Isotopes and Radiation Radioactivity and Nuclear Reactions

What’s an Isotope? Not all atoms of an element are exactly the same. The number of neutrons can vary from atom to atom. Still same element since protons do not differ. Carbon -12: 6 protons and 6 neutrons Carbon – 14: 6 protons and 8 neutrons.

Radioactivity

Nucleus Nucleus is incredibly small compared to the size of the atom. A marble in a football stadium. Nucleus has almost all the mass of the atom. Add football stadium picture

Strong Force Like charges repel. Strong force holds all the protons and neutrons together. Very strong, but limited range – less than the width of a few protons.

Size and Stability: Small Nuclei Small nuclei have less protons to repel each other. Small nuclei have particles that all fall with-in the width of a few protons. Small nuclei are very stable.

Size and Stability: Large Nuclei Large nuclei have many protons that contribute to a large repulsive force. Large nuclei have particles that are more than the width of a few protons. Particles only feel pull of closest neighbors. Large nuclei tend to be unstable and fall apart. All atoms beyond 83 protons are unstable.

Large Nuclei Picture

Stability and p+ to n ratio. In less massive atoms, a one to one ratio provides the best stability. Carbon -12 is stable, Carbon – 14 is not. In more massive atoms, the ration of neutrons to protons is 3 to 2 (1.5 neutron to every proton). The neutrons dilute the repulsive force of the protons.

Zone of Stability As the nucleus becomes bigger, you need more neutrons to keep it stable NOVA Science Video: Sea of Instability (www.pbs.org/wgbh/nova/sciencenow/3313/02)

Radioactivity When a nucleus falls apart (decays) it releases matter and energy. The starting atom is the parent, and new atom is called the daughter. This release is called radioactivity.

Nuclear decay

Alpha Particles A helium nucleus. 2 protons and 2 neutrons. Has a positive 2 charge (+2). Strongest electrical charge of all radiation, so most damaging. Least penetration of all radiation. Stopped by a sheet of paper or clothing. Add Alpha particle picture

Alpha Decay When an atom emits an alpha particle it changes to a new element. Atomic number is two lower. New mass number is four lower. Change is called transmutation. Transmutation diagrm

Alpha Decay Question So knowing this what would Radium become? Francium Astatine Radon Cerium

Beta Particles Negative charge – an electron is released. Very little mass and charge but very fast moving. More penetrating than big alpha particles. Pass through paper and clothes, but stopped by a sheet of metal.

Beta Decay Appears that a neutron becomes a proton, and an electron is released. Mass number stays the same, but atomic number is increased. Add Visual

Beta Decay Question 1 So what if Tungsten underwent Beta Decay what would it’s new atomic number be?

Beta Decay Question 2 What would Tungsten’s atomic mass be rounded to the nearest whole number?

Gamma Rays Pure energy. No mass and no charge. Example is an x-ray. No mass and no charge. Emitted with alpha and beta decay. Most penetrating – takes a thick block of lead or concrete to stop. Least damage producing form of radiation.

Half-Life Half-life is the time it takes for half of the parent nuclei to decay. Unaffected by heat, pressure, and other chemicals so makes an excellent clock. Start with 100 atoms. 1 half-life 50 remain. (1/2 of 100) 2 half-life 25 remain. (1/2 of 50) 3 half-lifes 12. 5 remain (1/2 of 25)

Half Life Question So if we started with 180 atoms of Sodium how many atoms would we have after 4 half lives? 60 11.25 90 22.5

Nuclear Reactions

Nuclear Fission Nuclear Fission- The division of a nucleus into smaller nuclei. This can be done by colliding a nuclei with a neutron. Only large nuclei can undergo fission. The mass of the daughter nuclei do not equal the mass of the parent nucleus. Some is lost to energy.

Energy equation The conversion of matter to energy is explained by Albert Einstein’s famous equation: E=mc2 According to this 1 gram of matter would become 100 trillion joules of energy.

Chain Reaction After nuclear fission the product causes the rest of the atoms to break apart also. 1 reaction causes a reaction in the rest of the nuclei in the element.

Nuclear Fusion Two small nuclei are combined to form 1 large nuclei. How can we get two nuclei to join when the repulsive forces don’t want them to? Speed through temperature. Ex. Sun Takes 4 Hydrogen nuclei and combines them to create a Helium atom.

Uses in Medicine We use radioactive elements (tracers) in science because they are easily detected through their radiation. PET scans Use radioactive fluorine because the brain absorbs it as it functions and we can measure the gamma rays. If low levels of gamma rays then the brain isn’t absorbing it so that part of the brain is not functioning correctly.