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9.3 Covalent Bonds

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    to lose one or more electrons, and for another to gain one or more electrons. However, some atoms will not give up or gain electrons easily. Yet they still participate in formation. How? There is another mechanism for obtaining a complete : electrons. When electrons are shared between two atoms, they form a . by using H atoms, with the understanding that H atoms need only two electrons to fill the 1 . Each H starts with a single electron in its : has two electrons around the , completely filling each 's : has a filled , this bond is stable, and we have made a diatomic hydrogen molecule. (This explains why hydrogen is one of the diatomic elements.) For simplicity's sake, it is not unusual to represent the with a dash, instead of with two dots: is called a . As another example, consider fluorine. F atoms have seven electrons in their : . has a complete octet around it now: . Each F has three other pairs of electrons that do not participate in the bonding; they are called . Each F has one bonding pair and three lone pairs of electrons. starts out with an odd number of electrons in its : : has a full with two electrons, while the F has a complete octet of electrons. will be between two atoms only. Consider H and O atoms: : has a complete . However, the O has only seven electrons around it, which is not a complete octet. This can be fixed by including a second H , whose single electron will make a second with the O : what side the second H is positioned on.) Now the O has a complete octet around it, and each H has two electrons, filling its . This is how a water molecule, HO, is made. by sharing electrons with an H . The electron dot diagram of NH is as follows: of a simple molecule. First, you must identify the central and the . The is the in the center of the molecule, while the are the atoms making bonds to the central . The central is usually written first in the formula of the (HO is the notable exception). After the central and have been identified, follow these steps: is the central , and the F atoms are the . There is a negative sign on the species, so we have an extra electron to consider. This uses up eight electrons, so we have 32 − 8 = 24 electrons left. This uses up 24 more electrons, leaving 24 − 24 = 0 electrons left. There are no additional electrons to add to the central . has eight electrons around it, as does each F . Each has a complete octet. This is a good for BF. formaldehyde (CHO), we would get the following: only has six electrons around it, not the eight electrons for an octet. How do we fix this? as a result: , we can now see that the O and C atoms have octets, while each H has two electrons: is full, so this is an acceptable . If we were to use lines to represent the bonds, we would use two lines between the C and O atoms: and represents two bonding pairs of electrons between the atoms. If the rules for drawing Lewis electron dot diagrams do not work as written, a may be required. only has four electrons around it. The way to solve this dilemma is to make a between carbon and O : still has eight electrons around it, but now the C also has a complete octet. This is an acceptable for CO. in which there are three pairs of electrons between two atoms. Good examples of this are elemental nitrogen (N) and acetylene (CH): are bonded together with covalent bonds. Because they are ions, however, they participate in ionic bonding with other ions. So both major types of bonding can occur at the same time.

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