proton massproton mass, , mmpp

= 1.672 6231 x 10= 1.672 6231 x 10-27 -27 kg = 1.007 2765 ukg = 1.007 2765 u

neutron massneutron mass, , mmnn

= 1.674 9286 x 10= 1.674 9286 x 10-27 -27 kg = 1.008 6649 ukg = 1.008 6649 u

electron masselectron mass, , mmee

= 9.109 3897 x 10= 9.109 3897 x 10-31 -31 kgkg

= 0.000 548 579 90 u= 0.000 548 579 90 u

1 u = 1.660 5402 x 101 u = 1.660 5402 x 10-27-27 kg kg

1 u = 931.494 32 MeV1 u = 931.494 32 MeV

The mass of a nucleus is always lessThe mass of a nucleus is always lessthan the sum of the uncombinedthan the sum of the uncombined

masses of the constituent particles.masses of the constituent particles.The difference is called theThe difference is called the

nuclear mass defectnuclear mass defect..The mass defect can be converted intoThe mass defect can be converted into

an equivalent an equivalent nuclear binding energynuclear binding energyusing the relationship using the relationship 1 u = 931 MeV1 u = 931 MeV..

Divide the binding energy byDivide the binding energy bythe number of nucleons to getthe number of nucleons to getthe the binding energy per nucleonbinding energy per nucleon..

This value is used to determine theThis value is used to determine thestability of that atom’s nucleus.stability of that atom’s nucleus.

RADIOACTIVITYRADIOACTIVITYthe spontaneous uncontrollable decaythe spontaneous uncontrollable decay

of an unstable atomic nucleus withof an unstable atomic nucleus withthe emission of particles and/or raysthe emission of particles and/or rays

An unstable atomic nucleus willAn unstable atomic nucleus willdecay naturally (emit particlesdecay naturally (emit particlesand/or rays from the nucleus)and/or rays from the nucleus)

until it becomes stable.until it becomes stable.

The danger of radioactive decayThe danger of radioactive decayproducts depends on theirproducts depends on their

charge and energy.charge and energy.

Alpha DecayAlpha Decay• alpha particlealpha particle: a “doubly: a “doubly ionized helium atom” or simply a “helium nucleus”ionized helium atom” or simply a “helium nucleus”• written written ,,,or ,or HeHe 44

224422

+2+2

• Z > 82Z > 82 for alpha decay possible for alpha decay possible• have relatively slow speeds (0.1 c)have relatively slow speeds (0.1 c)• can be stopped by a can be stopped by a few cm of airfew cm of air or or

an an ordinary sheet of paperordinary sheet of paper• natural natural ’s have energy between 4 and 10 MeV’s have energy between 4 and 10 MeV• half-lives from 10half-lives from 10-6-6 s to 10 s to 101010 yr yr

ZA

ZAP D 24

24 (link)

Beta DecayBeta DecayBeta DecayBeta Decay• electronelectron, written , written ee or or

or or positronpositron, written , written ee or or

00-1-1

00-1-1

00+1+1

00+1+1

• occurs primarily in light nucleioccurs primarily in light nuclei• penetrates penetrates many meters of airmany meters of air or or thin sheets of metalthin sheets of metal• high speed (approach speed of light)high speed (approach speed of light)

ZA

ZAP D 1 1

0oror Z

AZAP D 1 1

0

(link)

Gamma DecayGamma DecayGamma DecayGamma Decay• high energy photonshigh energy photons• (electromagnetic radiation)(electromagnetic radiation)• written written • penetrates penetrates 2 km air2 km air or or 30 cm lead30 cm lead• short lifetimesshort lifetimes• energy range of keV to MeVenergy range of keV to MeV• have short wavelength (have short wavelength (high frequencyhigh frequency))

Z ZAX XA * (link)(link)

Proton DecayProton DecayProton DecayProton Decay

ZA

ZAP D p 11

11

Neutron DecayNeutron DecayNeutron DecayNeutron Decay

nDP AZ

AZ

10

1

When balancing nuclear equations, When balancing nuclear equations, massmassnumbernumber and and nuclear chargenuclear charge must be must be

conserved. These equations are generallyconserved. These equations are generallysimpler to write than chemical equations.simpler to write than chemical equations.

The rate of radioactive decay dependsThe rate of radioactive decay dependson the amount of nuclei present.on the amount of nuclei present.

The equation for the number ofThe equation for the number ofradioactive nuclei present at anyradioactive nuclei present at anytime t is given by the equation:time t is given by the equation:

teNtN 0)(N(t)N(t) = # radioactive nuclei = # radioactive nuclei present at time tpresent at time t

NN00 = number initially present = number initially present

= the disintegration constant,= the disintegration constant, which is equal to which is equal to (ln 2)/T(ln 2)/T1/21/2 ,, where where TT1/21/2 is the is the half-life of the decaying nucleusof the decaying nucleus

Click Click here, and , and here,to runhere,to runsimulations ofsimulations ofradioactive radioactive decay.decay.

Often, the product of a Often, the product of a decaying nucleus is also decaying nucleus is also unstable and unstable and subsequently decays at subsequently decays at some other rate.some other rate.

The amount of each The amount of each nucleus present depends nucleus present depends on the amount of initial on the amount of initial nuclei present and on the nuclei present and on the decay rates of the parent decay rates of the parent and daughter nuclei.and daughter nuclei.