isotopes

Isotopes are atoms with the same number of protons, but differing numbers of neutrons. In other words, the isotopes have different atomic weights. Isotopes are different forms of a single element.

There are 275 isotopes of the 81 stable elements. There are over 800 radioactive isotopes, some of which are natural and some synthetic. Every element on the periodic table has multiple isotopes forms. The chemical properties of isotopes of a single element tend to be nearly identical. The exception would be the isotopes of hydrogen since the number of neutrons has such a significant effect on the size of the hydrogen nucleus. The physical properties of isotopes are different from each other since these properties often depend on mass.

With the exception of hydrogen, the most abundant isotopes of the natural elements have the same number of protons and neutrons. The most abundant form of hydrogen is protium, which has one proton and no neutrons.

The mass number of an isotope is the total number of protons and neutrons in an atomic nucleus.

Isotope Notation

There are a couple of common ways to indicate isotopes:

1. List the mass number of an element after its name or element symbol. For example, an isotope with 6 protons and 6 neutrons is Carbon-12 or C-12. An isotope with 6 protons and 7 neutrons is Carbon-13 or C-16. Note the mass number of two isotopes may be the same, even though they are different elements. For example, you could have carbon-14 and nitrogen-14.

2. The mass number may be given in the upper left side of an element symbol. For example, the isotopes of hydrogen may be written ¹H, ²H, and ³H.

Isotope Examples

Carbon 12 and Carbon 14 are isotopes of carbon, one with 6 neutrons and one with 8 neutrons (both with 6 protons). Carbon-12 is a stable isotope, while Carbon-14 is a radioactive isotope.

Uranium-235 and uranium-238 occur naturally in the Earth’s crust. Both have long half-lives. Uranium-234 forms as a decay product.

Hydrogen has three isotopes. 

• ¹H is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the formal name protium.
• ²H is the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleus.
• ³H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through β-decay with a half-life of 12.32 years.

Key Takeaways: Isotopes

• Isotopes are samples of an element with different numbers of neutrons in their atoms.
• The number of protons for different isotopes of an element does not change.
• Not all isotopes are radioactive. Stable isotopes either never decay or else decay very slowly. Radioactive isotopes undergo decay.
• When an isotope decays, the starting material is the parent isotope. The resulting material is the daughter isotope.

Parent and daughter isotopes

When radioisotopes undergo radioactive decay, the initial isotope may be different from the resulting isotope. The initial isotope is called the parent isotope, while the atoms produced by the reaction are called daughter isotopes. More than one type of daughter isotope may result.

As an example, when U-238 decays into Th-234, the uranium atom is the parent isotopes, while the thorium atom is the daughter isotope.

Stable isotopes

Stable isotopes have a stable proton-neutron combination and do not display any sign of decay. This stability comes from the number of neutrons present in an atom. If an atom has too many or too few neutrons, it is unstable and tends to disintegrate. Since stable isotopes do not decay, they do not produce radiation or its associated health risks.

Radioactive isotopes

Radioactive isotopes have an unstable combination of protons and neutrons. These isotopes decay, emitting radiation that includes alpha, beta and gamma rays. Scientists classify radioactive isotopes according to their creation process: long-lived, cosmogenic, anthropogenic and radiogenic.

• Long-lived radioactive isotopes emerged during the creation of the solar system.
• Cosmogenic radioactive isotopes occur as a reaction of the atmospheric to cosmic rays emitted by stars.
• Anthropogenic isotopes come from human-made nuclear activities such as weapons testing and nuclear fuel production.
• Radiogenic isotopes are the end result of radioactive decay.

Interesting Facts about Isotopes

• Many elements only exist in an unstable or radioactive form.
• All non-natural or man-made elements are radioactive isotopes.
• Heavier isotopes tend to react more slowly than lighter isotopes of the same element.
• Deuterium (the hydrogen isotope with one neutron) can form water with oxygen. This is called "heavy water" as deuterium has twice the mass of normal hydrogen (protium).
• Twenty-six elements only have one stable isotope. These elements are called 'monoisotopic'.