Nuclear Chemistry Brown, LeMay Ch 21 AP Chemistry

2 21.1: Radioactivity Result of unstable nuclei Nucleons: particles in the nucleus, n 0 and p + Radioisotopes: atoms that containing radioactive nuclei (or radionuclides) Nuclear reactions or equations: express products of radioactive decay, fusion, or fission Radioactive decay: process in which a radionuclide spontaneously decomposes

3 Most common types of radioactive decay TypeSymbolDescription Travels in air… Ex: alpha  or energized He nucleus He A few cm; cannot penetrate human skin U He 4 2 Th

4 Alpha decay

5 Ex: Same as: beta  or High energy electron e 0 ~300 cm; can penetrate skin, but rarely I Xe e 0 + n 1 0 p 1 1 e 0 + A neutron converts to a proton and electron

6 Ex: gamma  or photon  0 0 Very far; can be stopped by ~5 cm of Pb Pu  0 0 Pu * Represents energy emitted (i.e., radiation) when nucleons in an unstable radionuclide reorganize to become more stable Usually not written in a nuclear reaction.

7 Ex: Same as: positron Antimatter (positively charged) e; collides with e- and both are annihilated as gamma rays are created e 0 1 C 11 6 B 5 e p 1 1 n 1 0 e A proton converts to a positron and neutron

8

9 Ex: Same as: Electron capture Capture of inner shell e- by nucleus Rb Kr p 1 1 n 1 0 A proton and electron convert to a neutron e 0 e 0 e 0 +

: Nuclear stability Strong nuclear force: pulls nucleons together to form nuclei (actually acts on quarks) * Weak nuclear force: responsible for changes in flavor of quarks Nuclei become unstable (radioactive) if the neutron-to-proton ratio “strays” too far from “normal range” * Nuclear shell model: when p and n fill nuclear shells, atoms are unusually stable: “Magic numbers” 2, 8, 20, 28, 50, 82, 126

11 A radionuclide will decay until a stable ratio exists: If too many n, n will be converted to p by  emission. If too few n, p will be converted to n by positron emission or electron capture. Nuclei with p ≥ 84 undergo  emission 1

: Rates of Decay Half-life (t ½ ): Time for ½ a radioactive (i.e., having an unstable p/n ratio) material to decay (form 2 or more stable atoms)

: Mass-energy relationships  E =  m c 2 (mass in kg) Mass → energy  Mass lost during radioactive decay is released as energy Energy → mass  Mass defect (  m): mass difference between a nucleus and its constituent nucleons; the nuclear bonding energy must be added to a nucleus to break it into its nucleons  When energy is added, the nucleons separate and gain mass

& 21.8: Fission & Fusion Fission: splitting of a nucleus; some mass is lost, which results in release of energy (ex: nuclear power plants, “atomic” bombs) Ba Kr n + energy 1 0 U n + 1 0

15 Fusion: combination of 2 nuclei; some mass is lost, which results in release of energy (ex: stars, “H” bombs) H H 2 1 He n energy

16 * Fundamental Particles and the Standard Model of the Atom 6 flavors of quarks: Inc Mass ↓ Q = -1/3Q = 2/3 Down (d)Up (u) Strange (s) Charm (c): discovered 1974 at 1.5 GeV Bottom (b): discovered 1978 Top (t)

17 * Ordinary matter is made of: p + : u-u-d quark triplet n 0 : u-d-d quark triplet e-: one of 6 leptons  decay: d quark in a n changes into u quark, making a p