The radioactive carbon has six protons and eight neutrons in its nucleus, giving it a total atomic mass of 14.
This atom is not stable, and will break down, releasing nuclear energy in the process.
Radioactive carbon (Carbon 14) is formed in the upper atmosphere as a byproduct of cosmic radiation.
Cosmic rays are positively charged atoms moving at enormous speeds.
When they strike ordinary atoms in the upper atmosphere, the cosmic rays smash them apart. Some of these neutrons then collide with nitrogen atoms.