go to the AUS-e-TUTE homepage
home test Join AUS-e-TUTE game contact
 

 

Isotope Stability

Key Concepts

  • An unstable isotope emits some kind of radiation, that is it is radioactive.

  • A stable isotope is one that does not emit radiation, or, if it does its half-life is too long to have been measured.

  • It is believed that the stability of the nucleus of an isotope is determined by the ratio of neutrons to protons.

  • Observations of the atomic number of isotopes show us that:
  1. Isotopes with atomic number (Z) > 82 are unstable

  2. Of the elements with atomic number (Z) < 82, all have one or more stable isotopes except technetium (Z = 43) and promethium (Z = 61) which do not have any stable isotopes.

  3. Isotopes with atomic number (Z) ≤ 20 and with a neutron (n) to proton (p) ratio of about 1 are more likely to be stable (n ÷ p ~ 1)

  • Observations on whether the nucleus contains odd or even numbers of protons and neutrons leads us to believe that a nucleus with:

  1. odd numbers of protons and odd numbers of neutrons is most likely to be unstable

  2. even number of protons and even numbers of neutrons is most liklely to be stable

Composition of the Nucleii of Known Stable Isotopes
Protons Neutrons % Stable Isotopes Stability Trend
odd odd 1.5%* least stable
odd even 18%
even odd 20.5%
even even 60% most stable

*Stable nucleii with an odd number of protons and an odd number of neutrons are hydrogen-2, lithium-6, boron-10 and nitrogen-14. Each of these has Z < 20 and a neutron:proton ratio of 1.

Predicting the Stability of an Isotope

Example 1
Uranium-235 and uranium-238 both occur naturally.
Which of these isotopes is most likely to be unstable?
  1. Use the Periodic Table to find the atomic number (Z) for uranium
    Z = 92

  2. Predict the stability of each isotope:
    Since the atomic number for uranium is greater than 82, both isotopes are predicted to be unstable.

Example 2
Carbon-12 and carbon-14 both occur naturally.
Which of these isotopes is most likely to be stable?

  1. Use the Periodic Table to find the atomic number, Z, for carbon:
    Z = 6

  2. Predict stability:
    Since Z < 20, the most stable isotope is the one whose ratio of neutrons (n) to protons (p) is close to 1 (n ÷ p ~ 1)
    Isotope Atomic Number
    Z
    (No. protons)
    Mass Number
    A
    (no. protons + neutrons)
    No. neutrons
    (A - Z)
    n/p stability
    carbon-12 6 12 12-6=6 6/6=1 stable
    carbon-14 6 14 14-6=8 8/6=1.3 unstable

Example 3
Two isotopes of mercury are mercury-195 and mercury-196.
Which of these isotopes is most likely to be stable?

  1. Use the Periodic Table to find the atomic number, Z, for mercury (Hg):
    Z = 80

  2. Since Z for mercury is greater than 20 but less than 82, we will need to rely on a comparision of odd and even numbers of protons and neutrons in each nucleus:
    Isotope Atomic Number
    Z
    (No. protons)
    Mass Number
    A
    (no. protons + neutrons)
    No. neutrons
    (A - Z)
    odd:even stability
    mercury-195 80 195 195-80=115 protons:even
    neutrons:odd
    unstable
    mercury-196 80 196 196-80=116 protons:even
    neutrons:even
    stable


What would you like to do now?
advertise on the AUS-e-TUTE website and newsletters
 
 

Search this Site

You can search this site using a key term or a concept to find tutorials, tests, exams and learning activities (games).
 

Become an AUS-e-TUTE Member

 

AUS-e-TUTE's Blog

 

Subscribe to our Free Newsletter

Email email us to
subscribe to AUS-e-TUTE's free quarterly newsletter, AUS-e-NEWS.

AUS-e-NEWS quarterly newsletter

AUS-e-NEWS is emailed out in
December, March, June, and September.

 

Ask Chris, the Chemist, a Question

The quickest way to find the definition of a term is to ask Chris, the AUS-e-TUTE Chemist.

Chris can also send you to the relevant
AUS-e-TUTE tutorial topic page.

 
 
 

Share this Page

Bookmark and Share
 
 

© AUS-e-TUTE