These radioactive isotopes are often dangerous to biological systems. The electrons of an atom are found in orbits around the nucleus. The inner most orbit can contain only two electrons, the next two orbit can have up to eight electrons. An ion is an atom which has gained or lost electrons, thereby producing a negative or positive charge, respectively. Both of these are based on an atom's preference to have its outermost electron orbitals contain a complete set.
Covalent bonds are stronger that ionic bonds.