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