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In this video, we're going to learn about the octet rule and how to construct dot and cross diagrams to show the covalent bonding in hydrogen, oxygen and nitrogen.
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As seen in the video "How atoms bond" when two hydrogen atoms bond, the shells overlap.
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Because each atom only has one electron to share, there are two electrons being shared over two shells.
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These are represented by a dot and a cross.
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Which noble gas does this correspond with?
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Pause the video and continue when ready.
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The answer is helium, which has two electrons in its outer shell.
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As a result of electron sharing, each hydrogen atom now also has two electrons in its outer shell.
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In the next example, we are going to consider oxygen.
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It's in group six, therefore how many electrons does each oxygen atom need in its outer shell to fulfill a noble gas structure.
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Remember, oxygen has eight electrons in total, so we would need to fill the second shell.
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Pause the video and continue when you're ready.
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The answer is that each oxygen atom needs two more electrons before fulfilling a noble gas structure.
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This is also called the octet rule.
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In this example, the octet rule is fulfilled because each oxygen atom shares two electrons in the outer shell, forming an O2 molecule.
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If you count the number of electrons in each shell,
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you should notice that each shell has eight electrons shared over two shells shown by the dots for one oxygen atom and crosses for the other, which is the ideal.
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Whenever you draw a dot and cross diagram, wherever possible, you should ensure that a) the electrons are equally spaced, and b) that they are paired.
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The fact there are two shared pairs of electrons in the middle where the shelves overlap means that the oxygen molecule O2 has two covalent bonds connecting each oxygen atom.
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This is called a double bond. Each of the nitrogen atoms has five electrons in their outer shells.
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How many electrons therefore do you think each will need to share to fulfill the octet rule?
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Pause the video and continue when ready.
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If you said that three electrons available to form a shared pair, you'd be correct.
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You should see that there are three shared pairs of electrons in the overlap of the shells.
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This is called a triple bond. You should also notice that this example fulfills the octet rule.