|MadSci Network: Chemistry|
Hi Caroline! The polarity of a molecule can be predicted by knowing a couple of things: *what is the molecule's shape? *do the atoms in the molecule tend to pull electrons towards their nuclei, share them, or let them be pulled? So, let's start with the first point: how do you know a molecule's shape? "VSEPR," or valence-shell electron-pair repulsion theory, allows you to predict molecular shapes based on the number and geometry of electrons in an atom's outermost valence shell. For example, carbon has 4 unpaired electrons in its outer shell, which arrange themselves in a stable tetrahedral structure. Nitrogen has one electron pair + three unpaired electrons in its outer shell; these form a stable molecular structure called a trigonal pyramid (in other words, a triangle with a pyramidal base). There are some articles on VSEPR in MadSci Network archives, so I won't press this point too much further -- check out the archives if you need a VSEPR refresher. Given a molecule's shape, then, how do you know its polarity? Let's consider two carbon-based molecules, for simplicity's sake: methane (CH4) chloroform (CHCl3) H H | | H - C - H Cl - C - Cl | | H Cl We know that the central carbon atom has its outer electrons arranged in a tetrahedron. Now we need to think about the atoms bonded to the central carbon. In methane, 4 bonds are formed with hydrogen atoms, in a symmetrical structure. The molecule has no net polarity. Chloroform has 3 bonds with chlorine (two of them symmetrical; see the picture), and one with hydrogen. Chlorine, like many of its neighbors on the right side of the periodic table, is very electronegative: it pulls electrons towards its nucleus, like so: C .. Cl ----> partial + partial - ('loses' electron) ('gains' electron) Thus, it is a polar bond. The two symmetrical chlorine-carbon bonds "balance out" this pulling, so that no net pull occurs. The remaining chlorine-carbon bond is not "balanced out" by the less-polar hydrogen-carbon bond, resulting in a net polarity, with the "-" end towards the chlorine atom. If you know which atoms tend to be electronegative (F, Cl, O, N, S), you will be able to predict which bonds are polar. When two polar bonds are arranged equal and opposite (like the symmetrical Cl-C bonds in chloroform), no net polarity occurs. But, when polar bonds DON'T cancel, or if a polar bond is across from a nonpolar bond, a net molecular polarity ensues. Try sketching out these molecules/bonds and convincing yourself of their polarity: POLAR NOT POLAR water (H2O) ethane (C2H6) ammonia (NH3) oxygen (O2) ethanol (CH3CH2OH) carbon tetrafluoride (CF4) (for molecules with more than one central atom, like ethane or ethanol, it REALLY helps to draw the shape and consider the polarity and geometry of each bond). Good luck, Amanda firstname.lastname@example.org Resources: http:// www.ultranet.com/~jkimball/BiologyPages/ (information about electronegativity is shown under "E", including a good graphic) MadSci archives for "polarity, chemistry" and "VSEPR" has more in-depth answers that relate to this topic.
Try the links in the MadSci Library for more information on Chemistry.