Bond Polarity

So far, we have been proceeding under the assumption that the shared electrons in a covalent bond belong to the two bonded atoms equally. However, this assumption is often not warranted; sharing of electrons is frequently rather unequal, and this unequal sharing has important consequences, most of which will not become clear until Lesson 9. So let's dig into when electrons are and aren't shared equally.

Electronegativity

The nuclei of atoms are different from one another in a few ways. For starters, they all have different numbers of protons; antimony has more than arsenic, which has more than aluminum. Additionally, they are shielded from their valence electrons to different degrees, and are at different distances from those electrons.

For all these reasons, the nuclei of some atoms exert a much stronger pull on shared electrons than others. Fluorine, for example, exerts a very strong pull on its bonding electrons. Boron pulls much more weakly.

As with many properties of the elements, electronegativity has a trend from the lower left to upper right of the periodic table. It is greatest in the upper right and smaller as you go left and down. As usual, we ignore the noble gases, as they do not bond. As you can see, the values of electronegativity max out around four, and do not have units.

Polar and Non-polar Bonds

In any given covalent bond, you have two atoms, each exerting its own pull on the shared electrons.

If those atoms are the same element (or they happen to have the same exact electronegativity), then the pulls are equal, and the electrons get shared equally. This is known as a non-polar covalent bond.

When we say the sharing is equal, what we mean is that the cloud of electrons (the array of orbitals they occupy in the molecule) is equally dense on both atoms. This means that the charges on protons and electrons cancel each other out, and no part of the molecule has any net positive or negative charge, as in the Cl-Cl bond here.

However, if you have atoms with different electronegativities, then the more electronegative one pulls more strongly on the shared electrons. The electron density on that atom becomes larger, and the density on the other atom shrinks. This is called a polar covalent bond.

The result is that the more electronegative atom becomes partially negatively charged. This is not the same as a full ionic charge that results from transferring an electron - it's just a little extra negative charge. Naturally, the other atom gets partially positively charged as a result of having electron density pulled away from it. We represent these partial charges with lowercase Greek deltas, as shown at right.

For now, we will mostly just file away this information for later. It will become very important in Lesson 9. But you definitely want to assure yourself that you can categorize any bond as ionic, polar covalent, or non-polar covalent.

Ionic bonds, as you know, occur any time you have a metal and a non-metal bonded. Polar covalent bonds, for this class, will be any time two different non-metal elements bond together, with one exception: we will consider C-H bonds to be non-polar. Also, of course, any bond between atoms of the same element (H-H, O-O, etc.) is also non-polar.

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