IsotopesIsotopesMatter UnitMatter Unit

Learning Goal 5Learning Goal 5

Elemental NotationElemental Notation

Atomic NumberAtomic Number– The number of protons in an atom (# of The number of protons in an atom (# of

protons = # of electrons).protons = # of electrons).

Mass NumberMass Number– The total number of protons and The total number of protons and

neutrons in an atom’s nucleus.neutrons in an atom’s nucleus.

IsotopesIsotopes

Atoms with the same atomic number, but Atoms with the same atomic number, but different mass numbers.different mass numbers.– Atoms have the same number of protons, but Atoms have the same number of protons, but

a different number of neutrons.a different number of neutrons.

Isotopes of an element have the same Isotopes of an element have the same chemical properties but different weights.chemical properties but different weights.

Common IsotopesCommon Isotopes

Hydrogen IsotopesHydrogen Isotopes

Half-lifeHalf-life

The amount of time it takes for half of the atoms The amount of time it takes for half of the atoms to decay into a more stable form.to decay into a more stable form.

Cobalt-60 has a half-life of 5.3 years and is Cobalt-60 has a half-life of 5.3 years and is made in a reactor. Cobalt-60 is used for made in a reactor. Cobalt-60 is used for radiation therapy of cancer patients.radiation therapy of cancer patients.

Over 3,500 isotopes are known, and most are Over 3,500 isotopes are known, and most are made in the lab.made in the lab.

Smoke Detectors and Smoke Detectors and Americium-241Americium-241

How many of us have smoke detectors in our house? Chances are How many of us have smoke detectors in our house? Chances are that a great number of homes have had one or more of these that a great number of homes have had one or more of these devices installed as an early warning system in case of fire. What devices installed as an early warning system in case of fire. What most consumers don't know is that many of these units contain a most consumers don't know is that many of these units contain a small amount of americium-241. By utilizing the radioactive small amount of americium-241. By utilizing the radioactive properties of this material, smoke from a fire can be detected at a properties of this material, smoke from a fire can be detected at a very early stage. This early warning capability has saved many very early stage. This early warning capability has saved many lives. In fact, studies have shown that 80% of fire injuries and 80% lives. In fact, studies have shown that 80% of fire injuries and 80% of fire fatalities occur in homes without smoke detectors.of fire fatalities occur in homes without smoke detectors.

Archaeological DatingArchaeological DatingSignificant progress has been made in this field of study since the discovery of Significant progress has been made in this field of study since the discovery of radioactivity and its properties. One application is radioactivity and its properties. One application is carbon-14carbon-14 dating. Recalling that all dating. Recalling that all biologic organisms contain a given concentration of carbon-14, we can use this biologic organisms contain a given concentration of carbon-14, we can use this information to help solve questions about when the organism died. It works like this. information to help solve questions about when the organism died. It works like this. When an organism dies it has a specific ratio by mass of carbon-14 to carbon-12 When an organism dies it has a specific ratio by mass of carbon-14 to carbon-12 incorporated in the cells of it's body. (The same ratio as in the atmosphere.) At the incorporated in the cells of it's body. (The same ratio as in the atmosphere.) At the moment of death, no new carbon-14 containing molecules are metabolized, therefore moment of death, no new carbon-14 containing molecules are metabolized, therefore the ratio is at a maximum. After death, the carbon-14 to carbon-12 ratio begins to the ratio is at a maximum. After death, the carbon-14 to carbon-12 ratio begins to decrease because carbon-14 is decaying away at a constant and predictable rate. decrease because carbon-14 is decaying away at a constant and predictable rate. Remembering that the half-life of carbon-14 is 5700 years, then after 5700 years half Remembering that the half-life of carbon-14 is 5700 years, then after 5700 years half as much carbon-14 remains within the organism. as much carbon-14 remains within the organism.

Example : If an organism such as a tree contained 1 gram of carbon-14 while it was Example : If an organism such as a tree contained 1 gram of carbon-14 while it was living, then after 5700 years it would contain half that amount, or 0.5 grams of carbon-living, then after 5700 years it would contain half that amount, or 0.5 grams of carbon-14.14.

This method of dating using carbon-14 is only good for organisms or artifacts that are This method of dating using carbon-14 is only good for organisms or artifacts that are biological by nature and on the order of tens of thousands or years old.biological by nature and on the order of tens of thousands or years old.