Which is the Lewis structure for ammonia nh3

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H-N-H

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   H

Explanation:

Look at a periodic table to determine how many electrons you need to account for. Hydrogen (H) only has 1 electron, while Nitrogen (N) has 5. We have three Hydrogen atoms and one Nitrogen atom, so the total number of electrons will be 3 * 1 + 5 = 8 e-.

Now, place the center atom, which will be Nitrogen and place the three Hydrogens on three sides of it as above in the answer. You should use single bonds for this. Each single bond is a pair of electrons, so since we have three single bonds so far, we have accounted for 2 * 3 = 6 electrons. However, we need 2 more electrons for the total of 8. We put these electrons in as a lone pair above Nitrogen.

We check to see if everything follows the octet rule: Nitrogen has three single bonds, so that's 6 e-, as well as one lone pair, so that's another 2 e- for a total of 8 e-. Check. Now look at Hydrogen: H is the only element whose full orbital is 2 e-. Each H has a single bond with Nitrogen, so each does have 2 e-.

Thus, we know this is the correct diagram, and we are done.