11 chemistry: atoms first second edition julia burdge & jason overby copyright (c) the...

11

Chemistry: Atoms FirstSecond Edition

Julia Burdge & Jason Overby

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 20

Nuclear Chemistry

M. Stacey Thomson

Pasco-Hernando State College

What Is Radioactivity?What Is Radioactivity?• Radioactivity is the release of tiny, high-

energy particles or gamma rays from an atom

• Particles are ejected from the nucleus

2Tro: Chemistry: A Molecular Approach

Nuclear DecayNuclear DecaySome nuclei are unstable, and will,

over time, emit particles and/or electromagnetic radiation until they become stable.

The spontaneous emission of particles or electromagnetic radiation is known as radioactivity.

All elements with Z > 83 are radioactive.

Tro: Chemistry: A Molecular Approach

3

4

Nuclei and Nuclear ReactionsNuclei and Nuclear Reactions20.1

During a nuclear reaction, the products and reactants will contain different elements as the nuclei change.

There are several types of particles or forms of electromagnetic radiation that may be emitted during a nuclear reaction.

You should learn the names, symbols and the mass number and charge of each of the particles.

5

Nuclei and Nuclear ReactionsNuclei and Nuclear Reactions

The symbols for subatomic particles include:

11H

11pproton

10nneutron

01e

01electron

01e

01positron

42

42Heα particle

6

Nuclei and Nuclear ReactionsNuclei and Nuclear Reactions

In balancing a nuclear reaction, simply balance the total of all atomic numbers and total of all mass numbers for the products and reactants.

42He212

84Po 20882Pb +

Mass number:

Atomic number:

212 208 + 4 = 212

84 82 + 2 = 84

7

Example 20.1

Identify the missing species X in each of the following nuclear equations:

(a)

(b)

(c)

XPb Po 20882

21284

01-9038 X Sr

01-188 O X

8

Nuclear StabilityNuclear Stability20.2

Review the information from Chapter 2 on Nuclear stability.

Principle factor for nuclear stability is neutron-to-protonratio (n/p)

There are more stabile nuclei with 2, 8, 20, 50, 82, or 126 protons or neutrons

More with even #’s All with atomic number > 83 are radioactive All isotopes of Tc and Pm are radioactive

9

Nuclear StabilityNuclear Stability

The figure shows the number of neutrons vs. the number of protons in various isotopes.

Stable nuclei are located in an area of the graph known as the belt of stability.

Most radioactive nuclei lie outside the belt.

Above the belt of stability, the nuclei have higher neutron-to-proton ratio.

10

Types of Nuclear DecayTypes of Nuclear Decay

Above the belt, isotopes decay by:

beta emission

12

Types of Nuclear Decay Types of Nuclear Decay

Below the belt, isotopes decay by:

positron emission

electron capture

A Low Neutron to Proton RatioA Low Neutron to Proton Ratio

If the N/Z ratio is too low, and the nuclide has too many protons. These nuclides tend to undergo either positron emission or electron capture.


Positron EmissionPositron Emission


Positrons result from a proton changing into a neutron.

A positron has a charge of +1 and negligible mass.anti-electron

Positron EmissionPositron Emission


• When an atom loses a positron from the nucleus, itsmass number remains the sameatomic number decreases by 1

Electron CaptureElectron Capture


Electron capture occurs when an inner orbital electron is pulled into the nucleus. Aproton combines with the electron to make a neutron. This decreases the atomic number, and increases the N/Z ratio.

Electron CaptureElectron Capture


As a result of electron capture: mass number stays the same atomic number decreases by one

Particle Changes


Alpha (α) EmissionAlpha (α) Emission

Many nuclides that are too heavy to be stable (Z>83) undergo alpha emission.

An particle contains 2 protons and 2 neutrons, and is the same as a helium nucleus.


20

Alpha EmissionAlpha Emission


Loss of an alpha particle means:atomic number decreases by 2mass number decreases by 4

As a result, the N/Z ratio increases.

Gamma (γ) EmissionGamma (γ) Emission

During a nuclear reaction, high energy electromagnetic radiation, called gamma rays, is often emitted.

Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange.


