Nuclear Chemistry

Continued

Element Quiz Next Monday

•Elements: #1-36, plus Ag, Sn, I, Xe, Cs, Ba, W, Pt, Au, Hg, Pb, Rn, Fr, Ra, Os, Ir

• Transmutation: the process where one element changes into an atom of another element.

• A series of different types of decay• Induced transmutation: the artificial production of a nuclear reaction by striking the nuclei with high-velocity charged particles: “bombarding”

• Positron: a particle with the same mass as an electron but opposite charge.

Less common types of decay

• A proton becomes a neutron and a positron, then the positron is emitted.

• Positron emission: the emission of a positron from a nucleus.

Try it out!• Write a balanced nuclear equation for the

positron emission of sulfur-31.

S → P + β16

3115

31+1

0

• Electron capture: the nucleus of an atom takes an electron.

• The electron combines with a proton to form a…

Less common types of decay

neutron.

Try it out!• Write a balanced nuclear equation for the

electron capture of sulfur-31.

S + β → P16

31-10

15

31

NOTICE!Electron capture and positron emission have the same result, both decrease the atomic # by 1 but have no effect on the mass #

Chapter 25: Nuclear Chemistry

Half lives

Radioactive Decay Rates

• Radioactive decay rates are measured in half-lives.

• Half-life: the time required for ½ of a sample of a substance to radioactively decay.

• After each half-life, ½ of the original sample remains

• The rest becomes other elements

Radioactive Decay Rates

Radioactive Decay Rates

A radioactive substance has a half-life of 10 minutes. How many ½ lives are in…

- 20 minutes?- 50 minutes?- 100 minutes?

• Calculating half-life

2510

Radioactive Decay Rates

# ½ lives =total time

½ life

A radioactive substance has a half-life of 15.2 days. How many ½ lives are in…

- 45.6 days?- 76 days?- 91.2 days?

3 half-lives5 half-lives6 half-lives

Radioactive Decay Rates

How much of a 100 g sample is left over after…- 1 half-life?- 3 half-lives?- 5 half-lives?

• Calculating half-life Left over = total mass

21/2-lives

50 g12.5 g

3.13 g

Radioactive Decay Rates

• Calculating half-life

How much of a 33.45 mg sample remains after…- 1 half-life?- 3 half-lives?- 5 half-lives?

Left over = total mass

21/2-lives

16.73 mg

4.18 mg1.05 mg

Try it out!• 50.0 grams of molybdenum-91 decays for 62.0

minutes. What is the mass of the sample remaining? Its half-life is 15.49 minutes.

( )50.0 g2 x 2 x 2 x 2

n =tt1/2

remaining =

=62.0 min15.49 min

= 4 half lives

= 3.13 g

Try it out!• Iron-59 is used in medicine to diagnose blood

circulation disorders. The half-life of iron-59 is 44.5 days. How much of a 2.000-mg sample will remain after 133.5 days?

( )2.000 mg2 x 2 x 2remaining = = 0.250 mg

133.5 days44.5 days

= 3 half lives

Calculating Amount of Remaining Isotope

• Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with a mass of 225 grams of carbon-14.

• How long is 3 half lives?• What mass of the sample will remain?

17,190 years

28.1 g

Nuclear Stability

Nuclear Stability• The strong nuclear force: a force that holds

subatomic particles extremely close together• Nucleons: a general term for protons &

neutrons• Nuclear energy: Energy released by overcoming

the strong nuclear force

Nuclear Stability• All elements with atomic number 83 or

greater are radioactive• Neutrons are like glue in the nucleus - The

stability of a nucleus depends on its neutron-to-proton (n/p) ratio.

• Small nuclei: 1/1 ratio = stable(26 protons max - Iron)

• Large nuclei: 1.5/1 ratio = stable

Nuclear Stability• The band of

stability: The area on the graph within which all stable nuclei are found.

• If it’s outside this area, it decays to gain stability.

Too many neutrons

Too many protons

β decay

Positron emission or electron capture

Too bigAlpha

decay

Nuclear Stability• The steps in calculating the neutron-to-proton ratio

(the n/p ratio) for lead-206 are shown in this ex)

Try it out!

