Geologic Time and Absolute Dating

Review: Atomic Structure

• Atom– Basic unit of an element– Composed of protons

and neutrons (nucleus) surrounded by electrons

– The identity of an atom is determined by the number of protons the atom has

Example: Krypton

• Krypton’s atomic number is 36, therefore– A neutral krypton atom

has 36 protons and 36 electrons

– If atomic number ≠ 36 ≠ krypton

– The number of neutrons =atomic number-atomic weight (rounded up)=48

– The number of neutrons can vary without altering the identity of an atom—Isotopes

Isotopes

• Isotopes are like people—some are stable; some are not

• And, like people, it’s the unstable ones that attract our attention the most

Stable…

Seriously unstable…

Radioactivity• Unstable isotopes are

radioactive—their nuclei will decay over time

• A any radioactive isotope is called a “parent” isotope

• The decay product is called the “daughter” isotope

• When an isotope decays, they do so in one of three ways…

• Alpha emission– Nucleus emits two protons

and two neutrons• Plutonium-240 decays to

uranium-236

• Beta emission– Nucleus emits an electron

• Radium-228 decays to Actinium-228

• Electron capture– An atom’s nucleus captures

an electron which reacts with a proton creating a neutron• Carbon-11 decays to Boron-11

• In a nutshell: When the nucleus decays, a new, more stable isotope is created

Electron capture

Radioactive Decay and Popcorn…yummy

• Radioactive decay is a spontaneous and irreversible process• Ex. popcorn

If the Decay of an Atom Occurs Randomly, How is it Useful to Us?

Sampleof actinium

Even a small sample is composed of billionsof actinium atoms (Ac-227)

All isotopes of actinium are unstable and willdecay over time. Since every atom has a certain probability of decaying, on average, half of theatoms in a given sample will decay to a (more) stable daughter isotope over a set period of time

Actinium-227 has a half-life of 22 years

After 22 years, exactly half of the Atoms have decayed to thorium-227

Half-lives

• We can use the half-life of an isotope to figure out the age of a rock

• How can we do this?– Half-lives are constant

• Actinium-227 always has a half-life of 22 years

– As the parent decays the daughter accumulates• Older samples = higher

number of daughter isotopes

Example

• If we have a rock with 100 grams of a particular isotope (Bob-12)

• Bob-12 decays to Joe-11 and has a half-life of 400 Million years

• How old is our sample if only 25 grams of Bob-12 remain?– Our sample is 800 My old

100 grams of Bob-12

400 My(one half-life)

50 grams Bob-12

50 gramsJoe-11+

= 100 g

25 gramsBob-12 +

75 grams Joe-11 = 100 g

400 My years(now twohalf-lives have passed)

Good vs. Bad

• Isotopes with long half-lives are good for old rocks

• Young materials are best dated by short lived isotopes

Commonly Used Isotopes