What Is Formed When Atoms Share Electrons?

Orbitals and Valence Electrons

Atoms consist of a positively charged nucleus containing protons and neutrons surrounded by a cloud or series of orbitals of negatively charged electrons. Each successive orbital holds a different number of electrons at different energy levels. Covalent bonds are formed through the sharing of valence electrons, or the electrons in the outermost orbit.

Molecules are more stable when their valence shell is full. Covalent bonding that results in an electron configuration similar to that of the noble gases will be more stable and less reactive. One or more missing electrons make a compound unstable and more likely to react with other compounds in order to fill the shell.

When electrons are not shared equally between two atoms, but instead one of the atoms has a greater electronegativity and essentially pulls harder on the electrons, the bond is polar. It could also be said that such a bond is partially ionic, or has some ionic character. For example, oxygen has a higher electronegativity rating than hydrogen. The electronegativity of any atom can be found on certain periodic tables or in a textbook. The oxygen in water, or H2O, does covalently share electrons with the hydrogens, but the sharing is not totally equal -- the electrons spend more time on the oxygen side of the molecule, creating a local negative charge there. The hydrogens in turn have a local positive charge. This difference in charge is called a dipole moment.

Covalent bonds in which electrons are shared equally are called nonpolar covalent bonds because neither atom becomes more positively or negatively charged than the other. Truly nonpolar bonds occur only between atoms of equal electronegativity, which usually means two of the same atom. The covalent bond between the two oxygen atoms in O2, diatomic oxygen, is a nonpolar bond.