Chapter 19 The Nucleus: A Chemist’s View. Section 19.1 Nuclear Stability and Radioactive Decay Copyright © Cengage Learning. All rights reserved 2 Review

Chapter 19

The Nucleus: A Chemist’s View

Section 19.1Nuclear Stability and Radioactive Decay

Copyright © Cengage Learning. All rights reserved 2

Review

Atomic Number (Z) – number of protons Mass Number (A) – sum of protons and neutrons

XAZ

Section 19.2The Kinetics of Radioactive Decay

4 Forces in Nature

Force Equation notes

Gravity F = G m1 m2/r2 Works at an infinite distance. Weakest of all forces.

Electromagnetic F = K q1 q2/r2 Works for short distances, Holds the atom together.

Strong Nuclear force E =mc2 (mass defect) This is the glue that holds the nucleus together.

Weak nuclear force The force responsible for radioactivity. Pulls the nucleus apart.


Stable nuclei

When the strong force is greater than the weak force the nucleus is stable.

No nucleus beyond lead is stable indefinitely. Iron has the most stable nuclei. Most atoms have examples of stable and unstable

nuclei



The Zone of Stability


Unstable nuclei

When the weak force is greater than the strong force the nucleus is unstable.

Unstable nuclei become stable by changing the make up of their nucleus

This process is known as radioactive decay.



Radioactive Stability

Nuclides with 84 or more protons are unstable. Light nuclides are stable when Z equals A – Z

(neutron/proton ratio is 1). For heavier elements the neutron/proton ratio

required for stability is greater than 1 and increases with Z.



Radioactive Stability Certain combinations of protons and neutrons seem

to confer special stability. Even numbers of protons and neutrons are more

often stable than those with odd numbers.



Radioactive Stability

Certain specific numbers of protons or neutrons produce especially stable nuclides. 2, 8, 20, 28, 50, 82, and 126



Types of Radioactive Decay

Alpha production (α):

Beta production (β):




Gamma ray production (γ):

Positron production:




Electron capture:

Inner-orbital electron



Decay Series (Series of Alpha and Beta Decays)



Which of the following produces a particle?

electron capture

positron

alpha particle

beta particle

68 0 6831 1 30a) Ga + e Zn

62 0 6229 1 28b) Cu e + Ni

212 4 20887 2 85c) Fr He + At

129 0 12951 1 52d) Sb e + Te

CONCEPT CHECK!CONCEPT CHECK!


Rate of Decay

Rate = kN The rate of decay is proportional to the number of

nuclides. This represents a first-order process.



Half-Life

Time required for the number of nuclides to reach half the original value.


1/ 2 =

ln 2 0.693 = t

k k


Nuclear Particles


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Half-Life of Nuclear Decay





A first order reaction is 35% complete at the end of 55 minutes. What is the value of k?

k = 7.8 × 10-3 min-1

EXERCISE!EXERCISE!

Section 19.3Nuclear Transformations

Nuclear Transformation

The change of one element into another.


27 4 30 113 2 15 0Al + He P + n

249 18 263 198 8 106 0Cf + O Sg + 4 n


A Schematic Diagram of a Cyclotron

21


A Schematic Diagram of a Linear Accelerator


Section 19.4Detection and Uses of Radioactivity

Measuring Radioactivity Levels

Geiger counter Scintillation counter



Geiger Counter





Carbon–14 Dating

Used to date wood and cloth artifacts. Based on carbon–14 to carbon–12 ratio.



Radiotracers

Radioactive nuclides that are introduced into organisms in food or drugs and whose pathways can be traced by monitoring their radioactivity.



Radiotracers


Section 19.5Thermodynamic Stability of the Nucleus

Energy and Mass

When a system gains or loses energy it also gains or loses a quantity of mass.

E = mc2


Δm = mass defectΔE = change in energy

If ΔE is negative (exothermic), mass is lost from the system.


Mass Defect (Δm)

Calculating the mass defect for : Since atomic masses include the masses of the electrons, we

must account for the electron mass.

nucleus is “synthesized” from 2 protons and two neutrons.

29

42He

4 42 24.0026 = mass of He atom = mass of He nucleus + 2 em

1 11 11.0078 = mass of H atom = mass of H nucleus + em

42He

2 1.0078 + 2 1.0087 = 4.0026 2 e e m m m

= 0.0304 amu m


Binding Energy

The energy required to decompose the nucleus into its components.

Iron-56 is the most stable nucleus and has a binding energy of 8.79 MeV.



Binding Energy per Nucleon vs. Mass Number


Section 19.6Nuclear Fission and Nuclear Fusion

Nuclear Fission and Fusion

Fusion – Combining two light nuclei to form a heavier, more stable nucleus.

Fission – Splitting a heavy nucleus into two nuclei with smaller mass numbers.

Bombardment+ Parent Daughters + neutronsCopyright © Cengage Learning. All rights reserved 32

1 235 142 91 10 92 56 36 0n + U Ba + Kr + 3 n


Nuclear Fission





Criterion for a self sustaining reaction:

The target nuclei must produce at least one neutrons upon splitting.

The must be sufficient mass of target nuclei for the reaction to continue.

The mass defect must be sufficient in order to produce high energy emissions.


Fission Processes

A self-sustaining fission process is called a chain reaction.


Event

Neutrons Causing Fission Event

Result

subcritical < 1 reaction stops critical = 1 sustained reaction supercritical > 1 violent explosion


Schematic Diagram of a Nuclear Power Plant



Schematic Diagram of a Reactor Core



What does each part do? Fuel rods: Contains fissionable material Control Rods: Conic neutron “catchers” made of

boron or cadmium Heat exchanger: Web of pipes containing

superheated fluid that boils the water Reactor vessel: Contains reactor core Containment fluid: Fluid that surrounds

fissionable material also serves to control reaction Turbine: Spins when steam passes through


More parts

Generator: Connected to turbine. Converts mechanical Energy to electrical

Condenser: Returns steam to water. External water source: Provides water for

condenser. Cooling tower: Releases excess heat so water can be

returned to ecosystem Pumps: Move fluid around and create elevated

pressure conditions


High levelLow Level

Nuclear Waste


Nuclear Energy?

Pros Cons


Nuclear Fusion




Section 19.7 Effects of Radiation

Biological Effects of Radiation

Depend on:

1. Energy of the radiation2. Penetrating ability of the radiation3. Ionizing ability of the radiation4. Chemical properties of the radiation source



rem (roentgen equivalent for man)

The energy dose of the radiation and its effectiveness in causing biologic damage must be taken into account. Number of rems = (number of rads) × RBE

rads = radiation absorbed doseRBE = relative effectiveness of the

radiation in causing biologic damage

44


Effects of Short-Term Exposures to Radiation


Documents

Chapter 19 The Nucleus: A Chemist’s View. Section 19.1 Nuclear Stability and Radioactive Decay Copyright © Cengage Learning. All rights reserved 2 Review