Chapter 11 Nuclear Chemistry

Sec 11.1 Stable and Unstable Nuclides

Nuclear chemistry deals with the concept of radioactivity and particles that are given off by radioactive substances

The nucleus of an atom can be stable, which means that it does not undergo changeIf the nucleus of an atom is unstable, it will spontaneously undergo change

Some isotopes for an element are stable, while others are radioactiveRadioactive simply means that the substance emits radiation (such as alpha, beta or gamma radiation)

Radiation can occur when an isotope is imbalanced and emits a particle to become more stable

Sec 11.1 Stable and Unstable Nuclides

Sec 11.2 The Nature of Radioactivity

The field of study was pioneered by people such as Marie Curie in early 1900s

The three main types of radiation are: Alpha particles (positively charged

particles) Beta particles (stream of electrons, neg

charged) Gamma Rays (no particles, high energy)

Sec 11.2 The Nature of Radioactivity

The electromagnetic spectrumNote how small the spectrum of visible light truly is

Page 65

Sec 11.3 Radioactive DecayRadioactive decay is the process by which an unstable nucleus emits radiation and undergoes a change

One element can change into a different element through the process of radioactive decay

Sec 11.3 Radioactive DecayAlpha:

Normally given off by heavy elements We write this as the following

238 4 234

92U 2He (+ 90Th

Alpha Particles: Basically a helium nucleus with mass 4 and charge +2

Sec 11.3 Radioactive Decay

Alpha Emission If a heavy element is unstable it may

emit an alpha particle, which can be though of as a Helium Nucleus (He 4/2)

Note that Alpha Particles have a positive charge

Sec 11.3 Radioactive Decay

Alpha particles transform the nucleus into another element with a change of mass number by 4 and a change of atomic number by 2

Rule of thumb, alpha radiation converts an element two places to the left

Sec 11.3 Radioactive DecayBeta Emission:

If a nucleus has too many neutrons, it can convert a neutron to a proton and an electron

We write this as the following

1 1 0

0n 1H + -1e

Beta particles: e (-1) are basically electrons that are emitted from the nucleus

Sec 11.3 Radioactive Decay

Beta Particles transform the nucleus into another element with the same mass number but with an atomic number of +1Example: P S + e- (Page 67)

Rule of thumb, beta radiation converts an element one place to the right

Sec 11.3 Radioactive DecayGamma:

Gamma radiation doesn’t change the identity of the element

We write gamma as the following

11 0 11

5B*0 + 5B

Gamma emission: Basically gamma rays are energy from higher state atoms to ground state atoms. Gamma has no mass or charge

Sec 11.3 Radioactive DecaySummary of Types of Radiation

Sec 11.4 Rate of Radioactive Decay

Not all radioactive nuclei decay at the same rate, there is a large variation

Half-life is the time it takes for one half of any sample to decay

Logically, the faster the half life means that the nucleus is less stable

Sec 11.4 Rate of Radioactive Decay

It is important to realize that considering half-lives, a radioactive sample will never decay completelyAlso: **We do not currently know of any method to speed up or slow down radioactive decayHalf-Lives can be seconds, days, or years

Sec 11.4 Rate of Radioactive Decay

Figure 11.3 Page 271. An example of a half life of 8 days

Sec 11.4 Rate of Radioactive Decay

Sec 11.5 Bombardment Reactions

There are two main ways that a radioactive decay reaction takes place

Transmutation reactions, the type discussed in the previous section, describes a natural and spontaneous radioactive decay

Bombardment reactions are brought about by bombarding a stable nucleus with small particles, which then leads to radioactivity

Sec 11.5 Bombardment Reactions

Bombardment reactions were and still are used to discover the “synthetic” elements on the periodic table All the elements beyond uranium are

radioactive and were produced through this type of experiment

Many of the elements have a short half-life time, which makes them difficult to characterize, much less use

Sec 11.5 Bombardment Reactions

Table 11.2 Page 274

Sec 11.6 Radioactive Decay Series

In many cases, a radioactive substance with a high atomic number (the elements starting with uranium and beyond) undergo a series of radioactive decay steps to ultimately end with a stable form

Uraniun-238 for example, undergoes 14 steps including both alpha and beta emissions, to finally end up as Lead-206

Sec 11.7 Chemical Effects

In general, electrons of molecules are effected by radiation One, the electrons can be excited to a

higher energy state Or two, the electrons can be ionized to

actually make them leave the atom or molecule entirely

Examples of radiation capable of causing ionization are X rays and Ultraviolet light

