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Nuclear Reactions. The Nucleus. Nuclide: a unique atom represented by the symbol :. A. X. Z. Zone of Stability. Stability. Radioactive Decay: the probability that a nucleus will undergo decomposition to form a different nucleus.

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The nucleus
The Nucleus

Nuclide: a unique atom represented by the symbol:




Zone of Stability


Radioactive Decay: the probability that a nucleus will undergo decomposition to form a different nucleus

Isotopes: Atoms that have identical atomic numbers but different mass number values (neutrons have changed)

Important observations
Important Observations

  • All nuclides with 84 or more protons are unstable

  • Light nuclides (P to N ration =1) are stable

  • For stable heavy nuclides P to N ratio is >1

  • Special Stability: certain combinations confer special stability (even vs odd)

  • Magic Numbers: certain combinations confer especially stable nucleus 2,8,20,28,50,82,126 (parallels behavior for stable electrons for chemical stability)


The atomic mass number (A) and atomic number (Z) must remain constant across the reaction!

A: 238 = 234 +4

Z: 92 = 90 + 2

Types of decay
Types of Decay

Alpha Decay () : A very common mode of decay for heavy nuclides. (helium nucleus)

Types of decay1
Types of Decay

Beta Decay (-) : Most common, The mass number of the decaying remains constant, the net effect is the change of a neutron to a proton!

Types of decay2
Types of Decay

Positron Production (+) : Occurs for nuclides below the zone of stability, a particle with the same mass as an electron but opposite charge. The net effect is a change of a proton to a neutron.

Types of decay3
Types of Decay

Gamma ray () : Release of a high energy photon, frequently accompanies other reactions

Types of decay4
Types of Decay

Electron Capture : process in which one of the inner orbital electrons is captured by the nucleus

Interesting, and why do we not just turn mercury into gold?

Types of decay5
Types of Decay

Summary of Decay reactions

Decay series
Decay Series

Decay Series: a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.

Half life

  • Half-life (t½)

    • Time required for half the atoms of a radioactive nuclide to decay.

    • Shorter half-life = less stable.

Half life1

mf:final mass

mi:initial mass

n:# of half-lives

Mass defect
Mass Defect

When a system gains or loses energy it also gains or loses a quantity of mass.

E = mc2

m = mass defect

E = change in energy

If E =  (exothermic), mass is lost from the system.

Nuclear transformations
Nuclear Transformations

Nuclear Transformation: the change of one element to another.

F ission

  • splitting a nucleus into two or more smaller nuclei

  • 1 g of 235U = 3 tons of coal

F ission1

  • Chain reaction - self-propagating reaction

  • Critical mass - mass required to sustain a chain reaction

  • Subcritical: When the process dies out

  • Supercritical: rapid escalation of reaction

F ission2


  • combining of two nuclei to form one nucleus of larger mass

  • thermonuclear reaction – requires temp of 40,000,000 K to sustain

  • 1 g of fusion fuel = 20 tons of coal

  • occurs naturally in stars

Fission vs fusion

235U is limited

danger of meltdown

toxic waste

thermal pollution

fuel is abundant

no danger of meltdown

no toxic waste

not yet sustainable

Fission vs. Fusion