Chapter 21Nuclear Chemistry• Chemistry basically deals with electrons (bonding, ionization, molecular charges, electronegativity, etc.) • Nuclear chemistry (i.e. nuclear physics) involves processes that affect the nucleus of the atom.Overview• Review atomic structure• Introduce the types of spontaneous radiation:– Alpha, beta, gamma, positron• Review the band of stability and how it can be used to predict the most likely type of radiation from an atom.• Quantify the rates of radioactive decay– Or friend the exponential decay (first order process from kinetics) equation makes another appearance: At= Aoe−kt.• Calculate the energy of a nuclear reaction.• Define and quantify the mass defect of an atom and relate it to the nuclear binding energy.The Atom-- -- --NucleusElectronsprotonsneutrons-electronsProtons• Protons are found in the nucleus and they have a relative +1 charge.– The actual charge of a single proton is +1.602×10−19coulombs. • The number of protons is called the atomic number. The number of protons defines the different elements.• The mass of a proton is approximately 1.66×10−24g or 1.0073 amu– “amu” stands for atomic mass unit and it was originally defined as 1/12th the mass of 12C, so protons and neutrons have a mass of about 1 amu.Neutrons• Neutrons are found in the nucleus and they have no charge (hence neutral).• Atoms of a given element can have different numbers of neutrons.– Atoms with the same atomic number but different number of neutrons are called “isotopes”. • The atomic mass is approximately the number of protons + the number of neutrons.– For light elements, the number of neutrons often equals the number of protons. For heavy elements, there are more neutrons than protons.Electrons• The electrons are found in orbitals outside of the nucleus.• They have a relative charge of −1.– The actual charge of a single electron is −1.602×10−19coulombs.• The electrons are very small 5.486×10−4amu.• In neutral atoms, the number of electrons equals the number of protons.• The movement of electrons is electric current. – The SI unit for current is the ampere (A) = Coulombs per second (C/s)IsotopesC126Atomic mass(= atomic weight)= # of protons + # of neutronAtomic number= # of protons= # of electrons if atom is neutralThis is often omitted since “C” is defined as an atom with 6 protons.For this atom, there are: 6 protons6 neutrons (= mass − number)6 electronsx# atoms in a particular molecule(+ or −)Charge if presentIsotope ExamplesC146# of protons # of neutrons6= 14 − 68Cl3717=37 − 17= 20U23592= 235 − 92=143F199=19 − 9=10# of electrons6179210Nuclear Decay• Radiation is the result of nuclear decay where the nucleus breaks down in some fashion.• Nuclear decay processes are a function of the number of protons and neutrons in the nucleus. This is completely independent of whatever chemical processes (bonding, ionization, etc.) are going on.• Many elements have multiple isotopes (different number of neutrons). Often, some isotopes are stable and other isotopes are unstable.Types of Radiation• Alpha () radiation:  radiation is a particle that consists of two protons and two neutrons.– This is the same as a helium nucleus, so you will sometimes see it expressed as 4He2+.– The alpha particles have a +2 charge.– The loss of an alpha particle from a nucleus decreases the atomic number by two and the atomic mass by 4.– The alpha particles have almost no penetrating power. They can be stopped by a piece of paper.• Alpha radiation is very dangerous if it is inhaled or ingested. It was this type of radiation that was used to kill an ex-KGB spy in London in 2006. Po-210 was the poison.Types of Radiation• Beta () radiation:  radiation is a very high energyelectron, e−. – These electrons are emitted from the nucleus and are not associated with the electrons in the bonding orbitals. The particles have a −1 charge.– Beta decay is the result of a neutron decaying into an electron () and a proton. Therefore, the atomic number increases by one, but the atomic mass does not change.•1n 1p + e−– The beta particles have low penetrating power. They can be stopped by a piece of glass.Types of Radiation• Gamma () radiation:  radiation is a very high energyphoton. They have far more energy than X-rays. – Gamma radiation has no mass and no charge, so it is often left out of nuclear equations.– Gamma radiation is the result of the nucleus giving off energy after other radiation emissions have disturbed the energy state of the nucleus.• Gamma radiation almost always follows other radiation emissions. (e.g., the nucleus will emit an alpha particle and then rattles off the excess energy in the form of gamma rays)– The gamma radiation have very high penetrating power. You need a lot of lead shielding to stop it.Types of Radiation• Positron emission. This is similar to beta emission. – Positron emissions are the result of a proton decaying into a neutron and a positron:•1p 1n + e+– A positron is a positively-charged electron. They are anti-matter, so it will very quickly collide with a nearby electron a blow-up. The result is two gamma rays.– Atoms that are positron emitters are rather rare. The tend to be proton- rich atoms with small atomic weights.– For the pre-meds, the positron emitter of 18F is the central mechanism for the brain scans called PET scans (positron emission tomography)Types of Radiation• High energy neutron (1n). This type of radiation is often ignored, but it is rather important in nuclear reactors and bombs.– The radiation is just a neutron moving at very high speed. It has no charge and a mass of 1.– Neutrons are the only type of radiation that can make other materials radioactive.– Neutrons are given off when a nucleus splits apart in a fission processes.• The emission of a neutron from a nucleus is not a spontaneous event. The only time neutrons are emitted is fission when the nucleus breaks apart. Spontaneous fission is very rare in nature, but it can occur.Radiation Units• There are many different units to describe radioactivity and exposure to radiation.• The inherent “activity” of a substance is measured by:– becquerel (Bq): this is one disintegration per second (SI)– curie (Ci): the more traditional radiation measure. • 1 Ci = 3.7×1010Bq =