General ---------Biology ------------------Online------------------   ----wilcox@sunline.net Ch.  2 :  The Chemical Context of Life

The atoms in a molecule play tug-of-war with the bond electrons.  Most atoms have the same affinity (tug) for the electrons they share in a covalent bond.  For example, in the methane (CH₄) molecule below, the carbon and the four oxygen's all tug on the electrons with the same pull.  An atom's affinity or "tug" on electrons is called its "electronegativity."  Carbon and hydrogen have the same electronegativity; therefore, the resulting covalent bonds are called nonpolar.

Methane: showing orbitals and sub atomic particles.	Methane: CH4 showing atomic symbols.

Water, however, is different than methane, because the oxygen in water is much more electronegative than the hydrogen.  The result is a covalent bond where oxygen wins the tug-of-war over the electrons, leaving the oxygen with a partial negative charge and the hydrogen atoms with a partial positive charge.  A bond with a positive end and a negative end is called polar.

"Polar" Covalent Bonds in a Water Molecule

For another figure illustrating the nature of the polar covalent bond, click here.

• an atom's affinity or "tug" on electrons is called its "electronegativity"
• two atoms with the same affinity for electrons will form a covalent bond that is "nonpolar"
• oxygen is much more electronegative than most other atoms, and therefore
• oxygen bonded with other atoms form "polar covalent bonds"