Chapter 4 · Atoms and Elements
4.8Isotopes: When the Number of Neutrons Varies
10 min · two checks
Predict
Carbon-12 and carbon-14 are both carbon. What is different?
The idea
- Define isotope and write isotope symbols.
- Explain why chemical behavior stays nearly the same.
Isotopes are atoms of the same element with different mass numbers. The symbol can be carbon-14, C-14, or ¹⁴C. Chemistry is mostly an electron-and-proton story, so isotopes of one element react almost alike. The mass difference matters in a few precise places: rates of some bonds, and physical methods that separate by mass. It matters enormously in the nucleus: carbon-14 is radioactive and carbon-12 is not. Nuclear stability is a later chapter; the definition you need now is A differs, Z does not.
Natural samples are mixtures of isotopes. Chlorine’s atomic mass is about 35.45 because nature delivers mostly chlorine-35 and chlorine-37, not because any chlorine atom has a fractional proton. You cannot read the isotope mix from one mass number. You can read neutrons for a stated isotope: chlorine-37 has 17 protons and 20 neutrons.
Keep these
- Isotopes: same Z, different number of neutrons.
- Chemical properties nearly match; nuclear properties may not.
- A fractional atomic mass on the table is an average, not a single atom.
Worked path
Magnesium has isotopes Mg-24, Mg-25, and Mg-26. How many neutrons does each have? Z = 12.
Observe
All three have 12 protons.