Though similar to tendons, ligaments connect bone to bone and help to stabilize joints they surround. They are composed mostly of long, stringy collagen fibers that create bands of tough, fibrous connective tissue.
An electron is a negatively charged subatomic particle. It can be either free (not attached to any atom), or bound to the nucleus of an atom. Electrons in atoms exist in spherical shells of various radii, representing energy levels. The larger the spherical shell, the higher the energy contained in the electron.
Electron shell is a group of atomic orbital, so that their orbits formed electron shell with the same value of the principal quantum number. The 1, 2, 3, 4 electron shell can hold 2, 8, 18, 32 electrons respectively, since the electrons are electrically attracted to the nucleus.