Caption: A cartoon of radioactive isotope (AKA radionuclide) carbon-14 (C-14, radiocarbon, 6 p, 8 n, half-life =5700(30) y, t_e=8220 y) undergoing radioactive decay.

Features:

1. C-14 undergoes spontaneously radioactive decay (due to weak nuclear force) to nitrogen-14 (N-14, 7 p, 8, n, stable, terrestrial abundance 99.636 % out all nitrogen) by beta decay:
   
   
   
   
   
   
   
   

     C-14 → N-14 + e**- + ν**(bar) + 0.156476 MeV  ,

   where the last term is the decay energy in mega-electron-volts (MeV), e**- is negative charge beta particle (just a high kinetic energy electron) and ν**(bar) electron antineutrino. The ejected electron is high kinetic energy which makes it dangerous ionizing radiation.

2. C-14 is created naturally in the Earth's atmosphere by the (n-p) reaction (a nuclear reaction):

     N-14 + n → C-14 + p 

   where n stands for neutron and p for proton. The neutrons in the Earth's atmosphere are mainly produced by high kinetic energy cosmic rays (which come from space). The ejected proton is just a hydrogen ion (H**(+).

3. The (approximate) equilibrium C-14 abundance in the Earth's atmosphere is ∼ 1/10**12 out all carbon (C) atoms. The equilibrium abundance is maintained by cosmic ray creation and spontaneously radioactive decay destruction. However, the abundance does vary at bit over geologic time.

4. C-14 is from a nuclear physics perspective very different from carbon-12 (C-12, 6 p, 6 n, stable terrestrial abundance 98.93 % out all carbon), but from a chemistry perspective the 2 carbon isotopes are almost identical.

   Thus, plants (which get nearly all their carbon from the Earth's atmosphere) have the atmospheric abundance of C-14, and so do all the animals which derive their carbon at some remove from the plants.

5. However, when biota die, their C-14 radioactively decays away. The ratio of C-14 to carbon-12 in dead organic matter allows radioactive dating to infer the time since death.

6. There are uncertainties in radioactive dating by C-14 abundance, one of which is the variation in the atmospheric abundance of C-14 over geologic time. Also radioactive dating can only been done to ∼ 60,000 before present (PB) since too little C-14 remains from before then. (see Wikipedia: Carbon-14: Radiocarbon dating).

7. Longer lived radioactive isotopes (e.g., uranium-238 (U-238, half-life 4.468 Gyr, t_e=6446 Gyr)) can be used for the radioactive dating of rock, but NOT organic matter.