Nuclear Chemistry

Ch.18

(18-1) Nuclear Stability

• Nucleons: p+ & n0

• Nuclide: any combo of p+ & n0 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 n0 # v. p+ # in which all stable nuclei lie

Nuclei are more stable if they…

• Contain n0 ≥ p+

• Do not have too many or too few n0

• Have even #’s of nucleons• Have “magic #’s” of p+ or n0

– 2, 8, 20, 28, 50, 82, 126• Are not a neighboring isobar

(18-2) Types of Nuclear Change

• Spontaneous1. Radioactivity

– Artificial Transmutations2. Fission3. Fusion

• Nonspontaneous4. Transmutation

Radioactivity

• A nucleus decays & emits particles & electromagnetic waves

Converting n0 to p+

• Beta (β) decay: nuclei w/ too many n0 for the at.# become more stable by decaying & emitting radiation

• β particle: e- emitted from a nucleus when a n0 changes to a p+

• Ex:

Converting p+ to n0

• e- capture: nucleus has too few n0, so it absorbs an e-, which changes a p+ into a n0

• Gamma rays (γ): E produced by decaying nuclei

• Ex:

Converting p+ to n0 (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 n0 can decay by emitting α particles

• α particles: Helium-4 made when a n0 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