Calculate the neutron-to-proton ratio for . 1.6

What kind of decay is it likely to undergo? Explain.

The isotope has too many neutrons - Beta decay decreases the amount of neutrons

Chapter 25: Nuclear Chemistry

Table of Contents

Fission, Fusion, Nuclear issues

Nuclear Chemistry: Additional Concepts

1. Radiochemical Dating• Nuclear reaction rates remain constant• So…half-lives are constant. • Radiochemical dating:

determining the age of an object by measuring a radioisotope.

2. Nuclear fission• Heavy atoms (mass number > 60) tend to break into smaller

atoms, increasing their stability. • Nuclear fission: splitting an atomic nuclei into 2

approximately equal parts.

• releases a large amount of energy.

• Critical mass: The minimum amount of mass to sustain a chain reaction

Chain reaction

Nuclear reactors • Nuclear power plants use the

process of nuclear fission to produce heat in nuclear reactors.

• The heat is used to generate steam, which is then used to drive turbines that produce electricity.

7/8/10 –US Dept. of Energy announced $18.2 million for education in Nuclear Energy.

Understand the pros and cons

Nuclear Chemistry: Additional Concepts

Nuclear reactors 1) What is

the role of the fuel rods?

3) What keeps the process under control? How?

2) What controls the temp? How?

FISSION → energy

Water slows

neutrons

Control rods absorb neutrons

Nuclear Chemistry: Additional Concepts

Nuclear reactors • Fissionable uranium(IV) oxide (UO2) is commonly

used as fuel in nuclear reactors. • Cadmium and boron are used to keep the fission

process under control.

1 MWh = Energy for 650 houses, per hour.

1 ton of coal = 2.5 MWh

Breeder reactor reactors able to produce more fuel

than they use

• 3 mile island (1979) – US• Chernobyl (1986) – Ukraine

• several human errors & technical malfunctions

• Fukushima (2011) – Japan• Following earthquake

2. Nuclear fission Famous nuclear meltdowns

Nuclear Power:

65,100 MW produced per water tower.

US uses 4 GW/yr, from 104 operating units. (100 GW possible).

Upside: most energy produced, produces weapons-grade Pu

Downside: requires U, & produces waste that will be radioactive for 4.5 billion years.

3. Nuclear fusion • Nuclear fusion: nuclear reaction where small nuclei

combine to form larger ones. • release very large amounts of energy • require extremely high temperatures. (also called

thermonuclear reactions)

Solar Energy:

Sun = the source of all energy on our planet.

The sun makes ~35,000 times the total energy used by man.

~1/3 of this energy is either absorbed by the atmosphere or reflected back into space.

40,000 watts of light per sq. in. of its surface, per second.

What is the difference between nuclear fusion and nuclear fission?

Comprehension

Nuclear fusion is the combining of nuclei to form a single nucleus. Nuclear fission is the splitting of a nucleus into fragments.

Chapter 25: Nuclear Chemistry

Table of Contents

A bit more info (red is FYI)

Applications and Effects of Nuclear Reactions

• Geiger counters • use ionizing radiation • produces an electric

current in the counter• rate the strength of

the radiation on a scale.

• Geiger counters, scintillation counters, and film badges: devices used to detect and measure radiation

Applications and Effects of Nuclear Reactions• With proper safety procedures, radiation can be useful in industry, in

scientific experiments, and in medical procedures. • A radiotracer: a radioisotope that emits non-ionizing radiation and is

used to signal the presence of an element or of a specific substance. • Radiotracers are used to detect diseases and to analyze complex chemical

reactions.• Any exposure to radiation can damage living cells. • Gamma rays are very dangerous because they penetrate tissues and

produce unstable and reactive molecules, which can then disrupt the normal functioning of cells.

• The amount of radiation the body absorbs (a dose) is measured in units called rads and rems.

• Everyone is exposed to radiation, on average 100–300 millirems per year. A dose exceeding 500 rem can be fatal if received in a short amount of time.

Fin du ch 25

Practice Problems• p. 836-837,

oAlpha, beta, gamma: #38, 88oRadioactive decay: #68-72oHalf-Life: #77-79 (tough)

• p. 836, #55, 56, 60, 61, 62, 63, 64