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Nitrogen has 5 valence electrons.

So I would expect that when bonding with two hydrogens, two electrons would go into those bonds, and three would remain in place on the nitrogen.

Yet, all the diagrams I see for it look like this

nh2 diagram

Why are there 4 dots and not 3?

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As the compound is negatively charged, there is an extra electron present in the compound. Since the extra electron is held on nitrogen, there is 4 electrons on nitrogen instead of just 3. The extra electron will not be held on either hydrogens as it would be too unfavourable.

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  • $\begingroup$ Why would it be negatively charged? And how do you know this in advance? $\endgroup$ Jul 12 at 4:13
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    $\begingroup$ It is because in the compound you have drawn in the question, the compound is negatively charged. $\endgroup$ Jul 12 at 4:17
  • $\begingroup$ I got that from the site linked $\endgroup$ Jul 12 at 6:39
  • $\begingroup$ You have to start with a least some information. You either know it's an anion from whatever consideration you had before, say it's part of a compound like sodium amide and you have to draw the structure. Or you'll be given structure with the four dots and you'll have to figure out that it requires a negative charge to balance things. If none is given it would be like having ClO2 as Chlorine dioxide versus ClO2(-), the anion of the chlorous acid. $\endgroup$ Jul 12 at 7:47

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