives=
Value of 1 half life=
50g
12 years
60 years
?
nFinal
2
1
50
n
Initial
Final
2
1
60/12 = 5
5
2
1
50
Finalgx
x
6.132/50
5032
Nuclear Reactions and Energy
Nuclear Fission Atoms with a mass number greater than 60
tend to undergo nuclear fission in which an atom “splits” apart.
Nuclear Fusion Atoms with a mass number less than 60 tend to
undergo nuclear fusion in which two lighter atoms fuse together.
Binding Energy The amount of energy required to break one mole of nuclei into individual nucleons
Reactants and Products
Fission- 92235U fuel used
in a chain reaction Limited resource critical mass- minimum
mass to sustain chain reaction
Risk of runaway chain reaction
Produces radioactive waste products
Disposal concerns Reaction:
Fusion- 12H and 1
3H used as fuel
extracted from sea water not a chain reaction No risk of runaway
reaction Nonradioactive waste:
helium Problem: needs temp of
200 Million K Reaction:
Nuclear Power- generated by a controlled fission chain reaction
Control rods- absorb neutrons to slow the chain reaction
Made of cadmium Inserted or withdrawn
to keep temp of reaction steady
Moderators- slow neutrons down so they DO hit uranium fuel rods
Made of water, beryllium, or graphite
Intended to allow neutrons to be absorbed by uranium
Cooling and Shielding
Water- acts as a coolant and transfers heat between reactor and turbines that produce electricity
Steel & concrete- surround core and protect personnel by absorbing radiation
Other Uses of Radioactivity
TracersRadioactive isotopes used to track pathways
Chemistry/biology- pathways of reactions Industry and environment- path of groundwater,
durability of containers Medicine- diagnose malfunctions
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