What’s in an atom? ~ 1/1840 amu Flying around nucleus Electron ~ 1 amu0In nucleus Neutron ~ 1 amu+1In nucleusProton What’s its mass? What’s its charge? Where it’s found Particle

Basic Terminology Recall: Z = atomic number = # of protons N = number of neutrons A = mass number = Z + N Since each proton and neutron weighs roughly 1 amu, the mass of a nucleus (in amu) is roughly equal to its mass number.

In the Nucleus Most of the mass of the atom VERY small volume radius ~ m Number of protons determines identity Number of neutrons in an element can vary Isotopes: atoms with = protons but different numbers of neutrons Average atomic mass = weighted average of the masses of all naturally-occurring isotopes.

In the Nucleus  Electrostatic forces: protons repel each other  “Strong” nuclear forces: attract protons to neutrons  The balance between these two forces determines whether or not a given nucleus will be stable. Strong force explanation Strong force explanation More on strong and weak forces More on strong and weak forces

Unstable Nuclei wrong Z/N balance,  radioactive decay i.e. the atom will emit radiation. Radioactive nuclei spontaneously decay. The degree of unbalance determines the type of decay Radioactive nuclei (parent nuclei) decay into daughter nuclei.

Types of Radiation Alpha Radiation a positive charged particle is released a helium nucleus Beta Radiation a negatively charged particle is released an electron Gamma Radiation no particle is released high energy waves are released

Alpha (  )Decay The nucleus is too large for the strong force to hold it together, an  -particle ( 2 4 He nucleus) is emitted Daughter nucleus has 2 fewer protons and 2 fewer neutrons than parent. Alpha radiation is too weak to penetrate paper or skin Nuclear equation: U  2 4 He Th

Beta (  )Decay When a nucleus has too many neutrons, a  -particle ( -1 0 e) is emitted, a neutron in the nucleus splits into a p + and an e - The p + stays in the nucleus. The e - is ejected and called a  -particle. Daughter nucleus has 1 more proton and 1 fewer neutron than parent. Nuclear equation: 6 14 C --> 7 14 N e Beta radiation is unable to penetrate aluminum foil or wood

Gamma (  ) Radiation When a nucleus has too much energy, it can give off very high energy waves of light. the nucleus is unchanged - it still has the same # of p+ and # of n0. Nucleus goes from “excited state” to “ground state,” losing excess energy. Gamma rays are often given off with other types of radioactivity. Gamma radiation can pass through people

Radiation Shielding

Radiation - Summary Same nucleus, just lower energy  gamma emission Too much energy Atomic number +1 Mass number stays same  - or -1 0 e beta Too many n 0 in nucleus Atomic number -2 Mass number -4  or 2 4 He alpha Nucleus is too large Resulting DaughterSymbol Decay Type Problem

Electron Capture When a nucleus has too few neutrons, An electron falls into the nucleus unites with a proton to form a neutron. Electrons cascade in to fill in for the missing electron Daughter nucleus has 1 more neutron and 1 fewer proton than parent. Nuclear equation: 6 9 C e --> 5 9 B

Stable Nuclei For small isotopes  N ≈ Z e.g.: 16 O is most stable isotope of O: Z = N = 8 or N/Z=1 For larger isotopes, N/Z is between 1 and 1.5 e.g.: 208 Pb is most stable isotope of Pb: Z = 82; N = 126 or N/Z=1.5

Practice Nuclear Equations

Half-Life amount of time it takes for half of a given sample to decay. Each half-life, half of the sample decays and half remains. Half lives vary from billionths of a second to billions of years.

Half-Life: Equation Form How do we put this into equation form? Let t 1/2 = the half-life of the isotope Let A 0 = the amount you start with Let A(t) = the amount remaining at time t Then t/t 1/2 = the number of half-lives that have elapsed. A(t) = A 0 (1/2) t/t 1/2

Half-Life: Practice with the table

Half-Life: Pennium

Nuclear Reactions - Fission If nucleus too big - nuclear fission. “fissions” (breaks up) into several smaller nuclei ( and usually some extra neutrons as well ). not easily predicted. Often initiated by absorbing a neutron. Example: U n --> Ba Kr n

Nuclear Energy - Fission Energy can be harnessed U n --> Ba Kr n The neutrons collide with other atoms of 235 U, split, producing more neutrons... causing a chain reaction. The energy given off can be harnessed to produce electricity (or, unfortunately, for more destructive purposes).

Nuclear Energy - Fission

Nuclear Weapons

Nuclear Waste This pool at the Areva Nuclear Plant near Cherbourg, France, cools spent nuclear fuel rods before they are moved underground. Francois Mori / AP

Nuclear Fusion The opposite of nuclear fission is fusion, when smaller nuclei come together to form larger nuclei. Example: 1 1 H H --> 2 4 He The fusion of hydrogen to form helium is the source of energy for the sun and many other stars.

Nuclear Energy - Fusion Release even more energy than fission. emitted by stars (mostly hydrogen fusing to form helium). no safe way yet to harness fusion takes too much energy to get started

Energy in Radioactive Decay Radioactive Decay generally gives off large amounts of energy. Where does it come from? The answer lies in something called “mass defect.”

Mass Defect Law of Conservation of Mass Mass cannot be created or destroyed. Law of Conservation of Energy Energy cannot be created or destroyed. But, during nuclear reactions: Mass can be converted into energy and energy can be converted into mass.

Mass Defect During nuclear reaction, some mass is either lost or gained. This change in mass is called the mass defect (  m).  m = mass of products - mass of reactants The relationship between the mass defect and the amount of energy given off or absorbed (  E) is  E =  mc 2 where c = the speed of light = 3.0 x 10 8 m/s.

Mass Defect  E =  mc 2 Example: If 0.01g of mass is converted into energy, 9 x J of energy is given off. That’s enough to heat 2.7 million liters of water from room temperature to boiling! By comparison: you would need to burn 16 million grams of methane to give off the same amount of heat.

Ten most common elements in the Milky Way ZElement% Composition 1Hydrogen74% 2Helium24% 8Oxygen1% 6Carbon0.4% 10Neon0.1% 26Iron0.1% 7Nitrogen0.09% 14Silicon0.06% 12Magnesiu m 0.05% 16Sulfur0.04%

Stellar Nucleosynthesis – the Stellar Metals H, He, Li H is exhausted, He is burned, fission creates Li, B, Be, C carbon burning white dwarfs more massive stars use nuclear fusion to create N, O, F, Ar, Na and Mg most massive stars use nuclear fusion to create Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe

Stellar Nucleosynthesis – beyond Z=26... Most massive stars (12x our sun) burn through Mg, Si etc, only Fe core remains implode from its own gravity matter is crushed p’s and e’s form n’s explode during which Fe nuclei gain n’s then decay into protons new elements (thru 92) are formed

Common Uses of Radioactivity Food Irradiation Archaeological Dating Medical Detection Medical Treatment

Daily Exposure to Radiation Cosmic rays Radon gas Smoke detectors

Detecting Radiation Geiger Counters (beta) Scintillation Counters (alpha, beta and gamma) Film (beta, gamma)