A Lewis structure, or Lewis dot structure, is a diagram that shows the bonding between atoms in a molecule and the lone pairs of electrons that may exist. These diagrams represent the valence electrons as dots, with lines connecting atoms to show shared electron pairs (covalent bonds). Drawing a Lewis structure involves several steps, including calculating total valence electrons, identifying the central atom, and arranging electrons as bonds and lone pairs until all atoms have a stable electron configuration, usually an octet (or duet for hydrogen).
Ammonia (NH₃) is a simple covalent molecule composed of one nitrogen atom and three hydrogen atoms. It plays a vital role in chemistry, agriculture, and biology—most notably as a building block for fertilizers and a source of nitrogen in various compounds.
Valence Electrons
To draw the Lewis structure, we start by determining the total number of valence electrons in the molecule:
- Nitrogen (N) belongs to Group 15 of the periodic table and has 5 valence electrons.
- Hydrogen (H) belongs to Group 1 and has 1 valence electron.
Since there are three hydrogen atoms, the total number of valence electrons is: 5+(3×1)=8 valence electrons5 + (3 × 1) = 8 \text{ valence electrons}5+(3×1)=8 valence electrons
These 8 electrons will be arranged to satisfy the bonding requirements of all atoms.
Step-by-Step Construction
Step 1: Identify the central atom
Nitrogen (N) is the central atom because it is less electronegative than hydrogen and can form multiple bonds.
Step 2: Connect atoms with single bonds
Each hydrogen atom forms a single bond (a shared pair of electrons) with nitrogen.
So, three N–H single bonds are drawn.
Each N–H bond accounts for 2 shared electrons, giving:
3×2=6 electrons used
Step 3: Distribute remaining electrons
Out of 8 valence electrons, 6 are used for bonding.
This leaves 2 electrons (1 lone pair) remaining, which are placed on the nitrogen atom.
Step 4: Check octet rule
- Nitrogen now has 8 electrons around it (6 from bonds + 2 lone pair).
- Each hydrogen has 2 electrons (a full shell).
Thus, the structure satisfies the octet rule for nitrogen and the duet rule for hydrogen.
Lewis Structure Diagrammatically
- Each dash (–) represents a bonding pair of electrons.
- The two dots (:) on top of nitrogen represent its lone pair.
Molecular Geometry
The presence of three bonding pairs and one lone pair around the nitrogen atom gives ammonia a trigonal pyramidal molecular shape.
According to the VSEPR theory (Valence Shell Electron Pair Repulsion theory):
- The electron pairs repel each other to positions of minimum repulsion.
- The lone pair exerts slightly more repulsion than bonding pairs, causing the molecule to adopt a pyramidal structure.
The bond angle in ammonia is approximately 107°, slightly less than the tetrahedral angle (109.5°), due to the lone pair repulsion.
Polarity
Ammonia is a polar molecule because:
- Nitrogen is more electronegative than hydrogen, creating a dipole moment toward the nitrogen atom.
- The molecule’s asymmetric shape (due to the lone pair) prevents dipole cancellation.
As a result, NH₃ has a net dipole moment, making it capable of hydrogen bonding, which greatly influences its physical properties like high solubility in water and relatively high boiling point.
Summary Table
The Lewis structure of ammonia (NH₃) reveals a nitrogen atom bonded to three hydrogen atoms with one lone pair of electrons. This arrangement leads to a trigonal pyramidal geometry and a polar character. Understanding its Lewis structure helps explain ammonia’s reactivity, ability to form hydrogen bonds, and significance as a fundamental compound in both inorganic and biological chemistry.
| Property | Description |
|---|---|
| Molecular formula | NH₃ |
| Total valence electrons | 8 |
| Central atom | Nitrogen (N) |
| Number of bonds | 3 single N–H bonds |
| Lone pairs on central atom | 1 |
| Molecular shape | Trigonal Pyramidal |
| Bond angle | ~107° |
| Polarity | Polar molecule |