Nuclear Chemistry Chapter 22 Notes

The Nucleus Nucleons – the particles found in the nucleus of an atom; protons and neutrons. In nuclear chemistry, the atoms are called nuclides. Each one is identified by its name and mass number. –Radium-288 –

Mass Defect For a helium atom, we have 2 protons, 2 neutrons and 2 electrons. –2 protons: 2* amu = amu –2 neutrons: 2* amu = amu –2 electrons: 2* amu = amu The sum of these is amu.

Mass Defect We’d expect an atom of helium-4 to have a mass of amu. It actually has a mass of amu, which is amu less than expected. This difference is known as the mass defect. Where does this “lost” mass go? Isn’t mass a conserved quantity?

Nuclear Binding Energy We can use E=mc 2 to calculate the amount of energy “held” by the mass that was lost. –Convert the mass to kilograms first m = x kg –E = (5.0446x kg)*(3.00x10 8 m/s) 2 = 4.54x J The nucleus of each atom is held together by this nuclear binding energy.

Nucleons and Nuclear Stability What makes a nucleus stable? –It doesn’t undergo nuclear reactions. –It has enough neutrons to overcome the repulsion of the protons. –It lies on the band of stability. Band of Stability: the stable nuclei cluster over a range of neutron-proton ratios

Band of Stability

Nuclear Reactions Nuclear reaction – a reaction that affects the nucleus of an atom Transmutation – a change in the identity of a nucleus as a result of a change in the number of its protons

Radioactive Decay Radioactive decay – the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation or both

Radioactive Decay Nuclear radiation – particles or electromagnetic emitted from the nucleus during radioactive decay Radioactive nuclide – an unstable nucleus that undergoes radioactive decay

Alpha Particles Contain two protons and two neutrons Very low penetrating power – stopped by a piece of paper Written as:

Beta Particles High speed electrons Medium penetrating power – stopped by thin metal foil Released when a neutron decays into a proton and an electron Written as

Gamma Rays High speed radiation without mass or charge Strong penetrating power – partially stopped by thick lead and/or thick concrete Causes tissue damage Represented by

Nuclear Reaction Equations Shows the changes occurring within the nuclei The types of atoms will change, but two things must be constant: –The sum of the mass number –The sum of the atomic number

Nuclear Reaction Equations

Types of Radioactive Decay Alpha Decay Beta Decay Positron Emission Electron Capture

Alpha Decay Releasing alpha particles – very heavy nuclei getting rid of mass to increase stability Alpha decay of radium Ra He Rn

Beta Decay Elements above the band of stability are unstable because they have too many neutrons –One neutron is converted into a proton and an electron –The electron is released from the nucleus Beta decay of carbon C + 0 β 14 7 N

Positron Emission 15 8 O β ? 15 7 N ? ? Elements below the band of stability are unstable because they have too many protons –One proton is converted into a neutron by releasing a positron –A positron is a beta particle with a positive charge –The positron is released from the nucleus Positron emission of oxygen-15

Electron Capture Another way of stabilizing a nucleus with too many protons Capturing an electron from the inner orbitals –The electron combines with a proton to make another neutron Electron capture of rubidium Rb + 0 e Kr

Gamma Emission Gamma rays are released when nucleons drop to a lower energy level –Similar to the light released when electrons drop to a lower energy level Released whenever another type of radioactive decay occurs

Half-Life Half-life, t ½ : the time required for half the atoms in a radioactive material to decay Half-life is an indication of the stability of a nuclide –Short half-life means very unstable (microseconds) –Long half-life means very stable (billons of years)

Half-Life Equation mass Original mass Time Half-life

Example 1 The half-life of polonium-210 is days. How many milligrams of polonium- 210 remain after days if you started with 2.0 mg of the isotope?

Example 1 The half-life of polonium-210 is days. How many milligrams of polonium-210 remain after days if you started with 2.0 mg of the isotope?

Example 1 The half-life of polonium-210 is days. How many milligrams of polonium-210 remain after days if you started with 2.0 mg of the isotope?

Example 1 The half-life of polonium-210 is days. How many milligrams of polonium-210 remain after days if you started with 2.0 mg of the isotope?

Example 2 The half-life of uranium-238 is 4.46x10 9 years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

Example 2 The half-life of uranium-238 is 4.46x10 9 years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

Example 2 The half-life of uranium-238 is 4.46x10 9 years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

Example 2 The half-life of uranium-238 is 4.46x10 9 years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

Decay Series Decay series – a series of radioactive reactions that begins with a radioactive nuclide and ends with a stable one Parent nuclide – the heaviest particle in a decay series Daughter nuclides – all of the nuclides produced in a decay series Page 710 shows the decay of uranium- 238 into lead-206

Artificial Transmutations Artificial transmutations – causing a nucleus to undergo a radioactive reaction –Hit the nucleus with high speed particles –The particles go into the nucleus, causing it to become unstable –The unstable nucleus undergoes radioactive decay to return to stability

Fermilab (Illinois) 4 miles in diameter! *photo from Google Map

Transuranium Elements Elements with atomic numbers higher than 92 (past uranium) –Very unstable – they decay within microseconds –Only found in laboratories now –Were probably found in the very beginning of the universe, but they all decayed a long, long time ago

Radiation Exposure The energy in nuclear radiation can be transferred to atoms in your cells, causing damage to your body. The amount of radiation is measured in units called rems Being exposed to a higher amount of radiation = more damage, therefore you’d be more likely to get radiation poisoning or cancer

Radiation Detection Film badges – use exposure to approximate radiation exposure Geiger-Mϋller counters – count the ions made by radiation to measure its strength Scintillation counters – measure the light made by radiation to measure its strength

Applications of Nuclear Radiation Radioactive dating – use the concentration of radioactive nuclides to approximate the age of an object. Radioactive tracers – use radioactive atoms in medical procedures to follow their pathway through the body In agriculture, radioactive atoms can be used to test the effectiveness of fertilizers.

Nuclear Waste Nuclear waste – radioactive products of nuclear reactions It must be contained to protect humans from the radiation –Storage: store on-site if the half-life is short (ponds or dry casks) –Disposal: put the waste somewhere isolated, with no intentions of ever touching it again (Yucca Mountain)

Nuclear Fission Splitting of nucleus into fragments Involves heavy atoms breaking down into lighter atoms Fat man and little boy were fission bombs

Nuclear fission of uranium n U 1010 n + 36 Kr Ba + + Notice change from heavy atom to lighter atoms 3

Fission Chain Reaction Multiple fission reactions; initiated by new neutrons released LOTS OF ENERGY PRODUCED Example: atomic bomb, nuclear power plant

Critical Mass Minimum mass required to sustain a chain reaction Fission bomb has two pieces of fissionable material – a conventional explosive (TNT) forces them together so that it exceeds critical mass and starts a chain reaction

Nuclear Power Plant Generates steam by nuclear fission; steams drives turbines to produce electricity –Pros: less fuel, fewer pollutants –Cons: radioactive waste, more catastrophic plant accidents

Nuclear Fusion Combining two or more small nuclei into one stable nucleus Example: the Sun (or stars) hydrogen Helium + ENERGY FUSION

Hydrogen Bomb