Stability in bonding In covalent bonds, electrons arent always shared equally between the two nuclei. This is because some elements have a greater affinity for electrons than others. Electronegativity: Ability of an atom to attract electrons when in a molecule. The pull that an element has on electrons INCREASES as you go from LEFT TO RIGHT and from BOTTOM TO TOP. How does this affect bonds? H 2 has the same amount of pull in both directions. The bond would look something like: HH HF is made of elements from opposite sides of the periodic table. The fluorine has a much greater electronegativity than hydrogen (3.98 vs. 2.20). The electrons would concentrate around the fluorine, resulting in an asymmetrical electron distribution between the two nuclei. HF ++ -- When electrons in a covalent bond are concentrated near one of the nuclei, the bond is POLAR. HF Polar bond HHNonpolar bond If the electrons are shared equally between the nuclei, the bond is NONPOLAR. ++ -- Now, you try some... Do the following have polar or nonpolar bonds? F2F2 HCl BCl 3 S8S8 H2OH2O C 60 SO 2 BN nonpolar polar Electronegativity differences greater than 1.7 typically results in the formation of an ionic bond. Because of the bent structure of water, water is a polar molecule. O H H Use arrows along the bonds to indicate the direction of the pull on the electrons in the bond (the dipole moment). A plus-sign on the tail of the arrow shows that the bond is more positive at that end Both bonds in water are polar. How does this affect the polarity of the molecule? Add the two vectors (the arrows) together to find the net polarity (dipole moment) of the molecule All of the bonds in CCl 4 are polar. Is this molecule polar? C Cl Since the molecule is symmetrical, all of the dipoles along the bonds cancel each other out. CCl 4 is a nonpolar molecule and has no net dipole moment. Are these planar molecules polar or nonpolar? B F F F B F F H B H H H nonpolarpolarnonpolar Are these linear molecules polar or nonpolar? OC OCO SCO polarnonpolar polar