Isotope
Definition and meaning of Isotope in chemistry.
Isotopes are atoms of the same element that share the same number of protons but have different numbers of neutrons. The proton count sets an element's atomic number, so isotopes of an element share the same square on the periodic table. Their chemical behavior is nearly identical, while their mass and nuclear stability can differ.
In more detail
An atom's identity as an element depends only on its number of protons. Adding or removing neutrons changes the atom's mass but not which element it is. Chemical bonding depends mainly on the number and arrangement of electrons.
Isotopes of an element have the same electron count, so they react the same way in chemical reactions. Most elements found in nature exist as a mixture of two or more isotopes. The atomic mass listed on the periodic table is not the mass of any single atom.
It is a weighted average based on the natural abundance of each isotope. For example, chlorine's listed atomic mass of 35.45 reflects a mix of about 76% chlorine-35 and 24% chlorine-37. Some isotopes are stable and last essentially forever.
Others are radioactive, meaning their nuclei break down over time and release radiation. Radioactive isotopes have important uses. Carbon-14 lets scientists date ancient organic material, since it decays at a known, steady rate.
Other radioactive isotopes are used in medical imaging and cancer treatment. A common misconception is that isotopes of an element have different chemical properties. They do not. Only mass-dependent properties differ, along with how the isotope's nucleus behaves during radioactive decay.
Key facts
| Same | Number of protons (atomic number) |
|---|---|
| Different | Number of neutrons (mass number) |
| Chemical properties | Essentially identical |
| Atomic mass on table | Weighted average of natural isotopes |
| Radioactive example | Carbon-14, used in dating |
| Field | General Chemistry |
Carbon-12 and carbon-14 are both isotopes of carbon, since each has 6 protons. Carbon-12 has 6 neutrons and is stable. Carbon-14 has 8 neutrons and slowly decays, which is why scientists use it in radiocarbon dating.
Frequently asked questions
Do isotopes have the same chemical properties?
Yes. Isotopes of an element have the same number of protons and electrons, so they react the same way in chemical reactions. They mainly differ in mass and nuclear stability.
What is an example of an isotope?
Carbon has three natural isotopes: carbon-12, carbon-13, and carbon-14. All have 6 protons, but they have 6, 7, and 8 neutrons.
Why does the periodic table list decimal atomic masses?
The listed mass is a weighted average of all naturally occurring isotopes of that element. It is not the exact mass of any single atom.