Predicting a Molecule's Shape
Molecules with a central atom form predictable shapes that depend on the number of electron pairs residing in the outer shell of the central atom. The repulsive force between these outer electron pairs causes them to take positions as far from each other as possible. The position of the electron pairs in the outer shell then determines the angles at which the central atom bonds with the surrounding atoms in the molecule.
Another common type of bond, the covalent bond, results when two atoms share one or more pairs of electrons in an attempt to fill their outer shells and become more energetically stable. The atoms are held together by the mutual electrostatic attraction between the protons in their nuclei and these electrons. The bonded atoms form a stable unit called a molecule.
For example, because a chlorine atom is one electron short of completing its outer shell (and attaining a noble-gas configuration), two chlorine atoms combine to form a chlorine molecule by sharing two electrons. The atoms thereby complete each other’s outer shell: Cl + Cl → Cl2. Electron sharing distinguishes a covalent bond from an ionic bond (in which the electrostatic attraction of the ions results in bonding). In an ionic compound there are no molecules—only charged particles composing an extensive three-dimensional array.
Covalent bonds tend to form when the bonded atoms have nearly the same attraction for electrons; ionic bonds form when the electron-attracting power of the atoms differs markedly. If the valence electrons are represented by dots, the difference between bond types becomes more obvious:
