Unit 2 Lesson 3 Absolute Dating

Up and Atom! What are the parts of an atom? Atoms are made up of protons, neutrons, and electrons. Protons are the positively charged particles of atoms. The relative charge of a single proton is written as 1+. In the unified atomic mass unit (u), the mass of a proton is about 1 u. The number of protons in the nucleus of an atom is the atomic number of that atom. 2

What are the parts of an atom? Neutrons are particles that have no electrical charge. The mass of a neutron is slightly more than that of a proton, but it is still close enough to be considered 1 u. Most atoms contain at least as many neutrons as protons. 3

How can we describe atoms? Unit 3 Lesson 1 The Atom How can we describe atoms? The atoms of a certain element always have the same number of protons, but the number of neutrons may differ. Isotopes are atoms of the same element that have different numbers of neutrons. The total number of protons and neutrons in an atom’s nucleus is its mass number and each isotope has a different mass number. Example: Hydrogen has three natural isotopes: 1H, 2H, and 3H. Each isotope has a different mass number. 1H – 1 proton, 0 neutrons. Mass number = 1 2H – 1 proton, 1 neutron. Mass number = 2 3H – 1 proton, 2 neutrons. Mass number = 3 4

What are the parts of an atom? Together, protons and neutrons form the nucleus of the atom. The overall charge of the nucleus is equal to the charge on the total number of protons in the nucleus. The nucleus is small but very dense. More than 99% of an atom’s mass is in the nucleus. 5

What are the parts of an atom? The negatively charged particles of an atom are called electrons. The charge of a single electron is represented as 1-. Because they move so fast, it is not possible to determine the exact position and speed of an electron at the same time. So we picture the electrons as being in an electron cloud around the nucleus. 6

What are the parts of an atom? Compared with protons and neutrons, electrons have very little mass. The number of protons and electrons in an atom are the same, so their charges are balanced and the atom has an overall charge of 0. An atom can gain or lose electrons to become an ion, which has a net positive or negative charge. 7

Absolute Dating Determining the actual age of an event or object in years is called absolute dating. Scientists often use radioactive isotopes to find the absolute age of rocks and other materials. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Isotopes are formed from nuclear reactions. 9

What happens during a nuclear reaction? A nuclear reaction is a change that affects the nucleus of an atom. It differs from a chemical reaction in several ways. One difference is that chemical reactions do not change the mass of atoms, but nuclear reactions do so by a very small amount by changing the nucleus of an atom. They can change the number of neutrons They can change the number of protons 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).

- - - - Beryllium Lithium Atom Atom How a nuclear reaction can change an atom into another one. - Beryllium Atom 4 protons 4 electrons 5 neutrons Lithium Atom 3 protons 3 electrons 5 neutrons Nuclear Reaction Source + - N + + N N + - N N - Because the radiation decreased the number of protons and electrons from 4 to 3 in the atom, it changed from an atom of Beryllium to Lithium.

- - - - How a nuclear reaction can change an atom into an isotope. Beryllium-8 Isotope 4 protons 4 electrons 4 neutrons Beryllium Atom 4 protons 4 electrons 5 neutrons Nuclear Reaction Source - + N N + N + + - N N - Because the radiation decreased the number of neutrons from 5 to 4 in the atom, it changed from an atom of Beryllium to the Beryllium-8 isotope.

What happens during a nuclear reaction? Isotopes of the same element have different numbers of neutrons and from that, different mass numbers. Carbon atoms have 6 protons and 6 neutrons in their nucleus, giving them a mass number of 12. When carbon atoms lose or gain neutrons, they become isotopes. The mass number is added to the end of the name of an element to identify isotopes, such as carbon-11 and carbon-14.

How can the absolute age of rock be determined? Radioactive isotopes, also called radioisotopes, are isotopes that are unstable and break down into other, stable isotopes by a process called radioactive decay. As they break down, they release excess energy by emitting radiation in the form of alpha, beta and gamma rays. The radioactive isotope is called the parent isotope, and the stable isotope formed by its breakdown is called the daughter isotope. Half-life is the time needed for half of a sample of a radioactive element to undergo radioactive decay and form daughter isotopes. 14

Time Elapsed: 5730 years = Half-life 2865 years = Radiation After one half-life has passed, one-half of the parent isotope has changed into daughter isotopes. Time Elapsed: 5730 years = Half-life 2865 years = Radiation = Parent Isotope = Daughter Isotope 15

How can the absolute age of rock be determined? Scientists study the amounts of parent and daughter isotopes to date samples. Finding the absolute age of a sample by determining the relative percentages of a radioactive parent isotope and a stable daughter isotope is called radiometric dating. 16

What is the best rock for radiometric dating? Igneous rocks are the best types of rock samples to use for radiometric dating. When igneous rocks form, minerals in them often contain only a parent isotope and none of the daughter isotope. This makes the isotope percentages easier to interpret and helps dating to be more accurate. 17

Time for a Change What are some radiometric dating methods? Scientists use many different isotopes for radiometric dating. The type of isotope used depends on the type of material being dated. The half-life of the isotope used is also very important. It can’t be too short or too long compared to the age of the sample. 18

What are some radiometric dating methods? Radiocarbon dating is a method used for dating wood, bones, shells, and other organic remains. All living things have a constant ratio of radioactive carbon-14 to carbon-12. Once a plant or an animal dies, no more carbon is taken in. The ratio between the isotopes changes because carbon-14 undergoes radioactive decay. 19

What are some radiometric dating methods? The half-life of carbon-14 is 5,730 years. The number of half-lives of carbon-14 that have passed gives the absolute age. Radiocarbon dating can be used to date organic matter only. This method is used to date things that lived in the last 45,000 years. 20

What are some radiometric dating methods? Potassium-argon dating is often used to date igneous volcanic rocks that are 100,000 years to billions of years old. Uranium-lead dating is based on measuring the amount of the lead-206 daughter isotope in a sample. Uranium-lead dating can be used to determine the age of igneous rocks that are between 100 million years and a few billion years old. 21

Time Will Tell How is radiometric dating used to determine the age of Earth? Radiometric dating can be used to find the age of Earth. But there are no Earth rocks which can be directly studied that are as old as our planet. Meteorites are small, rocky bodies that have fallen from space to Earth’s surface. They are the same age as the solar system, including Earth. The absolute age of meteorites and other rocks in the solar system is about 4.6 billion years. 22

Showing Your Age How can fossils help to determine the age of sedimentary rock? Sedimentary rock layers and the fossils within them cannot be dated directly. But igneous rock layers on either side of a fossil layer can be dated radiometrically. Once the older and younger rock layers are dated, scientists can assign an absolute age range to the sedimentary rock layer containing the fossils. 23

How can fossils help to determine the age of sedimentary rock? Index fossils are 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.

How can fossils help to determine the age of sedimentary rock? To be an index fossil, the organism from which it formed must have lived during a relatively short geologic time span. Index fossils must be relatively common and must be found over a large area. Index fossils must also have features that make them different from other fossils.

How are index fossils used? Index fossils act as markers for the time that the organisms were alive on Earth. Index fossils can also be used to date rocks in separate areas. The appearance of the same index fossil in rock of different areas shows that the rock layers formed at about the same time. 26