Nuclear Chemistry Ch.18

(18-1) Nuclear Stability Nucleons: p + & n 0 Nuclide: any combo of p + & n 0 in a nucleus – Isotope: same at.#, but different mass Te-122, Te-124, Te-128 – Isobar: same mass, but different at.# Xe-124, Te-124, Sn-124

Binding Forces Strong nuclear force: attraction that holds nuclear particles together – Overcomes repulsive forces

Mass Defect Mass converted to E when a nucleus forms

Nuclear Binding E E emitted when nucleons come together (E needed to break a nucleus apart)

Band of Stability Area on a graph of n 0 # v. p + # in which all stable nuclei lie

Nuclei are more stable if they… Contain n 0 ≥ p + Do not have too many or too few n 0 Have even #’s of nucleons Have “magic #’s” of p + or n 0 – 2, 8, 20, 28, 50, 82, 126 Are not a neighboring isobar

(18-2) Types of Nuclear Change Spontaneous 1.Radioactivity – Artificial Transmutations 2. Fission 3. Fusion Nonspontaneous 4. Transmutation

Radioactivity A nucleus decays & emits particles & electromagnetic waves

Converting n 0 to p + Beta (β) decay: nuclei w/ too many n 0 for the at.# become more stable by decaying & emitting radiation β particle: e - emitted from a nucleus when a n 0 changes to a p + Ex:

Converting p + to n 0 e - capture: nucleus has too few n 0, so it absorbs an e -, which changes a p + into a n 0 Gamma rays (γ): E produced by decaying nuclei Ex:

Converting p + to n 0 (cont.) Positron emission: nuclei emit postitrons (antiparticles of e - ) Ex:

Annihilation of Matter Event when a particle collides w/ its anitparticle & both are changed into E

Losing Alpha Particles Alpha (α) decay: very large nuclei w/ too few n 0 can decay by emitting α particles α particles: Helium-4 made when a n 0 decays Ex: Decay series: many heavy nuclei must decay several times before reaching a stable state

Balancing Nuclear Eq.’s Total mass #’s & nuclear charges must balance on both sides of the eq. Ex: Masses = 234, Charges = 90 Ex: Masses = 238, Charges = 92

Nuclear Fission Rxn in which a large nucleus splits into 2 & produces a lot of E Chain rxn: nuclear rxn that sustains itself Critical mass: smallest mass of radioactive material needed to sustain a chain rxn

Nuclear Fusion 2 small nuclei combine to form 1 more stable nucleus & lots of E Reactants are plasmas (mixture of + nuclei & e - ) Need very high T’s & P’s

Transmutation Creating new nuclei by bombarding a nucleus w/ α particles Produces an unstable cmpd that stabilizes by emitting a p +

(18-3) How Nuclear Chem is Used Radioactive dating: using radioactive isotopes to determine an object’s age Half-life: time required for half of a radioactive material to decay

Smoke Detectors Have an α emitter, which attract e- from the gas, changing them to ions which conduct electricity When smoke particles mix w/ the gas, they reduce current flow & the detector’s circuits are set off

Neutron Analysis Used to determine composition of objects – Meteorite composition – Forensic science (gun residue)

Radiation Exposure rem: biological effect of exposure to nuclear radiation – Limit of 5 rems/yr