Isotopes and Isobars: Quick Revision

A brief note on Isotopes and Isobars. Read it before a test. Try each question at the end before you open its answer.

Isotopes and isobars

The average atomic mass

The average lies between the mass numbers, nearer to the more common isotope. It is rarely a whole number.

Uses of radioactive isotopes

• Uranium-235: fuel in nuclear reactors.
• Cobalt-60: treating cancer. Iodine-131: treating goitre.
• Carbon-14: finding the age of old wood and bone; its half-life is about $5700$ years.
• Americium-241: smoke detectors.

A worked example

Antimony is $57\ \%$ antimony-121 and $43\ \%$ antimony-123. Find its average atomic mass.

1. Weight each one. $121 \times 57 = 6897$ and $123 \times 43 = 5289$.
2. Add and divide. $\dfrac{6897 + 5289}{100} = 121.86\ \text{u}$.
3. Check. It is nearer to $121\ \text{u}$, the more common isotope.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Mixing up isotopes and isobars. Isotopes share $Z$; isobars share $A$.
• Taking a plain average instead of weighting by the percentages.
• Thinking that every isotope is radioactive. Most are stable.
• Thinking that a radioactive sample is gone after two half-lives. Each half-life only halves what is left.

Check yourself

1. Are ${}^{3}_{1}\text{H}$ and ${}^{3}_{2}\text{He}$ isotopes or isobars?

2. How many neutrons are there in uranium-235? Uranium has $Z = 92$.

3. An element is $80\ \%$ of the isotope with the mass number $24$ and $20\ \%$ of the isotope with the mass number $26$. What is its average atomic mass?

4. A sample holds $800$ atoms of a radioactive isotope. How many are left after three half-lives?