Ammonia (NH3) is a polar molecule. Its polarity arises from the types of bonds within the molecule and its distinct three-dimensional arrangement. This article explains the factors that contribute to a molecule’s polarity and how these apply to ammonia.
What Makes a Molecule Polar?
Molecular polarity stems from the uneven distribution of electrical charge within a molecule. This unevenness begins with electronegativity, an atom’s ability to attract shared electrons in a chemical bond. When two atoms with different electronegativities form a covalent bond, electrons are pulled more towards the more electronegative atom. This creates a partial negative charge on that atom and a partial positive charge on the less electronegative atom. This uneven sharing forms a polar covalent bond, characterized by a bond dipole moment.
For a molecule to be polar overall, it must possess these polar bonds, and their spatial arrangement must be asymmetrical. If individual bond dipoles are arranged symmetrically, they can cancel each other out, resulting in a nonpolar molecule despite having polar bonds. For instance, carbon dioxide has polar bonds, but its linear shape allows these dipoles to cancel, making the molecule nonpolar. Both bond polarity and molecular geometry play roles in determining a molecule’s overall polarity.
The Shape of Ammonia (NH3)
Ammonia (NH3) consists of a central nitrogen atom bonded to three hydrogen atoms. The nitrogen atom possesses five valence electrons; three form single covalent bonds with hydrogen atoms. The remaining two valence electrons on the nitrogen atom exist as a lone pair. This lone pair influences the molecule’s three-dimensional shape.
According to Valence Shell Electron Pair Repulsion (VSEPR) theory, electron pairs (bonding and lone pairs) repel each other and arrange themselves to minimize this repulsion. While the electron geometry around the nitrogen atom is tetrahedral due to four electron domains (three bonding pairs and one lone pair), the molecular geometry is different. The lone pair exerts a stronger repulsive force than bonding pairs, pushing the three N-H bonds closer together. This results in a trigonal pyramidal shape, resembling a pyramid with nitrogen at the apex and the three hydrogen atoms forming the base. The bond angle between the hydrogen atoms is approximately 107 degrees, slightly less than the ideal 109.5 degrees of a perfect tetrahedron.
Why Ammonia is a Polar Molecule
Ammonia is a polar molecule because it combines polar N-H bonds with an asymmetrical molecular shape. Nitrogen is more electronegative than hydrogen. This difference means that shared electrons in each N-H bond are pulled more towards the nitrogen atom, creating a partial negative charge on nitrogen and partial positive charges on the hydrogen atoms. Each N-H bond therefore has a bond dipole moment directed towards the nitrogen.
The trigonal pyramidal shape of ammonia is important for its overall polarity. Because the molecule is asymmetrical, the individual bond dipoles of the three N-H bonds do not cancel each other out. Instead, these dipoles add up, creating a net dipole moment for the entire molecule. This net dipole moment points towards the more electronegative nitrogen atom and the lone pair, making that region slightly negative and the hydrogen-bearing end slightly positive.