2. proton mass, mp neutron mass, mn electron mass, me
= x kg = u
neutron mass, mn
= x kg = u
electron mass, me
= x kg
= u
1 u = x kg
1 u = MeV

3. The mass of a nucleus is always less than the sum of the uncombined
masses of the constituent particles.
The difference is called the
nuclear mass defect.
The mass defect can be converted into
an equivalent nuclear binding energy
using the relationship 1 u = 931 MeV.
Divide the binding energy by
the number of nucleons to get
the binding energy per nucleon.
This value is used to determine the
stability of that atom’s nucleus.

4. Nuclear Binding Energy Web Links
Wikipedia: link
Tutorial: link
Mass Defect: link, link
Get atomic isotope masses at link

5. RADIOACTIVITY the spontaneous uncontrollable decay
of an unstable atomic nucleus with
the emission of particles and/or rays
An unstable atomic nucleus will
decay naturally (emit particles
and/or rays from the nucleus)
until it becomes stable.
The danger of radioactive decay
products depends on their
charge and energy.

6. Alpha Decay alpha particle: a “doubly
ionized helium atom” or simply a “helium nucleus”
written a, a ,or He 4 2 4 2 +2
Z > 82 for alpha decay possible
have relatively slow speeds (0.1 c)
can be stopped by a few cm of air or
an ordinary sheet of paper
natural a’s have energy between 4 and 10 MeV
half-lives from 10-6 s to 1010 yr
(link)

7. or Beta Decay electron, written e or b or positron, written e or b
(link) -1 -1
electron, written e or b
or positron, written e or b +1 +1
occurs primarily in light nuclei
penetrates many meters of air
or thin sheets of metal
high speed (approach speed of light) or

8. Gamma Decay (electromagnetic radiation) written g short lifetimes
high energy photons
(electromagnetic radiation)
written g
penetrates 2 km air or 30 cm lead
short lifetimes
energy range of keV to MeV
have short wavelength (high frequency)
(link)

9. Proton Decay Neutron Decay When balancing nuclear equations, mass
number and nuclear charge must be
conserved. These equations are generally
simpler to write than chemical equations.

10. The rate of radioactive decay depends on the amount of nuclei present.
The equation for the number of
radioactive nuclei present at any
time t is given by the equation:

11. N(t) = # radioactive nuclei
present at time t
N0 = number initially present
= the disintegration constant,
which is equal to (ln 2)/T1/2 ,
where T1/2 is the half-life
of the decaying nucleus

12. Click here, here,
and here to run
simulations of
radioactive decay.

13. Often, the product of a decaying nucleus is also unstable and subsequently decays at some other rate.
The amount of each nucleus present depends on the amount of initial nuclei present and on the decay rates of the parent and daughter nuclei.
View a typical decay chain here.