What is an isotope?
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have different masses but, because their electrons are the same, nearly identical chemistry.
Watch it explained
What is an isotope?
Weighs carbon-12 against carbon-14 on a balance, shows that six protons make an atom carbon whatever else changes, adds neutrons to take the mass number from 12 to 14, shows the six electrons bonding the same way, and ends on carbon-12, -13 and -14 with 6, 7 and 8 neutrons.
Scenes, in order:
- Same element, different weight
- Protons decide the element
- Add neutrons, change mass
- Chemistry stays the same
- Isotope: same protons, different neutrons
Same element, different mass
The number of protons (the atomic number) decides which element an atom is: every carbon atom has 6. The mass number is protons plus neutrons, and that is what changes between isotopes.
- Carbon-12: 6 protons, 6 neutrons. Stable, and about 99% of natural carbon.
- Carbon-13: 6 protons, 7 neutrons. Stable.
- Carbon-14: 6 protons, 8 neutrons. Radioactive, with a half-life of about 5,730 years, which is what radiocarbon dating measures.
Since all three have 6 electrons in a neutral atom, they form the same bonds. That is why isotopes are useful as tracers: a cell or a reaction treats them the same, but an instrument can tell them apart by mass.
Worked example: average atomic mass
The atomic mass on the periodic table is a weighted average of an element's natural isotopes. Chlorine is about 75.8% chlorine-35 (mass 34.97 u) and 24.2% chlorine-37 (mass 36.97 u).
(0.758 × 34.97) + (0.242 × 36.97) = 26.507 + 8.947 = 35.45 u, which matches the periodic table. No single chlorine atom has a mass of 35.45 u; the number describes the mix.
Common mix-ups
- Isotope vs ion: isotopes differ in neutrons; ions differ in electrons and carry a charge.
- Mass number vs atomic mass: the mass number is a whole-number count of particles in one nucleus; atomic mass is a measured, usually non-integer value.
- Assuming all isotopes are radioactive. Many, like carbon-12 and carbon-13, are stable.
- Writing carbon-14 as a different element because its mass is closer to nitrogen's. The protons decide; it is carbon.
The film on this page
The film weighs carbon-12 against carbon-14 on a balance, shows that six protons keep the atom carbon, adds neutrons to raise the mass number, and ends on carbon-12, -13 and -14 side by side. For the parts of the atom it builds on, see what an atom is; for unstable isotopes at work, see how nuclear power works. More on the chemistry page.
Questions
- How are isotopes written?
- As the element name or symbol with the mass number: carbon-14, C-14, or the symbol with 14 as a superscript before it. Sometimes the atomic number is written as a subscript below the mass number.
- Do hydrogen's isotopes have their own names?
- Yes. Protium has no neutrons, deuterium has one and tritium has two. Tritium is radioactive; the other two are stable.