พันธะเคมี อ. ศราวุทธ แสงอุไร A molecule with its center of positive charge in a location different from its center is polar. Polar Non-Polar You can figure out if a molecule is nonpolar or polar, by subtracting the electronegativity of each atom. If you get a zero, then the molecule is non polar. 2.1 – 3.0 = -0.9 3.0 – 3.0 = 0 Polar
พันธะเคมี อ. ศราวุทธ แสงอุไร Polar-what this means A polar molecule occurs when electrons from one atom ’ s electron cloud shifts to be closer to it ’ s neighboring atom(s). Polar δ + δ - These signs indicate that there is a partial positive charge on the hydrogen (it ’ s electrons are spending more time with fluorine) and a partial negative charge on Chlorine. If we were to put this in magnetic field, the molecule would twist, such that the hydrogens would be attracted to a negative charge and the chlorine would be attracted to a positive charge. (opposite attract.)
พันธะเคมี อ. ศราวุทธ แสงอุไร Dipole moment Who is going to win this tug of war contest? You may assume that every person possesses the same strength.
พันธะเคมี อ. ศราวุทธ แสงอุไร Dipole moment A dipole moment is kind of like a tug-of-war. You have two molecules with different electronegativities pulling against each other. F H Since Fluorine has a higher electronegativity, we would expect that electrons would be pulled towards the Fluorine. This results in something we call a dipole moment. dipole moment
พันธะเคมี อ. ศราวุทธ แสงอุไร Dipole moment Do dipole moments exist whenever you have atoms with different electronegativities? F H O C O Imagine that you have a tug-of-war game going on, but this time, both sides are of equal strength. Will the middle ever move? We can see in carbon dioxide that the dipole moments cancel each other out. Therefore, overall, this is a non polar molecule.
พันธะเคมี อ. ศราวุทธ แสงอุไร Dipole moments … why use them? In order to predict how something will bond, dissolve, react, vaporize, etc … you need to understand what makes molecules stick together in the first place. Notice, that now we are talking about molecule to molecule interaction.
แหล่งอ้างอิง Martin S. Silberberg, Chemistry: The Molecular Nature of Matter and Change, McGraw-Hill Higher Education, 2004 Raymond Chang, Chemistry, Williams College, McGraw-Hill Higher Education, 2002