Isobars and the uses of isotopes

Isobars

Calcium has $20$ protons and $20$ neutrons, so $A = 40$. Argon has $18$ protons and $22$ neutrons, so $A = 40$ also. They are different elements, but they have the same mass number.

Isobars are atoms of different elements that have the same mass number but different atomic numbers.

Isobars have different numbers of electrons, so their chemical properties are different.

The word "isobar" comes from Greek: "iso" means "same" and "baros" means "weight". Isobars have the same weight (mass number), but they are not the same element. Look at the three atoms above: they have different numbers of protons and neutrons, but each adds up to $40$.

Another pair is carbon-14 and nitrogen-14: carbon-14 has $6$ protons and $8$ neutrons, and nitrogen-14 has $7$ protons and $7$ neutrons. The sum is $14$ in both.

A way to remember: isoTOPes have the same TOPic (the same place, the same element). isoBARs have the same BAR, the same mass.

Radioactive isotopes

Some isotopes have unstable nuclei. They are radioactive: the nucleus changes by itself, and gives out rays. The time in which half of the atoms change is called the half-life. It can be a fraction of a second or billions of years.

Radioactive isotopes are very useful, but the rays can harm living things. So they are used only under strict safety rules, by trained people, with proper shielding.

The uses of isotopes

• A fuel in nuclear reactors. The isotope uranium-235 is used as a fuel. It gives a large amount of energy. A small amount of uranium gives as much energy as a very large amount of coal.
• Treating cancer. The rays of cobalt-60 can destroy cancer cells.
• Treating goitre. Iodine-131 is used for problems of the thyroid gland, which is in the neck. The thyroid takes up iodine, so the isotope goes just where it is needed.
• Finding the age of old things. The amount of carbon-14 in a bone or a piece of wood tells us how long ago the plant or animal lived. Carbon-14 has a half-life of about $5700$ years, so it can date things up to about fifty thousand years old.
• Smoke detectors. A tiny amount of americium-241 makes the air in the detector able to carry a small current. Smoke interrupts the current, and the alarm sounds.
• Tracers. A little of a radioactive isotope can be added to a pipe, a plant or the body. A detector then follows it, to find a leak or to see where something goes.

How does the carbon-14 method work? Living things take in carbon-14 from the air all through their life, so they have a steady small amount. When they die, they stop taking it in, and the carbon-14 slowly decays. After $5700$ years, half is left. After $11\,400$ years, a quarter is left. By measuring what is left, we find the age.

You should find that the carbon-14 halves in each 5700 years, so the curve falls quickly at first and then more and more slowly.

Practise with new numbers

Try it in Python

The program follows a dose of iodine-131 in a patient, halving every $8$ days.

Common mistakes

• Mixing up isotopes and isobars. Isotopes: same $Z$. Isobars: same $A$.
• Thinking that all isotopes are radioactive. Most are stable. Carbon-12 and chlorine-35 are not radioactive.
• Thinking that isobars have the same chemical properties. They are different elements.
• Thinking that radioactivity is only dangerous. It also has many uses, when it is controlled.

Remember these

• Isobars have the same mass number and different atomic numbers. Example: calcium-40 and argon-40.
• Isotopes have the same atomic number and different mass numbers.
• Isotopes are used as nuclear fuel, in treating cancer and goitre, in smoke detectors, and in finding the age of old objects.
• Radioactive isotopes must be handled under strict safety rules.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Are ${}^{14}_{6}\text{C}$ and ${}^{14}_{7}\text{N}$ isotopes or isobars?

2. Which isotope is used to treat goitre?

3. Why are isobars chemically different?

4. Why can carbon-14 not tell us the age of a rock that is a million years old?