To describe how a covalent bond forms. To apply the octet rule to covalent compounds

You have already seen examples of substances that contain covalent bonds. One substance mentioned previously was water (\(\ce{H2O}\)). You can tell from its formula that it is not an ionic compound; it is not composed of a metal and a nonmetal. Consequently, its properties are different from those of ionic compounds.

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Electron Sharing

Previously, we discussed ionic bonding where electrons can be transferred from one atom to another so that both atoms have an energy-stable outer electron shell. Because most filled electron shells have eight electrons in them, mmsanotherstage2019.comists called this tendency the octet rule. However, there is another way an atom can achieve a full valence shell: atoms can share electrons.

This concept can be illustrated by using two hydrogen atoms, each of which has a single electron in its valence shell. (For small atoms such as hydrogen atoms, the valence shell will be the first shell, which holds only two electrons.) We can represent the two individual hydrogen atoms as follows:


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The Lewis diagram of two hydrogen atoms sharing electrons looks like this:


This depiction of molecules is simplified further by using a dash to represent a covalent bond. The hydrogen molecule is then represented as follows:


Each fluorine atom contributes one valence electron, making a single bond and giving each atom a complete valence shell, which fulfills the octet rule:


The circles show that each fluorine atom has eight electrons around it. As with hydrogen, we can represent the fluorine molecule with a dash in place of the bonding electrons:



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In this molecule, the hydrogen atom does not have nonbonding electrons, while the fluorine atom has six nonbonding electrons (three lone electron pairs). The circles show how the valence electron shells are filled for both atoms.

Example \(\PageIndex{1}\)

Use Lewis diagrams to indicate the formation of the following:

Cl2 HBr


a. When two chlorine atoms form a chlorine molecule, they share one pair of electrons. In Cl2 molecule, each chlorine atom is surrounded by an octet number of electrons.

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The Lewis diagram for a Cl2 molecule is similar to the one for F2 (shown above).