Sec 11.7 Chemical EffectsThe radiation can strike the atom and cause ionization leading to an ion pair

Fig 11.7Page 277

Sec 11.7 Chemical Effects

Usually the ion pair formation is accompanied by the formation of a free radical A free radical is a molecule or ion that

has an unpaired electron, note that this is not common with normal molecules

Free radicals are dangerous and pose problems to cellular activity

Sec 11.8 Biochemical Effects

The three main types of radioactive particles (alpha, beta, gamma) have different amounts of penetrating power An alpha particle is slow and normally

do not penetrate the skin (ie stopped by a sheet of paper)

The primary danger from alpha particles arises from ingesting a substance that emits alpha particles

Sec 11.8 Biochemical Effects Beta particles are more penetrating than

alpha particles and can be stopped by a thick sheet of aluminum

Prolonged exposure to beta particles can cause harm, and once again ingesting a substance that emits beta radiation is harmful

Gamma radiation is the highest penetration of the three types and readily passes through the skin into tissues and organs

Gamma radiation can be stopped by thick lead

Sec 11.8 Biochemical Effects

Figure 11.8 Page 279

Sec 11.9 Detection of Radiation

Low levels of radiation cannot be felt, tasted, heard, seen, or smelled However, there are methods to detect

radiation levels, most famous is the Geiger Counter

The Geiger counter is relatively portable and can display the levels of radiation

Another way to detect radiation is by the use of photographic film that will darken when exposed

Sec 11.10 Sources of Radiation

Most sources of radiation are not the high energy dangerous sources referred to previously Humans are exposed to natural low

level dosages of radiation on a daily basis from the world around us

The levels of these radiation sources are much smaller than those generally thought to cause the health issues of radiation sickness

Sec 11.10 Sources of Radiation

Figure 11.11 Page 281

Sec 11.11 Nuclear Medicine

There are two main classes of uses of radiation in medicine: Diagnosis Therapy

Sec 11.11 Nuclear MedicineDiagnosis – radioactive isotopes are used to create an image of target tissuesMedical Imaging requires three things: Radioactive element that goes into

the tissue to be imaged Detection and mapping of the tissue

to see concentration levels Computer to translate the detection

map into a visual image

Sec 11.11 Nuclear MedicineNot all radioactive materials are suitable choices for use in medicineSome criteria used are: Detectable at low concentrations Short half-life to limit the time of exposure The radioactive material must have a known

mechanism for elimination from the body The chemical properties must be mostly

compatible with normal body biochemistry. It should be selective for the desired body tissue

Sec 11.11 Nuclear MedicineCommon choices for medical imaging

Table 11.4 Page 284

Sec 11.11 Nuclear MedicineAlternately, sometimes radioactive isotopes are used in therapy to selectively destroy diseased tissueThe radiation kills both cancer and normal tissue but the cancer cells are more effected because they are faster dividing. This is why people often have hair loss or stomach problems, fast dividing cells

Sec 11.11 Nuclear Medicine

Common Choices for Therapy

Table 11.5 Page 284

Sec 11.12 Fission and Fusion

Fission – nuclear fission is the opposite of fusion and involves causing an element to fragment into other elements, which also can release energy. Example:

Sec 11.12 Fission and Fusion

Fission reactions when controlled can be used to create atomic energy in power plants (“nuclear power”)Fission reactions when uncontrolled can be used in atomic weapons or nuclear explosions.

Sec 11.12 Fission and Fusion

Example of a Chain Reaction of Uranium

Figure 11.14 Page 285

Sec 11.12 Fission and FusionFusion – nuclear fusion is the process of smaller elements colliding and forming a larger element, which gives off a large amount of energyExample:

Sec 11.12 Fission and FusionFusion reactions are occurring in the sun and stars, giving off tremendous amounts of energy Fusion reactions are also responsible for the hydrogen bombThe elements that are “man-made” were discovered by controlled fusion reactions

Sec 11.13 Comparison of Reactions

Table 11.6 Outlines the differences between chemical and nuclear reactions

ProblemsAssigned problems from pages 289 - 292 11.5, 11.9, 11.11, 11.15 11.19, 11.23, 11.26, 11.36, 11.41,

11.42 11.52, 11.53, 11.55, 11.63

Practice Test page 292