Radioactive Half Life

Essential Standard 2.3

Understand the role of the nucleus in

radiation and radioactivity.

Learning Objective 2.3.2

Exemplify the radioactive decay of unstable

nuclei using the concept of half-life.

I Can StatementsAt the end of this lesson, you should be

able to say, with confidence:

• I can use the half-life or radioactive

materials to date a rock or a fossil

• I can describe how radioactivity is used in

medicine to detect or cure an illness

• I can describe how radioactivity is usually

detected

Radioactive DecayDuring radioactive decay particles are emitted from

parent isotopes resulting in smaller daughter isotopes.

During alpha decay, 2

protons and 2 neutrons are

emitted.

During beta decay, an

electron is emitted as a

neutron turns into a

proton.

Parent and Daughter

IsotopesDuring both alpha and beta decay, the number of

protons change resulting in the daughter isotope having

a different atomic number and thus being a different

element than the parent isotope.

Radioactive Half-LifeRadioactive half-life is the amount of time it takes

for half of radioactive parent isotope to decay into

a stable daughter isotope.

Short Half-LifeSome radioactive parent isotopes have very short

half-lives and decay into stable, non-radioactive

daughter isotopes fairly quickly.

Thalium-208 has a half-life of 3.05 minutes as it

decays into stable, non-radioactive Lead-208.

Long Half-LifeOther radioactive parent isotopes have a very long half-

life and take an extremely long time until they decay into

stable, non-reactive stable daughter isotopes.

Uranium -238 has a half-life of 4.5 billion years until

it decays into its stable, non-radioactive Lead-206.

Many StepsRadioactive isotopes that have long half-lives usually

have many in-between isotopes that are also

radioactive, before they reach the stable, non-

radioactive daughter isotope..

Radioactive DatingBecause Uranium – 238 has so many in-between

isotopes, also with long half-lives, it can be used to date

rocks that at one point contained Uranium – 238.

How they do this is

by comparing the

percentages of the

parent isotope to

that from any of the

daughter isotopes.

Radioactive DatingUranium – 238’s half-life is 4.5 billion years.

0

years

4.5

billion

years

9.0

billion

years

13.5

billion

years

18

billion

years

100% U

0% Pb

50% U

50% Pb

25% U

75% Pb

12.5% U

87.5% Pb6.25% U

93.75% Pb

It is much easier to just use a chart

Parent Isotope Daughter Isotope Half-Life

100% 0% 0

50% 50% 1

25% 75% 2

12.5% 87.5% 3

6.25% 93.75% 4

Parent Isotope – decreases by half

Daughter Isotope – Increases by whatever

amount the parent

isotope decreased

Radioactive Dating

Potassium – 40 decays into Calcium 40 and has a half-

life of 1.3 billion years.

Parent Isotope Daughter Isotope Half-Life

100% 0% 0

50% 50% 1

25% 75% 2

12.5% 87.5% 3

6.25% 93.75% 4

Radioactive Dating

If you find a sample that contains 12.5 grams of

Potassium – 40 and 87.5 grams of Calcium – 40, how

old is that sample?

3 half-lives have passed, so it is 3(1.3) = 3.9 billion

years old.

Carbon DatingAll living things contain carbon. But carbon has three

isotopes: Carbon – 12, Carbon – 13, and Carbon - 14.

Carbon-12, with 6 protons

and 6 neutrons, is stable

and non-radioactive.

Carbon-14, with 6 protons

and 8 neutrons, is

unstable or radioactive.

Carbon – 13 is very rare and not used for

carbon dating.

Carbon DatingCarbon – 14 decays into Nitrogen – 14 through

beta decay, when a neutron turns into a proton

and an electron (beta particle) is emitted.

The half-life for Carbon-14 is 5,730 years.

Carbon DatingWhen an organism dies, it usually has equal amounts of

carbon-12 and carbon 14.

However over time, the

carbon-14 decays into

nitrogen–14, while the

amount of carbon-12

remains the same.

By comparing the amounts of carbon-12 to the amount

of carbon-14, scientists can date fossils, cloth, wood or

any other organic thing.

Carbon DatingWhen an organism dies, it usually has equal amounts of

carbon-12 and carbon 14.

Scientists compare

the amounts of

MedicineRadioactivity is used in the medical field both to detect

illness and to help cure illnesses.

An isotope of Barium is used

during upper and lower GI’s to

detect abnormalities in the

digestive system.

MedicineRadiation therapy shoots radiation at tumor cells, in

an effort to destroy the cancerous cells.

Detecting Radiation

Radioactivity can be easily

detected using Geiger counters.

Geiger counters make a clicking sound when they

detect anything radioactive.

The more clicks,

the more

radioactive the

material.

The End