Chapter 6: Problem 47

Draw the Lewis structure of \(\mathrm{NH}_{4}^{+} .\) Examine this structure to explain why this five-atom group exists only as a cation.

Short Answer

Expert verified

The Lewis structure for the \(\mathrm{NH}_{4}^{+}\) cation can be drawn with Nitrogen at the centre surrounded by four Hydrogen atoms, each bonded to Nitrogen. It's a cation because the formal charge calculation yields a +1 charge on Nitrogen atom and zero on each Hydrogen, resulting in a net positive charge (+1), which is consistent with the cationic nature of \(\mathrm{NH}_{4}^{+}\)

Step by step solution

Draw the Lewis structure of the atom

Firstly, count the total number of valence electrons of all the atoms present in the compound. Nitrogen (N) has 5 valence electrons and Hydrogen (H) has 1. Since there are 4 Hydrogen atoms, they will contribute 4 electrons (1*4=4). Also, since it’s a cation with a positive charge, we subtract one electron. Hence, total valence electrons are 5+4-1=8. Now, start by placing Nitrogen in the center (as it's less electronegative) and Hydrogen atoms around it. Use the valence electrons to form bonds - here, each N-H bond means two shared electrons. This way 8 electrons are used up in 4 bonds.

Check for Octet rule

In step 1, all Hydrogen atoms have reached their octet because they require only 2 electrons to fill their outermost shell. Also, Nitrogen's outer shell has 8 electrons now (2 from each bond). Hence, the octet rule is satisfied for all atoms.

Calculate formal charge

The formal charge of an atom in a molecule can be calculated by the formula: Formal Charge = (no. of valence e- in free atom) - (no. of valence e- in the bonded state). Here, valence electrons in the bonded state include half of the bonding electrons (since the other half belongs to the other atom in the bond) and all non-bonding (lone pair) electrons. For Nitrogen, Formal Charge = 5 - (4 + 0) = +1. For Hydrogen, every atom has a formal charge of zero. Therefore, considering all atoms, the entire molecule carries a +1 formal charge. This explains why \(\mathrm{NH}_{4}\) exists only as a cation, \(\mathrm{NH}_{4}^{+}\).

Check for exceptions and finalize the structure

There are a few exceptions to the octet rule. However, Nitrogen and Hydrogen follow the rule strictly. So, the final Lewis structure remains the same.

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Draw the Lewis structure of \(\mathrm{NH}_{4}^{+} .\) Examine this structure to explain why this five-atom group exists only as a cation.

Short Answer

Step by step solution

Draw the Lewis structure of the atom

Check for Octet rule

Calculate formal charge

Check for exceptions and finalize the structure

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