Valence electrons are fundamental to understanding how atoms interact and form molecules. These electrons dictate an atom’s chemical behavior, including its capacity to bond with other elements. For the molecule ammonia (\(\text{NH}_3\)), determining the total number of these outer-shell electrons is the first step in predicting its chemical structure and properties. The arrangement of these electrons defines the molecule’s shape, which influences its physical and chemical functions.
Understanding the Role of Valence Electrons
Valence electrons are the electrons located in the outermost occupied shell of an atom, and they are the participants in the formation of chemical bonds. These electrons are more weakly attracted to the positive nucleus than the inner electrons, allowing them to be shared or transferred during chemical reactions. The number of valence electrons an atom possesses determines its reactivity and the number of bonds it is likely to form.
For main-group elements in the periodic table, the group number corresponds directly to the number of valence electrons an atom has. Nitrogen (N) is in Group 15, indicating it has five valence electrons. Hydrogen (H), located in Group 1, has only one valence electron. Atoms often seek to achieve a stable, full outer shell, typically containing eight electrons, known as the octet rule. Hydrogen is an exception, requiring only two electrons for stability.
Calculating the Total for Ammonia (\(\text{NH}_3\))
Calculating the total number of valence electrons in the ammonia molecule is a process based on the atoms involved. The chemical formula \(\text{NH}_3\) tells us the molecule contains one Nitrogen atom and three Hydrogen atoms. Each element contributes its specific number of valence electrons to the total pool available for bonding.
The single Nitrogen atom contributes five valence electrons, as it belongs to Group 15. The three Hydrogen atoms each contribute one valence electron, for a total of three electrons from the hydrogen component. To find the total number of valence electrons for the entire \(\text{NH}_3\) molecule, we sum the contributions from all atoms: five from Nitrogen plus three from the three Hydrogens (1 \(\times\) 3). This calculation yields a total of eight valence electrons (5 + 3 = 8) for the ammonia molecule.
This total count is used to construct the Lewis structure, which maps out the location of all bonding and non-bonding electrons within the compound.
Structure and Bonding in Ammonia
The eight total valence electrons in \(\text{NH}_3\) are distributed to achieve the most stable electron configuration for the central Nitrogen atom and the three surrounding Hydrogen atoms. A single covalent bond is formed by a shared pair of two electrons. Since Nitrogen is bonded to three Hydrogen atoms, a total of six electrons (three bonding pairs) are used to form the three N-H single bonds.
The remaining two valence electrons (8 total electrons minus 6 bonding electrons) form a non-bonding electron pair, called a lone pair, which resides exclusively on the central Nitrogen atom. This distribution results in a tetrahedral arrangement of electron domains around the central Nitrogen atom. The molecular shape, which only considers the positions of the atoms, is described as trigonal pyramidal.
The lone pair causes repulsion, pushing the three N-H bonding pairs closer together. This decreases the bond angle from the ideal \(109.5^\circ\) to approximately \(107^\circ\). The uneven distribution of electron density, caused by the lone pair, also makes the ammonia molecule polar.