Radioactivity Chapter 25

Nuclear chemistry study of the structure of atomic nuclei changes they undergo. Nuclear Radiation

Wilhelm Roentgen (1845–1923) 1895-invisible rays were emitted when electrons bombarded the surface of certain materials. The Discovery of Radioactivity caused photographic plates to darken. named the invisible high-energy emissions X rays.

Henri Becquerel (1852–1908) was studying phosphorescence minerals that emit light after being exposed to sunlight The Discovery of Radioactivity phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates.

Marie Curie (1867–1934) and her husband Pierre (1859–1906) took Becquerel’s mineral sample (called pitchblende) and isolated the components emitting the rays. The Discovery of Radioactivity darkening of the photographic plates was due to rays emitted specifically from the uranium atoms present in the mineral sample.

The Discovery of Radioactivity Marie Curie named the process by which materials give off such rays radioactivity the rays and particles emitted by a radioactive source are called radiation.

isotopes are atoms of the same element that have different numbers of neutrons. Types of Radiation Isotopes of atoms with unstable nuclei are called radioisotopes emit radiation to attain more stable atomic configurations in a process called radioactive decay lose energy by emitting one of several types of radiation.

Why do some atoms decay? The nucleus contains tightly packed protons and neutrons (nucleons) The strong nuclear force keeps the nucleons packed together even though protons want to push each other away Stable atoms have a neutron to proton ratio of about 1:1

As atomic number increases, more neutrons are required to have enough of a strong force to keep the protons pushed together The neutron to proton ratio for stable atoms increases to 1.5:1

Band of Stability When the number of protons and neutrons are plotted, the stable nuclei are found within the “band of stability” Radioactive isotopes are outside the band of stability –They will undergo nuclear reactions to become more stable –All elements higher than atomic# 83 are radioactive

Basic Assessment Questions Question 3 Topic 26 Topic 26 Calculate the neutron-to-proton ratio for.

Types of Nuclear Radiation Alpha Beta Gamma

Alpha Radiation Release of 2 protons and 2 neutrons –Equivalent to a He nucleus –Charge of 2+ –Mass = 4 amu Largest and slowest –Least penetrating  can be stopped by paper Changes to a different element with a lower atomic mass and lower atomic number Example: Polonium-212 (atomic# 84) is converted to Lead-208 (atomic# 82)

Beta Radiation Decay of a neutron into a proton and electron –Electron is emitted, proton stays –Forms a new element b/c of addition of proton Decay of the proton into a neutron and positron (like a positive electron) –The positron is emitted as a beta particle Faster than alpha particles  can be stopped by aluminum foil

Gamma Radiation Not a particle Electromagnetic wave with short wavelength and high frequency & energy No mass, no charge Very fast  speed of light Stronger than X-ray Stopped by several centimeters of lead

Transmutation: changing one element into another through radioactive decay –Adding or removing a proton changes the atomic number, resulting in a different element Half-Life: amount of time for half of a sample of a radioactive element to decay into something else –Can range from a fraction of a second to billions of years –Amount remaining=initial amount(1/2) t/T t=total time T=half-life

Half-life m f :final mass m i :initial mass n:# of half-lives

Half-life Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s?

Nuclear Decay Alpha Emission parent nuclide daughter nuclide alpha particle Numbers must balance!!

Nuclear Decay Beta Emission electron Positron Emission positron

Nuclear Decay Electron Capture electron Gamma Emission –Usually follows other types of decay.

Types of Transmutation Induced transmutation –Nucleus of an unstable isotope (radionuclide) is struck with a high velocity charged particle Particle accelerator Need lots of energy and unstable nucleus –Elements atomic 93 and higher (transuranium elements) Natural transmutation –Occurs naturally as a radioisotope decays to become more stable

Basic Assessment Questions Question 1 Topic 26 Topic 26 What element is formed when polonium-214 ( ) radioisotope undergoes alpha decay? Give the atomic number and mass number of the element.

Basic Assessment Questions Question 2 Topic 26 Topic 26 What element is formed when undergoes beta decay? Give the atomic number and mass number of the element.

Basic Assessment Questions Question 4 Write a balanced nuclear equation for the beta decay of the following radioisotope. Topic 26 Topic 26

Nuclear Fission Fission = divide Neutron hits an unstable atom Nucleus splits into two fragments of about the same mass –Some single neutrons are released (energy) –These neutrons can smash into other atoms Causes a chain reaction

Fission Reaction

Nuclear reactors Nuclear power plants use the process of nuclear fission to produce heat in nuclear reactors. The heat is used to generate steam, which is then used to drive turbines that produce electricity.

Atomic Bomb- uncontrolled fission reactions

Little Boy: $2billion in research; made of Uranium-235; equal to 20,000 tons of TNT; 140,000 people died; 2/3 of the city destroyed Fat Man: Plutonium-239; 70,000 people died; 40% of the city destroyed

Hydrogen Bomb 1000 times more powerful than atomic bomb March 1, 1954; Bikini Atoll in Pacific –Never in war Fission reaction triggers fusion of Hydrogen isotopes

Nuclear Fusion Opposite of fission Two nuclei fuse together to form one nucleus with a larger mass –Not simple sum of masses –Some mass lost as energy Requires high temperature: Thermonuclear reaction Occurs in the sun and stars –4 H combine to form one He, 2e- and energy

Nuclear Fusion

Solar Flare

Radiation Detectors Cloud Chamber –supersaturated water or ethanol –radioactive particle flows through and knocks e- off –vapor condenses showing path –alpha: short/thick trails; beta: long/thin Bubble Chamber –superheated liquid –e- knocked off again –bubbles are formed

Measuring Radiation Geiger Counter –produces electric current when near radiation –Results in clicks or a digital reading

Using Nuclear Reactions in Medicine Tracers: monitor body processes –Iodine-131 emits beta particles used to detect tumors in thyroid gland –also used: Carbon-11 and Sodium -24

Cancer Treatment damage cancer cells Gold -198 or Iridium implanted in or near tumor Cobalt-60 –outside body –emits gamma rays

Positron Emission Tomography (PET) – Fluorine-18 attached to molecules that go to brain –positrons are emitted and collide with electrons forming 2 gamma rays –the gamma rays are detected and indicate brain activity

opes/radioactive_decay3.htmlhttp:// opes/radioactive_decay3.html jects/Fission/frames.htmlhttp:// jects/Fission/frames.html xperience/the.bomb/history.science/