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How Earth’s Age is Inferred

How Earth’s Age is Inferred. Isotopic Dating. 1. _______________ (Numerical) Age: Determination of the actual age of a rock unit in years. Absolute. a. ___________________ uses radioactive elements in earth materials.

alexandra-york
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How Earth’s Age is Inferred

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  1. How Earth’s Age is Inferred Isotopic Dating

  2. 1. _______________ (Numerical) Age: Determination of the actual age of a rock unit in years. Absolute a. ___________________ uses radioactive elements in earth materials. b. Previously called_________ datingby geochronologists. Isotopic Dating radiometric

  3. 2. Elements and Atoms Element a. _____________: A substance that cannot be broken into simpler substances by ordinary chemical means.

  4. b. ______ Atom • Smallest particles of an element that can enter a chemical reaction • The smallest chemically indivisible particle of an element.

  5. c. The Traditional Model of an Atom i. Central nucleus containing __________ and ______________. protons neutrons

  6. ii. Developed by Danish physicist Niels Bohrin 1913 • Referred to as the “Bohr Model” of the atom.

  7. iii. Electrons are in________ orbits • Called shells • Are specific distances from the nucleus.

  8. d. The Modern Model of the Atom(__________________ Model) Electron Cloud i. Electrons are in a cloud-like region around the nucleus ii The shells are referred to as _________. (1) Three-dimensional regions around the nucleus. (2) Each can hold a different number of electrons and has a different shape. (3) Most stable if the outermost shell has _____ electrons. orbitals 8

  9. Nobel Gases • Have 8 electrons and are inert (unreactive)

  10. e. The Size of the Atom i. An atom of iron is about 25 ten-millionths of a meter in diameter • Most of its volume is empty space. ii. Its nucleus has a diameter of about one-hundred-thousandths of the diameter of the space in which its electrons move.

  11. Sizes of Most Common Ions in Minerals • Sizes are in angstroms. • One angstrom = 1.0 x 10-8 cm (one hundred millionth of a cm) • Ions close in size are in the same row. • Similar sized ions can replace one another in a crystal structure.

  12. f. Subatomic Particles Protons i. ____________: Positively charged and equal to the unit charge of the electron. Protons are 1836 times heavier than electrons. ii. ____________: Electrically neutral and equal in mass to the proton. iii. _________________: The number protons contained in the nucleus and identifies the atom. iv__________________: Sum of an atom’s protons and neutrons. Neutrons Atomic Number Mass Number

  13. Plotting Information around the chemical symbol

  14. Isotopes v. ____________: Atoms of the same element with the same number of protons but different numbers of neutrons.

  15. Isotopes of Oxygen

  16. The Mass of an Atom • Measured using a mass spectrometer • An electron beam fragments gas atoms or molecules into positively charged ions • Ions are • Accelerated • Deflected by a magnet • More massive particles are deflected by smaller amounts than the less massive particles

  17. Standard Mass • Determined using a neutral atom of carbon-12 • It is set at exactly 12 mass units (12 u) • 1 atomic mass unit (amu) = 1/12 of this mass which is1.66 X 10-26 g. • All atomic masses are set to this value • Only carbon-12 has a mass in amu that is exactly the same as its mass number. • For other atoms, it is not precisely the same as the mass of one proton and one neutron. For example . . . • 35Cl has a mass of 34.969 amu • 37Cl has a mass of 36.966 amu

  18. vi. _____________: The average number of protons and neutrons in an atom Atomic Mass weight (1) Also called atomic ___________. (2) Given in ____________________(amu) (3) Also considers the ______________________ of each isotope. Example using chlorine:  Of the two isotopes of Cl, 75% (75.53%) are the lighter isotope 35Cl 25% (24.47%) are 37Cl  Finding the atomic mass (34.969 amu x .7553) + (36.966 amu x .2447) = 35.45 amu atomic mass units fractional abundance

  19. g. ____: A charged atom Ion • Atoms that gain electrons are ____________charged. • Atoms that lose electrons are ____________ charged. negatively positively

  20. 3. _________________________ Radioactive Decay a. Provides a “clock that starts when radioactive elements are sealed into newly crystallized minerals. The_______ of decay is known. rate

  21. b. ______________ Radioactivity unstable i. Isotopes of some elements have ________ nuclei and spontaneously change or “_______” into new elements which are often unstable and decay into new ____________. ii. _________and _________leave these atoms producing energy. decay elements Protons neutrons

  22. Radioactive Decay iii. There are three types of radioactive decay. The original isotope is referred to as the______ isotope and the new isotope formed is called the _________ product. parent daughter

  23. (1) _________ ( ) Emission Alpha α (a) Two protons and two neutrons (He atom) leave the nucleus. (b) Reduction in the atomic number results in a new element. (c) U Th + He 238 234 4 Alpha particle is emitted 92 90 2 New element (thorium)

  24. (2) ________ ( ) Emission Beta β (a) Release of an electron from the nucleus. (b) A neutron is actually a proton with an electron inside it making it electrically neutral.

