10_Nuclear Chemistry.pdf

Nuclear Chemistry

Radioactivity - radiation emitted from an unstable isotope of an

Types of radiation

 Alpha rays (

a

charged alpha particles 4He+2. An alpha particle has two protons and two

neutrons like a 4He nucleus.

 Beta rays (

b

charged particles with charge and mass the same as an electron.

Atomic Symbols Review

210

Po

84

mass #

atomic #

Mass # = protons + neutrons

Atomic # = protons (from periodic table)

Isotopes of an element have

Alpha Decay

 An alpha particle is always released in an

alpha decay process.

 222Rn --> 4He + 218Po

86 protons --> 2 protons + 84 protons 136 neutrons --> 2 neutrons + 134 neutron  The mass numbers must add up to the

Alpha Decay

 An alpha particle is always released in an

alpha decay process.

 146Sm --> 4He + 142Nd

atomic # 62 --> 2 + 60 84 neutrons --> 2 neutrons + 82 neutrons  The atomic numbers must add up to the

Alpha Decay

 What are the products of alpha decay for

210_Pb?

 210Pb --> 4He + ?

atomic # 82 --> 2 + ? 128 neutrons --> 2 neutrons + ? neutrons  The mass and atomic numbers must both

Alpha Decay

 What are the products of alpha decay for

210_Pb?

 210Pb --> 4He + 206Hg

atomic # 82 --> 2 + must be 80

128 neutrons --> 2 neutrons +126 neutrons  mass numbers: 210 = 4 + 206

Beta Particle Symbol

0

e

-1

mass #

atomic #

Mass # = 0 , no protons or neutrons

Atomic # = - 1 for nuclear math to work

Produced by the breakdown of

Beta Decay

 A beta particle is always produced in

beta decay. The mass number is zero and the atomic number is -1.

 67Cu --> 0e + 67Zn

atomic # 29 --> -1 + 30 mass # 67 --> 0 + 67

 In beta decay, the mass number of the product is the same and the atomic

number is one number greater.

Beta decay process

1 1 0

n --> p + e

0 1 -1

neutron --> proton + electron

The electron is ejected from the

nucleus (the beta particle). The

atom produced has one more

Beta Decay

 What are the products of beta decay for

99_Mo?

 99Mo --> 0e + 99Tc

atomic # 42 --> -1 + must be 43

 mass numbers: 99 = 0 + 99

Are you understanding?



What are the alpha decay

products for

Bi?



What are the beta decay

Answers

210

Bi -->

He +

Tl

alpha

particle

45

Ca -->

e

+

Sc

beta

What about gamma rays?

 Gamma rays are energy that is

produced when an alpha or beta decay takes place.

 Gamma rays are the most hazardous

Three Factors for Radioactivity

Safety:



Dosage

0 to 25 rem: no effect,

25 to 100 rem: short term effect,

100 to 300 rem: nausea (3

month recovery), 300 to 600

rem: nausea, hemorrhage,

death for 50%, over 600 rem:

death for nearly 100%.

Three Factors for Radioactivity

Safety:



Duration

the shorter the

exposure time the less harm is

done.



Distance

the amount of

What if you have to work with

radiation? USE SHIELDING!

 Alpha Particles - shielded by skin.

Avoid getting them in the body by ingestion or breathing.

 Beta Particles - aluminum foil 1 cm thick

Radon gas - a natural source of

radiation exposure, 55% ave.

 222

Rn -->

He + ?



Radon is formed from uranium

containing minerals in the soil.



Radon gas seeps into homes

and is breathed into the lungs.



Smokers have an increases risk

Other sources of exposure.

 Cosmic radiation, 8%  Rocks and soils, 8%

 Minerals in the body, 11%

 Medical x-rays, 11%

 Nuclear medicine, 14%

 Consumer products, 3%

In-class assignment

 1. Write alpha decay equations for:

a) 223Fr b) 209At c) 226Ra

 2. Write beta decay equations for: a) 140Ba b) 110Ag c) 198Au

 3. A person builds a bomb shelter with thick cement walls. What type of

Nuclear Equations & Half-Lives

FISSION – part of the nucleus splits out (one piece forms two or more).

FUSION – two nuclei come together into a larger, heavier atom .

Fusion powers the sun!

1

H +

H



He

Atomic # 1 + 1 = 2

Nuclear fusion keeps

the temperature of the

sun at about

Transmutation – how new

elements are made

 209

Bi +

Cr



_____ +

n

(neutron)

 242

Cm +

He



_____ +

n

What state was this element

made in?

