1 Atomic and Nuclear Structure (2019_08_29 12_06_50 UTC).ppsx

Editor's Notes

• #3: Nuclide: refers to the composition of the nucleus (number of protons and neutrons). Nucleons: include protons + neutrons. Z determines the number of electrons, and therefore the chemical properties of the atom (high-Z materials have fewer electrons/gram (electron density) than low-Z materials have& high-Z materials have more tightly bound electrons) The outer shell called valence shell, Valence electrons are not strongly attracted and their movements are responsible for all chemical reactions. # of electrons in the shell =2 n2 the maximum # of e- in the “outer” shell of an atom = 8 Neutrons are the neutrally charged particles that enable the formation of stable large atomic nuclei by decreasing the repulsion between the protons in the nucleus. However, neutrons, like protons, actually consist of particles called quarks; a neutron is one up quark and two down quarks, while a proton is two up quarks and one down quark. Isotope: Same number of protons, different neutrons. Same chemical behavior, different mass and different nuclear decay properties. Ex: 125I and 131I both behave like iodine, but have different half-lives. Isotone: Isotone: same number of neutrons, different protons. Rarely used. Isobar: Isobar: same number of nucleons, different nuclide. (more protons and less neutrons, or vice versa) Beta decay and electron capture always result in an isobar. Ex: 131I decays to 131Xe, which has the same mass number but is a different nuclide. Isomer: Isomer: same nuclide, different energy state. (excited vs. non-excited) Isomers release their energy through gamma decay Ex: 99mTc decays to 99Tc, releasing its excess energy without changing the number of protons or neutrons.
• #4: In order of descending strength these are: Strong Nuclear Force: The strongest force in nature; “glues” the nucleus together. Holds the nucleus together, counters the repulsive effect of protons’ positive charge. Electromagnetic (Coulombic) Force: ~1/100 as strong as the strong force. Opposites attract. Electrons are attracted by the positively charged nucleus and are more attracted as they get closer; Valence electrons are not strongly attracted and their movements are responsible for all chemical reactions. Protons repel each other within the nucleus but are held in place by the strong force. Weak Nuclear Force: ~1/1,000,000 as strong as the strong force. Works inside particles (between quarks) and is responsible for radioactive decay. Gravity: ~1 X10-39 as strong as the strong force. Not important on the atomic scale.
• #5: Einstein’s E = mc2 Mass and energy are always interchangeable. Energy can be converted to mass and mass can be converted to energy by multiplying by c2 (speed of light squared). As particles approach the speed of light, the velocity must remain constant so as the particle gains energy, it actually gains mass. There are two common ways to measure mass: Atomic mass units (AMU): Defined as 1/12 the mass of a Carbon-12 atom. This is slightly less than the mass of the component particles, due to the binding energy of the carbon atom. (see below) Proton mass = 1.0073 AMU Neutron mass = 1.0087 AMU (slightly larger than a proton) Electron mass = .0005 AMU (approx. 1/2,000) Energy equivalent (MeV/c2, may be shortened to just “MeV”): Defined as the equivalent amount of energy (mc2), measured in mega electron volts. Proton mass = 938.3 MeV Neutron mass = 939.6 MeV Electron mass = 0.511 MeV (or 511 keV) 1 AMU = 931.5 MeV
• #6: When particles are bound to each other they give off energy. Stars shine as they perform fusion and synthesize nuclei! Nuclear binding energy is the energy from binding neutrons and protons into a nucleus. • This energy is “paid for” in mass, according to E=mc2. This “mass deficit” is equal to the binding energy. Ex: Carbon-12 (12C) contains 6 protons and 6 neutrons. • The sum of masses should be 12.09565 AMU, but 12C has a mass of 12.00000 AMU. The mass deficit is .09565 AMU, or 89.1 MeV and this is the binding energy that holds the nucleus together. • In order to un-bind something, you need to spend at least as much energy as the binding energy. You cannot split a carbon nucleus with 18 MeV photons from an average linac, but you could with a cyclotron throwing 200+ MeV protons.
• #8: Neutron-to-proton (n/p) ratio: Protons generally hate each other due to their charge; they need neutrons to keep the peace. Too many neutrons and the nucleus just becomes uncomfortable. Unstable nuclei will decay toward more stable products. The mode of decay depends on the n/p ratio. • See Chapt. 2 for more detail on nuclear decay. • For elements up to Z=20 (Calcium), the magic n/p ratio is 1:1. Ex: Stable carbon (12C) has n=6, p=6. • For elements heavier than Z=20, the magic n/p ratio is >1:1. Ex: Stable gold (Au-197) has n=118, p=79 Stable atoms: # neutrons ≈ # protons (A ≈ 2Z) when Z ≤ 20 # neutrons > # protons when Z ˃ 20 Unstable atoms (radionuclides, radioactive atoms): Generally # protons > # neutrons Likely to undergo radioactive decay Gives off energy and results in a more stable nucleus
• #10: Electron Orbits (Energy Levels) Each electron fits into energy levels in an orderly fashion with a particular address. Principle quantum number (n)=1, 2, 3, etc. or K, L, M, N, etc. Orbital quantum number (l) – can have (n -1) values. Named s, p, d, f for sphere, peanut, dumbbell, fan Ex: if n=3, then there are l orbitals 0,1,2 • Magnetic quantum number (ml) – can have 2 l + 1 values. Numbered negative (n-1) through positive (n-1). Ex: n=3, l=2, therefore ml can be -2,-1,0,+1,+2 • Spin quantum number – for our purposes, either +1/2 or -1/2. • Outer (valence) shell can have up to eight electrons. These are generally s2 and p6.
• #11: valence electrons, which are responsible for chemical reactions and bonds between atoms as well as the emission of optical radiation spectra, normally occupy the outer shells If energy is imparted to one of these valence electrons to raise it to a higher energy (higher potential energy but lower binding energy) orbit, this will create a state of atomic instability The electron will fall back to its normal position with the emission of energy in the form of optical radiation. The energy of the emitted radiation will be equal to the energy difference of the orbits between which the transition took place
• #12: Whenever an electron absorbs energy, it becomes uncomfortable. The electron may move to a higher shell, or it may be ejected from the atom. If it absorbed energy less than his binding energy, it absorbs the entire amount and therefore is knocked into a higher orbital level, the electron will fall back to its normal position with the emission of energy in the form of optical radiation If it absorbed energy higher than his binding energy, it might have been knocked completely out of the atom (ionization). If an electron moves to a lower energy shell, excess energy may be carried away as the electron’s kinetic energy, or it may be emitted as a photon
• #13: Some how, electron is kicked out of the inner orbital and completely disappeared (many things can cause this) electron in a higher energy orbital level move closer to the nucleus which had a lower energy level (higher binding energy) Since it moved to a lower energy level, it had to give up some of that energy There are two ways that an electron can give off energy when it drops to a lower energy level. It can emit a characteristic x-ray This is known as “characteristic=fluorescent radiation)” because the energy levels are unique to a given nuclide and orbital. it can transfer that energy to the entire orbital which makes everyone in the orbital angry until they actually kick out another electron (called an Auger electron). The energy of the Auger electron is equal to the energy transferred, minus the binding energy that had to be overcome in order to eject an electron