Absolute Dating and Radiometric Methods

Atoms consist of protons, neutrons, and electrons.
Protons:
- Positively charged particles.
- Relative charge is 1+.
- Mass is approximately 1 unified atomic mass unit (u).
- The number of protons defines the atomic number.
Neutrons:
- Electrically neutral particles.
- Mass is slightly more than a proton (approximately 1 u).
- Most atoms have at least as many neutrons as protons.
Nucleus:
- Formed by protons and neutrons.
- Overall charge equals the total charge of the protons.
- Small and dense; contains over 99% of the atom's mass.
Electrons:
- Negatively charged particles (charge of 1-).
- Move too fast to determine exact position and speed simultaneously.
- Exist in an electron cloud around the nucleus.
- Compared to protons and neutrons, electrons have very little mass.
- In a neutral atom, the number of protons equals the number of electrons, balancing the charge.
- Ions are formed when atoms gain or lose electrons, resulting in a net positive or negative charge.

Bohr Model: A simplified representation of an atom showing electrons in defined orbits around the nucleus.
Electron Cloud Model: Represents the probability of finding electrons in certain regions around the nucleus.

Atoms can gain or lose electrons to form ions.
Cations: Positively charged ions (e.g., $Na^+$ , Sodium ion).
Anions: Negatively charged ions (e.g., $Cl^-$ , Chloride ion).
Example: Sodium chloride (NaCl) is formed by ionic bonding between $Na^+$ and $Cl^-$ .

Atoms of the same element have the same number of protons but may have different numbers of neutrons.
Isotopes: Atoms of the same element with different neutron numbers.
Mass Number: The total number of protons and neutrons in an atom's nucleus; varies among isotopes.
Example: Hydrogen has three isotopes: 1H (1 proton, 0 neutrons), 2H (1 proton, 1 neutron), and 3H (1 proton, 2 neutrons) with mass numbers 1, 2, and 3, respectively.

Absolute Dating: Determining the actual age of an event or object in years.
Radioactive isotopes are used to find the absolute age of rocks and other materials.
Isotopes are formed from nuclear reactions.

Nuclear Reaction: A change that affects the nucleus of an atom.
- Differs from chemical reactions, which do not change the mass of atoms.
- Nuclear reactions can change the number of neutrons and protons, thus changing one type of atom into another.
- Example: A nuclear reaction can decrease the number of protons, turning a beryllium atom into a lithium atom.
Isotopes of the same element have different numbers of neutrons and, therefore, different mass numbers.
Carbon atoms typically have 6 protons and 6 neutrons (mass number 12).
When carbon atoms gain or lose neutrons, they become isotopes (e.g., carbon-13, carbon-14).
Atoms can change to become isotopes of different elements.
Cosmic rays create carbon-14 by colliding with nuclei in the upper atmosphere, liberating neutrons that interact with nitrogen nuclei, replacing a proton with a neutron.

If an atom's nucleus has too many neutrons, it can become unstable and radioactive.
Radioactive Isotopes (Radioisotopes): Unstable isotopes that break down into stable isotopes through radioactive decay.
Radioactive decay involves releasing excess energy by emitting radiation in the form of alpha, beta, and gamma rays.
Half-Life: The time required for half of a radioactive element sample to undergo radioactive decay and form daughter isotopes.
Parent Isotope: The original radioactive isotope.
Daughter Isotope: The stable isotope formed by the breakdown of the parent isotope.

Scientists study the amounts of parent and daughter isotopes to date samples.
Radiometric Dating: Finding the absolute age of a sample by determining the relative percentages of a radioactive parent isotope and a stable daughter isotope.
Igneous rocks are the best samples for radiometric dating because they often contain only the parent isotope and none of the daughter isotope when formed.
The half-life of the isotope used must be appropriate for the age of the sample (neither too short nor too long).

Radiocarbon Dating:
- Used for dating wood, bones, shells, and other organic remains.
- Living things maintain a constant ratio of radioactive carbon-14 to carbon-12.
- After death, no more carbon is taken in, and the carbon-14 decays, changing the isotope ratio.
- The half-life of carbon-14 is 5,730 years.
- Effective for dating organic matter from the last 45,000 years.
Potassium-Argon Dating:
- Used to date igneous volcanic rocks from 100,000 years to billions of years old.
Uranium-Lead Dating:
- Based on measuring the amount of lead-206 daughter isotope in a sample.
- Used for igneous rocks between 100 million and a few billion years old.

Radiometric dating can estimate Earth's age, but no Earth rocks are as old as the planet itself.
Meteorites, which are the same age as the solar system (including Earth), are used.
The absolute age of meteorites and other rocks in the solar system is approximately 4.6 billion years.

Sedimentary rock layers and the fossils within them cannot be directly dated radiometrically.
Igneous rock layers around sedimentary layers can be dated.
This provides an absolute age range for the sedimentary rock layer containing the fossils.

Index Fossils: Fossils used to estimate the absolute age of the rock layers in which they are found.
Once the absolute age of an index fossil is known, it can be used to determine the age of rock layers containing the same fossil anywhere on Earth.
To be an index fossil, the organism must have:
- Lived during a relatively short geologic time span.
- Been relatively common.
- Been found over a large area.
- Have unique features.
Index fossils act as markers for the time organisms lived on Earth and can date rocks in different areas, indicating that the rock layers formed at about the same time.