A chemical compound in which the molecule has no permanent dipole moment. Depending on the relative electro-negativities of the two atoms sharing electrons, there may be partial transfer of electron density from one atom to the other. When the electro-negativities are not equal, electrons are not shared equally and partial ionic charges develop. The greater the difference, the greater the ionic bond. Bonds that are partly ionic are known as polar covalent bonds. Non-polar covalent bonds have an equal share of the bond electrons and arise when the electro-negativities of the two atoms are equal. There are many non-polar substances. Some are completely non-polar while others are considered to be non-polar since they lack any significant polarity. Completely non-polar compounds include nitrogen, oxygen, and chlorine gases, diatomic molecules such Br₂, I₂, and F₂, acetylene, and carbon tetrachloride. They are all perfectly symmetrical in which the dipole moments of any polar bonds is completely annulled by equal and opposite dipole moments from the other bonds. Hydrocarbons such as propane, butane, pentane, hexane, cyclohexane, octane, as well as fats and oils are mostly non-polar.