Isotopes

Atoms of the same element, with different masses

Hydrogen has three kinds of atom. All have one proton and one electron. But their masses are different.

These are called the isotopes of hydrogen.

Almost all the hydrogen in nature is protium ($99.98\ \%$). Deuterium is rare, about $1$ atom in $6500$, and tritium is extremely rare.

Isotopes

Isotopes are atoms of the same element that have the same atomic number but different mass numbers. They have the same number of protons and electrons, and a different number of neutrons.

The word comes from Greek: "iso" means "same" and "topos" means "place". They have the same place in the periodic table, because they are the same element.

We write an isotope by the name of the element and the mass number, such as chlorine-35 and chlorine-37, or carbon-12 and carbon-14.

More examples

• Carbon: carbon-12 has $6$ neutrons, and carbon-14 has $8$ neutrons. In nature, about $99\ \%$ of carbon is carbon-12.
• Chlorine: chlorine-35 has $18$ neutrons, and chlorine-37 has $20$.
• Uranium: uranium-235 has $143$ neutrons, and uranium-238 has $146$.
• Oxygen: oxygen-16 has $8$ neutrons and oxygen-18 has $10$. A third isotope of oxygen is very rare.

How to find the neutrons: neutrons $= A - Z$. For uranium-235, $Z = 92$, so $235 - 92 = 143$.

You should find that sliding the neutrons moves the red dot up its dashed line, changing the mass number but never the element.

Chemical and physical properties

The chemical properties of an element depend on the electrons, mainly the outer ones. Isotopes have the same number of electrons, so they have the same chemical properties. Both chlorine-35 and chlorine-37 join with sodium to make salt in the same way.

The physical properties that depend on mass, such as density, are slightly different. Heavy water, made of deuterium, is denser than ordinary water. Ordinary water has a density of $1.00\ \text{g/mL}$, and heavy water has about $1.11\ \text{g/mL}$. A cube of ice made from heavy water sinks in ordinary water.

Because isotopes of an element are chemically the same, we cannot separate them by a chemical reaction. They have to be separated by their slightly different masses, for example in a machine that spins them.

Practise with new numbers

Try it in Python

The program lists the protons, neutrons and electrons of some isotopes.

Common mistakes

• Thinking that isotopes are different elements. They are the same element, with the same atomic number.
• Changing the number of protons. All isotopes of an element have the same number of protons.
• Thinking that isotopes differ in the number of electrons. In a neutral atom, they have the same number.
• Mixing up the mass number and the atomic mass. Each isotope has a whole mass number. The atomic mass in the table is an average (the next lesson).

Remember these

• Isotopes have the same $Z$ and different $A$, so a different number of neutrons.
• They have the same chemical properties, because they have the same electrons.
• Hydrogen has three isotopes: protium, deuterium and tritium.
• Some physical properties, such as density, differ slightly.

Check yourself

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

1. How many neutrons are there in carbon-14?

2. Why do isotopes have the same chemical properties?

3. Are ${}^{35}_{17}\text{Cl}$ and ${}^{37}_{17}\text{Cl}$ isotopes? Why?

4. An ice cube made from heavy water sinks in ordinary water. Why?