23

Gamma EmissionGamma Emission


• Gamma () rays are high energy photons of light

• No loss of particles from the nucleus• No change in the composition of the nucleus

same atomic number and mass number

Other Properties of Other Properties of RadioactivityRadioactivity


• Radioactive rays can ionize mattercause uncharged matter to become chargedbasis of Geiger Counter and electroscope

• Radioactive rays have high energy

• Radioactive rays can penetrate matter

• Radioactive rays cause phosphorescent chemicals to glowbasis of scintillation counter

Penetrating Ability of Radioactive Rays

0.01 mm 1 mm 100 mm

Pieces of Lead


Ionizing Ability of Radiation


27

Highly energetic radiation interacts with molecules and atoms by ionizing them. This can have serious biological effects on cells in living systems. Cell damage, or abnormal cell replication can occur.

α particles are highly ionizing, but not very penetrating. They can be stopped by a sheet of paper, clothing, or air. As a result, they are not very damaging unless ingested or breathed into the lungs.

β particles have lower ionizing power, but are more penetrating. A sheet of metal or a thick piece of wood will stop them.

Ionizing Ability of Radiation


28

γ rays have the lowest ionizing power, but are the most penetrating. Several inches of lead or slabs of concrete are needed to stop gamma rays.

29

Nuclear Binding EnergyNuclear Binding Energy

A quantitative measure of nuclear stability is the nuclear binding energy.The nuclear binding energy is the energy required to break up a nucleus into its component protons and neutrons.

The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons is called the mass defect.

The measured mass of 19F = 18.99840 amu

mass of 9 protons = 9 x 1.007825 amu = 9.070425 amumass of 9 electrons = 9 x 5.4858x10-4 amu = 0.0049372 amumass of 10 neutrons = 10 x 1.008665 amu = 10.08665 amu

The calculated mass of 19F = 19.16201 amu

Mass defect of 19F = 19.16201 amu – 18.99840 amu = 0.16361 amu

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Nuclear Binding Energy Nuclear Binding Energy

The loss in mass is converted to energy and can be quantified with Einstein’s mass-energy equivalence relationship.

ΔE = energy of product – energy of reactant Δm = mass of product – mass of reactant

For 19F, Δm = 18.99840 – 19.16201 amu = –0.16361 amu

ΔE = (Δm)c2

kgxamux

kgamum 28

261071682

1002214186

1163610

.

..

31


The loss in mass is converted to energy and can be quantified with Einstein’s mass-energy equivalence relationship.

ΔE = (–2.7168 x 10–28 kg)(2.99792458 x 108 m/s)2

ΔE = (Δm)c2

ΔE = –2.4417 x 10–11 kg (m/s)2

ΔE = –2.4417 x 10–11 J

32


Plot of nuclear binding energy per nucleon versus mass number.

33

Nuclear RadioactivityNuclear Radioactivity

The disintegration of a radioactive nucleus often is the beginning of a radioactive decay series, which is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.

The beginning radioactive isotope is called the parent and the product isotope is called the daughter.

20.3

34

Kinetics of Radioactive DecayKinetics of Radioactive Decay

All radioactive decays obey first-order kinetics.

The corresponding half-life of the reaction is given by:

0

ln tNkt

N

1/2

0.693t

k

35

Nuclear RadioactivityNuclear Radioactivity

A piece of linen cloth found at an ancient burial site is found to have a 14C activity of 4.8 disintegrations per minute. Determine the age of the cloth.

Assume that the carbon-14 activity of an equal mass of living flax (the plant from which linen is made) is 14.8 disintegrations per minute. The half-life of carbon-14 is 5715 years.

SolutionStep 1:Determine the rate constant from the equation below:

1/2

0.693t

k

4 1

1/2

0.693 0.6931.21 10 yr

5715 yrk

t

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Dating Based on Radioactive DecayDating Based on Radioactive Decay

A piece of linen cloth found at an ancient burial site is found to have a 14C activity of 4.8 disintegrations per minute. Determine the age of the cloth.

Assume that the carbon-14 activity of an equal mass of living flax (the plant from which linen is made) is 14.8 disintegrations per minute. The half-life of carbon-14 is 5715 years.

