Radioactivity and Nuclear Reactions Ch 9.1-9.2, 9.4

Nucleus and the Strong Force Protons and neutrons are packed tightly together Two positives normally repel each other, so why don’t the protons in the nucleus repel? Strong force = one of four basic forces that causes protons and neutrons to be attracted to each other 100 times stronger than electric force Short-range force, so it weakens with distance

Small vs Large Nuclei Protons and neutrons are held together less tightly in large nuclei. Why? Small nuclei have few protons, so the repulsive force on a proton due to other protons is small In a large nuclei, the attractive strong force is exerted only by the nearest neighbors. All the protons exert repulsive forces making the repulsive force large.

Radioactivity In many nuclei, the strong force keeps the nucleus together (STABLE) When it can’t, the nucleus can decay and give off matter and energy in a process of radioactivity Larger nuclei tend to be unstable – all nuclei containing more than 83 protons are radioactive All elements with more than 92 protons are synthetic and decay soon after they are created (UNSTABLE)

Stable and Unstable Nuclei Smaller elements neutron to proton ratio is 1:1 to be stable isotopes Heavier elements neutron to proton ratio is 3:2 to be stable isotopes Nuclei of any isotopes that differ much from these ratios are unstable, whether heavy or light

Nuclear Radiation When an unstable nucleus decays, particles and energy are emitted from the decaying nucleus Alpha Particles – (2 p and 2 n lost) massive, comparatively speaking; loses energy quickly; can’t pass through paper; changes the element (transmutation); mass changes; can damage the body Beta Particles – (n turns into p and emits e) e emitted from n; transmutation changes the element; mass doesn’t change; much faster and penetrating; damage body Gamma Rays – electromagnetic waves that carry energy; most penetrating form; cause less damage to biological molecules

At a glance…

Radioactive Half-Life Some radioisotopes decay in less than a second, while others take millions of years Half-life: the amount of time it takes for half the nuclei in a sample of the isotope to decay

Radioactive Half-Life cont

Ch 21.3: Absolute-Age Dating of Rocks Relative-age dating vs. Absolute-Age Dating Relative-age dating: compares past geologic events based on the observed order of strata in rock record Absolute-age dating: determines actual age of a rock, fossil, or other object

Radioactive Decay Radioisotopes are found in igneous and metamorphic rocks, some fossils, and organic remains Emission of radioactive particles and the resulting change into other elements over time is called radioactive decay This decay stays constant regardless of the environment, pressure, temperature, or any other physical changes So, these atomic particles become accurate indicators of the absolute age of an object

I love you Mrs. Sjuts! 

Radioactive Dating Fossils and rocks can be dating using radioactive isotopes Amounts of the radioisotope and its daughter nucleus are measured in a sample Then, the number of half-lives that need to pass to give the measured amounts of the isotope are calculated The number of half-lives is the amount of time that has passed since the isotope began to decay AND usually is the same as the age of the object.

Carbon Dating The radioactive isotope C-14 is often used to find the ages of once living objects It is naturally found in most all living things An atom of C-14 eventually will decay into N-14 with a half-life of 5,730 years By measuring the amount of C-14 in a sample and comparing it to the amount of C-12, scientists can determine the approx age of plants and animals that lived within the last 50,000 years

Uranium Dating Some rocks contain uranium, which has two radioactive isotopes with long half-lives, both decaying into isotopes of lead By comparing the uranium isotope and the daughter nuclei the number of half-lives since the rock was formed can be calculated U-235  0.7 billion years U-238  4.5 billion years

Ch 9.4 Nuclear Reactions Nuclear Fission – the process of splitting a nucleus into two nuclei with smaller masses Chain reaction – ongoing series of fission reactions Critical mass – the amount of fissionable material required so that each fission reaction produce approximately one more fission reaction Nuclear Fusion – two nuclei with low masses are combined to form one nucleus of larger mass

Nuclear Fission Large elements need a TON of energy in order to hold their nucleus together. When the large nucleus is split into smaller nuclei, those smaller nuclei don’t require as much energy to stay together… So, that leftover energy is released! Atomic bomb – used in Hiroshima and Nagasaki Fission - Chain Reaction Nuclear Fission: Pros and Cons Nuclear Meltdown Cooper Nuclear Station near Brownville, NE Fort Calhoun Nuclear Generating System between Ft. Calhoun and Blair

Nuclear Fusion Need very high temperatures in order to overcome the repulsive forces. Sun's Fusion Scientists cannot control fusion reactions for the purpose of power. We can, however, use it to make nuclear weapons. Large ones. Hydorgen Bomb - Fusion

Nuclear Decay vs. Nuclear Reactions Decay happens spontaneously Reactions are controlled and self-sustaining and release much more energy

Nuclear Reaction: Plutonium Pu-239 Used to make nuclear weapons like the one dropped on Nagasaki in 1945