  25. (b) ________ ( ) Emission Beta β (c) When a neutron emits an electron, it becomes a proton which increases the atomic number by one. (d) After the 238U undergoes alpha emission to become 234Th, the 234Th undergoes beta decay to become 234Pa (the atomic mass number is unchanged because the lost electron’s weight is negligible. Th Pa + β Particle (electron) 234 234 90 91 emitted New element (protactinium)

  26. (3) _________ Capture Electron (a) A proton captures an orbiting electron and becomes a neutron. (b) The atomic number decreases by one, thereby changing it into another element. (c) The potassium-argon system is an example. K +β Particle (electron)Ar 40 18 Emitted New Element

  27. Uranium 238 Decay

  28. c. ________ Half Life • The time required for _____the amount of atoms of radioactive isotope to decay. (1) The length of half-lives for different isotopes of different elements can vary from: - less than 1/billionth of a second to - 49 billion years half

  29. (2) Radioactive decay is not linear (a) Linear change has a constant rate • The same percentage of the candle is burning away every minute • The same amount of water drips into the glass every hour.

  30. (b) Radioactive Decay is Geometric • In radioactive decay, during each equal time unit the proportion of parent atoms decreases by ½ • This produces a curved graph

  31. ii. Method: (1) Chemical analysis determines the amount of parent isotope and daughter isotope present in a rock using a mass spectrometer. (2) Determine the parent/daughter __________. (3 Age is calculated mathematically on the basis of it’s known half-life. (4) Whenever possible, more than one isotope pair will be used. ratio

  32. Example • Analysis of the biotite crystals in a granite determines that they contain • 25.0% potassium-40 (40K) atoms • 75.0% argon-40 (40Ar) atoms • This is a 1:3 ratio. • What is the age of the granite?

  33. The potassium has gone through two half-lives Lab Manual Figure 8.10 on page 183 (9th edition)

  34. 2 half-lives = 2.000 x T½ • The half life of 40K is 1.3 x billion years. • 2 x 1.3 billion years = 2.6 billion years.

  35. iv. Reliability Closed System • _________________: When the rock or mineral was sealed off so that neither isotope could enter or leave the environment. • Must be able to infer no __________________were present at time of closure. • There must have been _________________for measurable result by a mass spectrometer. • Half-Life is not affected by: • ______________ • ______________ • ______________ • If a rock _______, it’s radioactive clock is _______and the age will be the time of solidification daughter products sufficient time heat chemical action pressure melts reset

  36. Igneous Crystallization • Crystallization of magma separates parent atoms • from previously formed daughters • This resets the radiometric clock to zero. • Then the parents gradually decay.

  37. Radioactive Isotopes Commonly Used • Uranium-Lead (Rocks must be at least 10 Ma (million years old). • Potassium-Argon (Argon gas becomes trapped in different crystal structures) • Carbon 14 (Radiocarbon) • Used for organic matter • Short half-life (5,730 years) • Useful only in dating objects accurately back to 40,000 years.

  38. iii. Carbon 14 (Radiocarbon) • Used for organic matter • Short half-life (5,730 years) • Useful only in dating objects accurately back to 40,000 years. • Fundamentally different from parent-daughter systems because 14C is continuously created in the atmosphere by bombardment of nitrogen by cosmic rays • Cosmic radiation bombards nitrogen. • A neutron strikes and is captured by a 14N atom. • A proton is expelled from the nucleus and becomes 14C

  39. Radiocarbon Dating (d) While 14C eventually reverts to 14N because its nucleus is unstable, the rate of 14C production provides a balance so that the amount of 14C remains constant. • Living matter incorporates 12C and 14C into its tissues. The ratio of 12C to 14C remains constant while it’s alive. • Upon death, 14C decays and no further 14C replacement occurs.Age is estimated from the ratio of 14C to all other carbon in the sample

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