Answers for transmutations

Atomic numbers must add up

Bi + Cr



? + n

83 + 24 = 107 + 0

Element 107 has just been named

Bohrium (Bh) after Niels Bohr.

Mass numbers must add up.

209 + 54 = 262 + 1

Answers for transmutations

Atomic numbers must add up

Cm + He



? + n

96 + 2 = 98 + 0

Element 98 is named

Californium (Cf) after the state CA.

Mass numbers must add up.

242 + 4 = 245 + 1

Answers for transmutations

Atomic numbers must add up

Am + He



? + 2 n

95 + 2 = 97 + 0

Element 97 is named

Berkelium (Bk) after the city.

Mass numbers must add up.

Back to fission (alpha and beta

decay)



The amount of

radioactivity

may be in many

units: counts

per minute

(cpm), rem,

How fast does fission take place?

45

Ca has a half-life of 165 days.



If we start with a 160 cpm

sample of

Ca, after 165 days

only 80 cpm will remain.



160 x ½ = 80



165 days later the 80 cpm

Graph of decay of %

Fr with a

half-life of 15 minutes.

0 10 20 30 40 50 60 70 80 90 100

0 min 15 min 30 min 45 min 60 min

How much of a radioactive isotope

remains after a given time?

 90Sr has a half-life of 28.8 years. How

much of a 48 rem sample remains after 86.4 years?

 FIRST : find how many have half-lives for the time given in the problem.

86.4 years / 28.8 years = 3 half-lives

RECOGNIZING THE “HOW

MUCH” QUESTION

GIVEN Half-life

A medical lab receives a 50.0 g

vial of

Ba (half-life = 11.6

days). How much remains after

46.4 days?

 # of half-lives = time given / half-life = 46.4 days / 11.6 days = 4 half-lives

 50.0 g x ½ x ½ x ½ x ½ = 3.13 g 131Ba

A more visual approach…

50.0 g

25.0 g

12.5 g

6.35 g

3.13 g

1

2

3

4

How much time does it take for the

amount of radioactivity to change?

 90Sr has a half-life of 28.8 years. How

much time does it take for a 48 rem sample to decrease to 12 rem?

 FIRST : find how many have half-lives by

taking ½ the higher activity to get to lower. 48 rem x ½ = 24 rem x ½ = 12 rem It took

2 half-lives ( x ½ twice)

RECOGNIZING THE “HOW

LONG” QUESTION

GIVEN Half-life

Amount of radioactivity before Amount of radioactivity after

How old is a sample of wine with 26

cpm from

H if new wine has 104 cpm?

 3H has a half life of 12.26 years.

 104 x ½ = 52 x ½ = 26 2 half-lives

 2 half-lives x 12.26 years = 24.52 years  The wine was bottled in 1975. After it

was bottled no new 3H could enter the wine, so the level decreased through radioactive decay. While the grape

Radioactive Carbon Dating

 14

C has a half-life of 5730 years



Living things maintain a

constant level of

C because

they constantly take it in in food

(for animals) or CO

(for plants)



Once the animal / plant dies the

How old is a piece of wood found in a tomb

11 cpm, if new wood is 88 cpm from

C?

 88 cpm x ½ = 44 x ½ = 22 x ½ = 11 cpm

so 3 half-lives.

 3 half-lives x 5730 years = 17200 years

Nuclear Medicine

PET scans – Positron Emission Treatment of hyperthyroid and

Fast Facts about Nuclear Medicine

(from Society of Nuclear Medicine)

 10 to 12 million nuclear medicine

imaging procedures are performed in the U.S. each year.

 There are 100 different imaging procedures available today.

 The amount of radiation is about the same as a diagnostic x-ray.

 Nuclear medicine procedures are

Positron Emission Tomagraphy

 Some radioactive isotopes convert a

proton into a neutron in the nucleus by emitting a positron.

18 18 0

F



O + e

9 8 +1

Positron Emission Tomagraphy

 A positron and an electron will

annihilate each other producing two gamma rays that are used for the

imaging.

0 0

http://www.epub.org.br/cm/n01/p

et/pet.htm



PET scans

In class assignment

 Radioactive iodine has a half-life of 8.0

days.

 How much of a 100 mrem sample remains after 24 days?

 How long would it take for 120 grams of radioactive iodine to decay to 15