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Covalent Bonding

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In this lesson we'll continue exploring chemical bonds, focusing now on covalent bonds. Specifically, we'll answer these questions: What is a covalent bond, how do these bonds form, and what are nonpolar and polar covalent bonds? Recall from basic atomic structure that bonds between atoms always involve the atom's outermost electrons, and these are called the valence electrons. Let's look at the simplest atom, hydrogen. Hydrogen's atomic number is one. Again from basic atomic structure, this tells me that it has one proton and, thus, also one electron. So I'll draw the nucleus and then surround it by an electron orbital. That first orbital can hold up to two electrons, but hydrogen only has one, so I'm going to add that. And that is the valence electron. Hydrogen is unstable because it has only one valence electron. To be stable that first orbital would need two. "If I add a second atom of hydrogen, of course it's going to be the same, so again, one orbital, one valence electron. And that means that, of course, it's also not stable. Hydrogen could become stable by giving up its valence electron, and in fact, sometimes it does just that, forming a hydrogen ion. But there is another option. Instead of losing electrons the atoms can move close together and share their electrons, like this. You see the two electrons now residing in an overlapping orbital. The atoms now share both electrons and claim them as their own, effectively gaining a full outermost electron orbital to become stable. When this happens a covalent bond is formed. Let's consider the name covalent: co means with or together, like in the word cooperate, and valent refers to those outermost electrons, the valence electrons. A covalent bond forms when atoms share one or more pairs of valence electrons. These bonds are very strong because the atoms are stable but they can only remain stable if they stay close enough together to continue sharing those electrons. Covalent bonds are often indicated by a line between the two atoms that share a pair of electrons. If they share one pair of electrons, as in our example, they form a single covalent bond. If they share two pairs of electrons, it's a double covalent bond. And atoms can even share three pairs of electrons to form a triple covalent bond. Let's look at another example and go a little bit bigger this time. Oxygen's atomic number is eight, and that means it has eight protons and, thus, also eight electrons. I'll draw the nucleus and start adding electrons. The first orbital holds two and the second orbital holds the remaining six electrons, and those are going to be the valence electrons. By the octet rule, the second orbital needs eight electrons to be stable so, like hydrogen, oxygen is unstable. It needs to either gain two electrons or share two pairs. Remember that hydrogen has one lone valence electron, and it will eagerly share it. So let's slide a hydrogen atom up next to our oxygen atom. As you can see we've formed a single covalent bond between the hydrogen and the oxygen. They share one pair of electrons. And if it worked here, I'm betting it's going to work on the other side, too, so let's put another hydrogen over there. And now we have a second, single covalent bond. Another pair of electrons is being shared, forming another covalent bond. We have two hydrogen atoms, each covalently bonding to an oxygen, and I believe you know that as water. The two main types of covalent bonds are polar covalent bonds and nonpolar covalent bonds. What would you say is the relationship between Earth's North and South Poles or between the poles of a magnet? In general, polar means opposite. Let's take a closer look at how this concept relates to molecules. In covalent bonds electrons are always shared between atoms. The electrons may be shared equally, meaning that they spend equal time around each atom. When electrons are shared equally, the bond is called a nonpolar covalent bond. In this example if you looked at the electron cloud depicted by the shaded area, you see that it is rather evenly distributed around both nuclei. The cloud represents the probability that an electron is there at any given time. But not all covalent bonds are created equally. Electrons may be shared unequally, producing a polar covalent bond. In this water molecule the electrons are being shared unequally. The electron cloud is much larger around the oxygen than around either hydrogen. In polar covalent bonds the electrons spend more time around one atom than around the others. They are unequally distributed. This produces a slight imbalance in the electrical charge across the molecule because the negative electrons tend to stay more on one side of the molecule than on the other. In this case they tend to stay around the oxygen more. Thus, the oxygen takes on a very slight negative charge because it tends to have extra electrons hanging around more often than not. And because of that each hydrogen takes on a slight positive charge because the electrons hang around them less often. This is the hallmark of polar covalent bonds. Unequal electron sharing produces slight electrical charges on the atoms. Part of the molecule is slightly negative and part of it is slightly positive. These slight charge imbalances are indicated by the use of the Greek letter delta and a plus or minus sign. Delta with a minus sign indicates the area that is more negative because it more strongly attracts the electrons. One of the most important polar substances is water, which is found in abundance both inside and outside of our cells. Polar substances dissolve easily in water but nopolar substances, such as fats and oils, do not. Covalent bonds are common in organic compounds that we use in the human body. Carbohydrates, which include starches and sugars like glucose, are held together by covalent bonds. The amino acids that make up proteins, which we use to build structures such as our muscles also contain covalent bonds, as do lipids, such as fats and cholesterol.