Question

What is the geometric structure of the ammonia molecule? How many pairs of electrons surround the nitrogen atom in \(\mathrm{NH}_{3} ? \mathrm{What}\) is the approximate \(\mathrm{H}-\mathrm{N}-\mathrm{H}\) bond angle in ammonia?

Short answer

The geometric structure of the ammonia molecule (\(\mathrm{NH}_{3}\)) is trigonal pyramidal. There are 4 pairs of electrons surrounding the nitrogen atom: 3 bonding pairs with hydrogen atoms and 1 lone pair. The approximate H-N-H bond angle in ammonia is 107°.

Step-by-step solution

Identify the central atom and note its valence electrons

In ammonia (NH₃), the central atom is Nitrogen (N). The nitrogen atom has 5 valence electrons, as it belongs to group 15 in the periodic table.

Count the total number of bonding and non-bonding electron pairs

There are three hydrogen atoms bonded to the nitrogen atom. Each hydrogen atom contributes one electron to form a shared electron pair (a bond). So, there are three bonding pairs so far with the three hydrogen atoms. As nitrogen has 5 valence electrons and we already counted for 3 when we formed bonds with hydrogen through sharing, there are 2 electrons left on the nitrogen atom. These 2 electrons form one non-bonding (lone) pair. So, there is a total of 4 electron pairs (3 bonding pairs and 1 lone pair) surrounding the nitrogen atom.

Determine the electron domain geometry

Since there are 4 electron pairs around the central nitrogen atom, the electron-domain geometry of ammonia is tetrahedral.

Determine the molecular geometry and bond angle

Considering the 3 bonding pairs and 1 lone pair, the molecular geometry of the ammonia molecule is trigonal pyramidal. In a trigonal pyramidal geometry, the bond angle between the bonding pairs is less than the bond angle in a perfect tetrahedron (which is 109.5°) due to the greater repulsion caused by the lone pair. The approximate H-N-H bond angle in ammonia is 107°. To summarize: 1. The geometric structure of the ammonia molecule is trigonal pyramidal. 2. There are 4 pairs of electrons (3 bonding pairs and 1 lone pair) surrounding the nitrogen atom in ammonia. 3. The approximate H-N-H bond angle in ammonia is 107°.

Key concepts

Electron Pair Geometry

In the study of molecular geometry, understanding the electron pair geometry is fundamental. This geometry considers both bonding and lone electron pairs around a central atom. For ammonia (\(\text{NH}_3\)), the central atom is nitrogen, which is surrounded by four pairs of electrons - three bonding pairs shared with hydrogen atoms and one lone pair. This distribution results in an electron domain geometry that is **tetrahedral**. The tetrahedral shape maximizes the distance between electron pairs, minimizing repulsion.

Note: While the \(\text{NH}_3\) molecule's electron pair distribution is tetrahedral, its actual shape or molecular geometry differs due to the lone pair's effect. The lone pairs play a crucial role in determining the overall shape and angles we observe in the molecule.

Bond Angle

When discussing bond angles, it’s important to consider the molecular geometry, as it significantly influences these angles. In the case of ammonia (\(\text{NH}_3\)), the molecular shape is **trigonal pyramidal** due to the presence of a lone pair on nitrogen. This configuration leads to an H-N-H bond angle that is **less than the typical 109.5°** found in an ideal tetrahedral molecule.

The presence of the lone pair introduces additional electron pair repulsion. This repulsion is stronger for lone pairs than bonded pairs, thus compressing the bond angle to approximately 107°. This subtle yet vital change underscores the impact of lone pairs on molecular structure, highlighting why bond angles differ from expected values based purely on electron domain geometry.

Ammonia Structure

Ammonia (\(\text{NH}_3\)) is an essential compound with a unique structure that significantly influences its properties and interactions. The **trigonal pyramidal** geometry of ammonia arises due to one lone pair and three bonding pairs of electrons situated around the nitrogen atom.

The nitrogen is centrally located with hydrogen atoms forming a pyramidal base. This geometry allows ammonia to have distinct polarity and molecular characteristics due to uneven charge distribution. The presence of the lone pair not only affects the bond angles but also contributes to ammonia's ability to act effectively as a base by accepting protons from acids in chemical reactions.

Valence Electrons

Understanding valence electrons is key to predicting the behavior and structure of molecules like ammonia. In the periodic table, elements in the same group share the same number of valence electrons, influencing their chemistry. For nitrogen, located in group 15, there are **five valence electrons**.

In ammonia (\(\text{NH}_3\)), these valence electrons determine how nitrogen bonds with hydrogen atoms. Three electrons are shared with hydrogen atoms, forming three single covalent bonds. The remaining two valence electrons form a lone pair, influencing both the molecule’s geometry and chemical properties. By studying both the distribution and impacts of valence electrons, one gains insight into not just ammonia, but also broader trends across nitrogen-containing compounds.