Unit 2 Notes – Radioactivity Mr Nelson

Facts about the nucleus Radius = 10-13cm (0.0000000000001 cm) Radius = 1/100000th of an atom Density = 1.6x1014g/cm3 That’s 160000000000000 g/cm3 A sphere the size of a ping pong ball = 2.5 billion tons

Why does the nucleus stay together?

4 basic forces Gravity Weak Nuclear Electromagnetic Strong Nuclear

What holds nucleus together? The answer was proposed by Hideki Yukawa in 1935, he called it a STRONG NUCLEAR FORCE. It is exerted by the p+ and n0 on each other SNF overwhelms the electrostatic charge (++)

SNF The SNF acts over very short distance, so larger nuclei the force does not reach far enough If the SNF is not enough, the electrostatic forces will dominate and the nucleus will break apart. Instability occurs when nucleus is too big

Nuclear symbols Chapter 19 requires that you understand nuclear symbols and how to manipulate them. A X Z

Nuclear vs Chemical Reactions In a chemical reaction: Atoms are rearranged to form new substances In nuclear reactions: We are creating new elements! ALCHEMY!

Radioactive Decay Radioactive – something is radioactive if the nucleus spontaneously decomposes When it does this a different nucleus is formed Extra particles are usually formed also Example: The symbol represents a beta (b) particle

b particle production The mass of a beta particle is 0, and the atomic number is –1: So, beta particles change neutrons to protons.

Where did the e- come from? A decaying neutron!!!

Nuclear equations & Laws Notice that in the equation, atomic mass (A) and atomic # (Z) are the same on both sides of the arrow. Law of conservation of Mass!

Types of Radioactive Decay Alpha (a) particle: An alpha particle is a Helium Nucleus. The nuclear symbol is:

Gamma Rays (g) A gamma ray is a high energy photon of light. Released along with other types of radiation

Positrons A particle with the same mass as an e- but a positive charge: Positrons are anitmatter! Example:

Electron capture A nucleus CAPTURES an inner electron Gamma rays are always produced. Explanation: NOTE that in electron capture the beta particle is always on the LEFT of the arrow!!

Decay Series An unstable nucleus undergoes a series of changes until it becomes a stable nucleus

Band of Stability Predicts whether or not an atom’s nucleus will be stable. For atoms less than 20 p+, n0:p+ are 1:1 n0/p+ ratio greater than 1.5/1 will be unstable

Binding Energy & Mass Defect Binding Energy – energy that holds nucleus together Mass defect – the ‘missing mass’ when the mass of nucleons are added together. Example: Iron – 56

E = mc2 Einstein’s idea E = energy m = mass in kilograms c = speed of light 3x108 m/s

Hadronization (hadrons produced) Reconstructed Collision of e+e- Hadronization (hadrons produced) e+ e- q e+ e -

Half Life ½ life: the time for ½ of a sample of nuclei to decay For U-238, it is 4.5 billion years Pa-234 has a ½ life of 1.2 minutes!!

Transuranium elements & Radioactivity Transuranium elements are just elements #93-11? (anything after Uranium!) All elements after Bismuth (Z = 83) are radioactive

Fission and Fusion Combining two light nuclei to make a heavier nucleus is called fusion Splitting a heavy nucleus to form two nuclei with smaller masses is called fission.

Fission Releases a lot of energy! 26 million times that of burning methane Can cause chain rxns

Chain Rxns At least one neutron must go on to split another nucleus

Nuclear Reactors

Nuclear Fusion Produces more energy than fission! Takes very high temperatures to get protons close enough to “stick” to each other Why is “cold fusion” a hollywood fad?

Effects of Radiation Different particles penetrate in different ways energy mass How easily they make ions Chemical properties These things make radiation more or less harmful

Detecting Radiation High energy particles from radioactive decay make ions when they travel through matter. The fast particles “knock” electrons off the atoms they hit. A geiger counter can detect these ions through the flow of electricity

A geiger counter

Carbon Dating (Radioactive Dating) Based on the radioactivity of carbon-14, which decays through b-particle production. Carbon-14 is continuously made in the atmosphere when neutrons from space hit the nitrogen in the air.

More on carbon dating: Living things always have the same amount of C-14 because they take it in through the air. As soon as a living thing dies it stops taking in carbon from the atmosphere. The C-14 decays. The ½ life of C-14 is 5730 years. If a piece of wood has ½ the C as a living tree, it is 5730 years old. What are some of the problems with this method?

Making new elements Scientists make new elements by bombarding existing nuclei to create elements with larger and larger atomic numbers.

Changing one element into another Radioactive decay is when a nucleus decomposes and turns in to another nucleus Bombarding nuclei with particles can form new nuclei also.