Differences in electronegativity affect bond strength and properties. Think of a chemical bond as a tug-of-war match, in which the electrons are the rope and electronegativity is the strength of each team. Case 1: Nonpolar bonds If two elements in a bond have the exact same electronegativity, the tug-of-war is tied, and the electrons are equally shared between elements. This only ever happens in bonds between two of the same element, since each element's electronegativity is unique. i.e. F2, O2, N2. Case 2: Polar covalent bonds If two elements in a bond have slightly different electronegativity, the element with greater electronegativity pulls the electrons closer to itself, leading to an unequally distributed, or polar bond. For example, in HCl, the electronegative Cl holds more of the negative charge, and the H has more of a positive charge. Case 3: Ionic bonds If two elements in a bond have extremely different electronegativity (usually a metal bonded with a non-metal), this leads to a bond so polar that essentially one element has a whole negative charge (-1), and the other element has a whole positive charge (+1). In a molecule, polarity is evaluated by the symmetry of the molecule. For example, even though CF4 is composed all of polar bonds, the molecule overall is nonpolar because the central carbon receives equal pull from each of the F's in every direction. For the purposes of organic chemistry, the more carbons and hydrogens there are, the less polar the molecule.