Question

Simple Lewis Structures, Formulas, and Names fo Use covalent Lewis structures to explain why the compound that forms between nitrogen and hydrogen has the formula \(\mathrm{NH}_{3} .\) Show why \(\mathrm{NH}_{2}\) and \(\mathrm{NH}_{4}\) are not stable.

Short answer

\(\mathrm{NH}_{3}\) is stable because nitrogen can share three pairs of electrons with three hydrogen atoms, completing nitrogen's octet and hydrogen's duet. \(\mathrm{NH}_{2}\) and \(\mathrm{NH}_{4}\) are not stable as they would leave nitrogen with an incomplete octet or exceed the nitrogen's capacity to hold 8 electrons, violating the octet rule.

Step-by-step solution

Determine the Valence Electrons

First, determine the number of valence electrons for nitrogen (N) and hydrogen (H). Nitrogen has 5 valence electrons and each hydrogen atom has 1 valence electron.

Draw the Lewis Structure for \(\mathrm{NH}_{3}\)

To form the most stable structure, nitrogen needs 3 more electrons to complete its octet. It shares a pair of electrons with each of the 3 hydrogen atoms, each of which needs 1 more electron to complete its duet, forming 3 N-H single covalent bonds. The total of the shared and lone pair electrons on nitrogen should be 8, fulfilling the octet rule.

Evaluate the Stability of \(\mathrm{NH}_{3}\)

The resulting molecule \(\mathrm{NH}_{3}\) has a complete octet around nitrogen and each hydrogen has a complete duet. This is a stable arrangement which is why \(\mathrm{NH}_{3}\) is a common and stable compound.

Analyze the Potential Structure for \(\mathrm{NH}_{2}\)

If the compound \(\mathrm{NH}_{2}\) were to form, nitrogen would have only 6 electrons in its valence shell, which is not enough to complete its octet. Moreover, there would be an unpaired electron on nitrogen, resulting in a very reactive and unstable species.

Analyze the Potential Structure for \(\mathrm{NH}_{4}\)

With the hypothetical \(\mathrm{NH}_{4}\) compound, nitrogen would have to share its 5 valence electrons with 4 hydrogen atoms, resulting in a total of 9 valence electrons around nitrogen. This violates the octet rule, as there would be no way for nitrogen to accommodate 9 electrons, leading to an unstable structure.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how the atom can bond with others. They are essentially the actors in the chemistry theater, participating in bonding performances that dictate the stability and reactivity of molecules. For example, in the case of nitrogen (\textbf{N}), it has 5 valence electrons and it's looking to either gain 3 more or share them to feel complete - think of it as having an inner desire to achieve a full outer shell, much like filling up the last seats in a movie theater for the perfect view.

In the context of the ammonia molecule (\textbf{NH}\(_3\)), understanding that hydrogen (\textbf{H}) has just 1 valence electron and is looking to partner up to achieve a stable duet - similar to finding that one dance partner who complements your moves perfectly - helps explain the formation of this compound. Think of \textbf{NH}\(_2\) and \textbf{NH}\(_4\) as attempts at creating dance groups that either lack members or are overcrowded, making the choreography unstable. Hence, counting valence electrons is a bit like preparing for a group dance performance on stage - you need just the right number of dancers to nail the routine.

Octet Rule

The octet rule is like the golden rule of a stable electronic relationship in chemistry. It suggests that atoms are most stable when they have eight electrons in their valence shell, akin to having a complete set of best friends. In our nitrogen (\textbf{N}) and hydrogen (\textbf{H}) example, this plays out in the formation of ammonia (\textbf{NH}\(_3\)). Nitrogen is like a person with five good friends, and it's seeking three more to fill out its friendship circle, while each hydrogen is a lone individual seeking to join a duo.

An \textbf{NH}\(_2\) molecule fails to satisfy everyone's social needs, leaving nitrogen two friends short of a party. On the flip side, an \textbf{NH}\(_4\) molecule is like trying to squeeze an extra friend into an already full car - there's just not enough room. Thus, we can consider the octet rule as a strategy for maintaining harmony in the molecule's social circle - keeping everyone happily engaged without feeling lonely or overcrowded.

Covalent Bonding

Covalent bonding is akin to a strong friendship formed when two atoms share valence electrons. They aren't given away or taken (like in ionic bonding) but rather shared equally, building a bond of mutual benefit. In ammonia (\textbf{NH}\(_3\)), nitrogen shares three pairs of electrons with three hydrogen atoms, forming three covalent bonds - a gesture of sharing its valence electrons, being a good friend to hydrogen atoms that need one more to achieve fullness in their outer shell.

Why not an \textbf{NH}\(_2\) or an \textbf{NH}\(_4\)? Well, \textbf{NH}\(_2\) would be like a friendship where someone is left feeling a little empty inside - nitrogen hasn't shared enough to feel complete. The \textbf{NH}\(_4\) scenario would be like someone trying to be friends with too many people at once, leading to a crowded situation where the bonding is just too much to handle. Covalent bonds create a balance where each atom gets to have a satisfying and stable electron-sharing relationship.