Electrons are shared differently in ionic and covalent bonds. Covalent bonds can be non-polar or polar and react to electrostatic charges.
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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.
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.
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Non polar covalent bond analogy. Both puppies have an equal hold on both bones. Neither puppy has a charge; they are neutral.
Polar covalent bond analogy. One puppy is able to pull more on the bones, but both puppies still have a hold on both bones.
Fig. 3-1: Bonding using a puppy analogy. In this analogy, each puppy represents an atom and each bone represents an electron.
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.