SolutionStep 2:Use the equation below to calculate time:

t = 1.0 x 104 yr

4 14.8ln 1.21 10 yr

14.8t

0

ln tNkt

N

37

Worked Example 20.3

Strategy The activity of a radioactive sample is proportional to the number of radioactive nuclei. Thus, we can use ln([A]t/[A]0) = –kt with activity in place of concentration:

ln = –kt

To determine k, though, we must solve t½ = 0.693/k, using the value of t½ for carbon-14 (5715 years) given in the problem statement.

A wooden artifact is found to have a 14C activity of 9.1 disintegrations per second. Given that the 14C activity of an equal mass of fresh-cut wood has a constant value of 15.2 disintegrations per second, determine the age of the artifact. The half-life of carbon-14 is 5715 years.

14C activity in artifact14C activity in fresh-cut wood

38

Nuclear transmutation differs from radioactive decay in that transmutation is brought about by the collision of two particles.

Particle accelerators made it possible to synthesize the so-called transuranium elements, elements with atomic numbers greater than 92.

Nuclear TransmutationNuclear Transmutation20.4

147N

42

178O 1

1p+ +

39

Nuclear TransmutationNuclear Transmutation

Write an equation for the process represented by:

SolutionStep 1:Determine the bombarding particle and the emitted particle:

Step 2:Write the equation:

AgpPd 10947

10646 ),(

AgpPd 10947

10646 ),(

emitted particlebombarding particle

p Ag He Pd 11

10947

42

10646

40

Worked Example 20.5

Strategy The species written first is a reactant. The species written last is a product. Within the parentheses, the bombarding particle (a reactant) is written first, followed by the emitted particle (a product).

Write the balanced nuclear equation for the reaction represented by where d represents a deuterium nucleus.

Mn)Fe(d, 5425

5626

42

5425

21

5625 Mn H Fe

Solution The bombarding and emitted particles are represented by and , respectively.

H21 42

Think About It Check your work by summing the mass numbers and atomic numbers on both sides of the equation.

41


Schematic of a cyclotron particle accelerator.

42


Section of a particle accelerator.

43

Nuclear FissionNuclear Fission

Nuclear fission is the process in which a heavy nucleus (mass number > 200) divides to form smaller nuclei and one or more neutrons.

20.5

23592U

10n 90

38Sr 103 n+ +

14354 Xe +

44


Relative yields of the products resulting from the fission of 235U as a function of mass number.

45


235U is capable of a self-sustaining sequence of nuclear fission known as a nuclear chain reaction.

46


The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass.

47


Schematic of a nuclear fission reactor.

48

Nuclear FusionNuclear Fusion

Nuclear fusion is the process of combining small nuclei into larger ones.

The following reactions are believed to take place in the sun:

Because fusion reactions take place at very high temperatures, they are often called thermonuclear reactions.

20.6

49


Promising fusion reactions include:

Due to the high temperaturerequirements, containmentis an issue.

50


A promising design employs high power lasers

A small scale fusion reaction was carried out at the Lawrence Livermore National Laboratory (right)

Technical difficulties still need to be overcome before it can be put to practical use

51

Use of Isotopes: Chemical AnalysisUse of Isotopes: Chemical Analysis

Radioactive and stable isotopes have applications in science for molecular structure determination.

20.7

Two proposed structures for thiosulfate ion:

By using radioactive sulfer-35 isotope, the isotope acts as a “label” for the S atoms.

Based on studies, structure 2 has been confirmed.

1)

2)

52

Isotopes in MedicineIsotopes in Medicine

Radioactive and stable isotopes have applications in science and medicine.

Radioactive isotopes are used as tracers.

Use of tracers for diagnosis include:

Sodium-24 – blood flow Iodine-131 –thyroid conditions Iodine-123 – brain imaging

53

Biological Effects of RadiationBiological Effects of Radiation

The fundamental unit of radioactivity is the curie (Ci)

1 Cu = 3.70 x 1010 disintegrations per second.

20.8

54


A common unit for the absorbed dose of radiation is the rad (radiation absorbed dose).

1 rad = 1 x 105 J/g of tissue irradiated

The rem (roentgen equivalent for man) is determined from the number of rads:

Number of rems = number of rads x 1 RBE

RBE = the relative biological effectivness.

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