Electrons are shared differently in ionic and covalent bonds. Covalent bonds can be non-polar or polar and react to electrostatic charges.
Ionic bonds, like those in table salt (NaCl), are due to electrostatic attractive forces between their positive (Na+) and negative charged (Cl-) ions. In unit two, we compared atoms to puppies and electrons to bones in our analogy of how bonding works. In ionic bonding, each puppy starts out with an electron bone, but one puppy acts like a thief and steals the other puppy’s bone (see Fig. 3-1a). Now one puppy has two electron bones and one puppy has none. Because the electron bones in our analogy have a negative charge, the puppy thief becomes negatively charged due to the additional bone. The puppy that lost its electron bone becomes positively charged. Because the puppy who lost his bone has the opposite charge of the thief puppy, the puppies are held together by electrostatic forces, just like sodium and chloride ions!
In covalent bonds, like chlorine gas (Cl2), both atoms share and hold tightly onto each other’s electrons. In our analogy, each puppy again starts out with an electron bone. However, instead of one puppy stealing the other’s bone, both puppies hold onto both bones (see Fig. 3-1b).
Some covalently bonded molecules, like chlorine gas (Cl2), equally share their electrons (like two equally strong puppies each holding both bones). Other covalently bonded molecules, like hydrogen fluoride gas (HF), do not share electrons equally. The fluorine atom acts as a slightly stronger puppy that pulls a bit harder on the shared electrons (see Fig. 3-1c). Even though the electrons in hydrogen fluoride are shared, the fluorine side of a water molecule pulls harder on the negatively charged shared electrons and becomes negatively charged. The hydrogen atom has a slightly positively charge because it cannot hold as tightly to the negative electron bones. Covalent molecules with this type of uneven charge distribution are polar. Molecules with polar covalent bonds have a positive and negative side.
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a.Ionic bond analogy. The thief puppy has both bones (i.e. both electrons). The other puppy has lost its bone (electron). The puppies are held together because of the electrostatic force caused by their charge difference.
Water (H2O), like hydrogen fluoride (HF), is a polar covalent molecule. When you look at a diagram of water (see Fig. 3-2), you can see that the two hydrogen atoms are not evenly distributed around the oxygen atom. The unequal sharing of electrons between the atoms and the unsymmetrical shape of the molecule means that a water molecule has two poles - a positive charge on the hydrogen pole (side) and a negative charge on the oxygen pole (side). We say that the water molecule is electrically polar.
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Fig. 3-2: Different ways of representing the polar sharing of electrons in a water molecule. Each diagram shows the unsymmetrical shape of the water molecule. In (a) & (b), the polar covalent bonds are shown as lines. In part (c), the polar covalent bonds are shown as electron dots shared by the oxygen and hydrogen atoms. In part (d), the diagram shows the relative size of the atoms, and the bonds are represented by the touching of the atoms.
The polar covalent bonding of hydrogen and oxygen in water results in interesting behavior, suc
Molecule Orientation
Fig. 3-4. Water stream bending due to electrostatic force generated by rubbing a plastic comb on dry hair.
Photo Credit: Kanesa Seraphin
Water is attracted by positive and by negative electrostatic forces because the liquid polar covalent water molecules are able to move around so they can orient themselves in the presence of an electrostatic force. (see Fig. 3-4).
These forces can be observed in the following video:
Fig. 3-5: Water molecules are normally randomly oriented (left) unless they are orienting themselves in their presence of an electrostatic force (right).
Although we cannot see the individual molecules, we can infer from our observations that in the presence of a negative charge, water molecules turn so that their positive hydrogen poles face a negatively charged object. The same would be true in the presence of a positively charged object; the water molecules turn so that the negative oxygen poles face the positive object. See Fig. 3-5 for an artist interpretation.
Symmetry and Asymmetry
Remember that in a polar molecule, one atom’s pull is stronger than the other’s. Polar covalent molecules exist whenever there is an asymmetry, or uneven distribution of electrons in a molecule. One or more of these asymmetric atoms pulls electrons more strongly than the other atoms. For example, the polar compound methyl alcohol has a negative pole made of carbon and hydrogen and a positive pole made of oxygen and hydrogen (see Fig. 3-6).
Fig. 3-6: Polar molecules (top) and nonpolar molecules (bottom). Note that carbon dioxide has two covalent bonds between each oxygen atom and the carbon atom, which is shown here as two lines and referred to as a double bond.
When molecules are symmetrical, however, the atoms pull equally on the electrons and the charge distribution is uniform. Symmetrical molecules are nonpolar. Because nonpolar molecules share their charges evenly, they do not react to electrostatic charges like water does. Covalent molecules made of only one type of atom, like hydrogen gas (H2), are nonpolar because the hydrogen atoms share their electrons equally. Molecules made of more than one type of covalently bonded nonmetal atoms, like carbon dioxide gas (CO2), remain nonpolar if they are symmetrical or if their atoms have relatively equal pull. Even large compounds like hexane gasoline (C6H14), is symmetrical and nonpolar. Electrostatic charges do not seem to have much, if any, effect on nonpolar compounds. See Fig. 3-6 for examples of polar and nonpolar molecules.
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.
