Unstable: definition in chemistry
Unstable: Describes a substance, nucleus or particle that tends to change, decompose, react or decay rather than stay as it is.
An unstable compound breaks down easily, sometimes slowly and sometimes explosively, because its products have lower energy. An unstable nucleus has an unfavorable balance of protons and neutrons, so it gives off radiation as it decays into a more stable nucleus. Stability is always relative: diamond is less stable than graphite at room conditions, yet the change is so slow that diamonds never turn into graphite in practice.
Examples
- Carbon-14 — an unstable isotope that decays with a half-life of about 5,730 years
- Hydrogen peroxide (H₂O₂) slowly decomposes into water and oxygen
- Carbonic acid (H₂CO₃) in soda breaks down into water and carbon dioxide
- Nitroglycerin can explode when it is jolted
On the periodic table
Every element after lead (atomic number 82) has only unstable, radioactive isotopes, and so do technetium and promethium among the lighter elements.