Question

Write the Lewis structure for each of the following species, and describe its geometry: (a) \(\mathrm{HNO}_{2}\), (b) \(\mathrm{N}_{3}^{-}\), (c) \(\mathrm{N}_{2} \mathrm{H}_{5}{ }^{+}\), (d) \(\mathrm{NO}_{3}^{-}\).

Short answer

The Lewis structures and geometries for the given species are as follows: (a) \(\mathrm{HNO}_{2}\): Trigonal pyramidal geometry with the Lewis structure: H - N = O : ↔: O : (with 3 lone pairs on each O atom). (b) \(\mathrm{N}_{3}^{-}\): Linear geometry with the Lewis structure: : N ≡ N ≡ N :. (c) \(\mathrm{N}_{2} \mathrm{H}_{5}{ }^{+}\): Trigonal pyramidal geometry for each N with the Lewis structure: H - N - H ↔ H - N - H (with one lone pair on each N atom). (d) \(\mathrm{NO}_{3}^{-}\): Trigonal planar geometry with the Lewis structure: O - N = O ↔ O - (with 3 lone pairs on the single bonded O atoms and 2 lone pairs on the double bonded O atom).

Step-by-step solution

(a) \(\mathrm{HNO}_{2}\) Lewis structure and geometry

Step 1: Find the total number of valence electrons. H - 1 valence electron N - 5 valence electrons O - 6 valence electrons (there are 2 O atoms) Total valence electrons: 1 + 5 + (2 × 6) = 18 Step 2: Arrange the atoms and distribute the electrons. The central atom will be N. Place the O atoms and H atom around the central N atom. First, place single bonds between N and each atom. Each bond has 2 electrons, so we have used 6 valence electrons. Now place the remaining electrons on O atoms as lone pairs to meet the octet rule. Each O atom will have 3 lone pairs (6 electrons). All electrons are used: 6 (bonds) + 12 (lone pairs) = 18 electrons. Lewis Structure: H - N = O : ↔: O : (with 3 lone pairs on each O atom) Step 3: Determine the molecular geometry using the VSEPR theory. The molecule is AX3E, where A is N, X is the bonded atoms (H or O), and E is a lone pair on N. The molecular geometry is trigonal pyramidal.

(b) \(\mathrm{N}_{3}^{-}\) Lewis structure and geometry

Step 1: Find the total number of valence electrons. N - 5 valence electrons (there are 3 N atoms) 1 extra electron due to the -1 charge Total valence electrons: (3 × 5) + 1 = 16 Step 2: Arrange the atoms and distribute the electrons. The central atom is N. Put one N atom in the middle, one N atom on each side, and draw triple bond between middle N and each of the other two N atoms. Remaining 2 electrons go on the terminal N atoms as lone pairs. Lewis Structure: : N ≡ N ≡ N : Step 3: Determine the molecular geometry using the VSEPR theory. The central N atom has 2 double bonded atoms, and the terminal N atoms have 1 triple bonded atom each. The molecular geometry for each N is linear.

(c) \(\mathrm{N}_{2} \mathrm{H}_{5}{ }^{+}\) Lewis structure and geometry

Step 1: Find the total number of valence electrons. N - 5 valence electrons (there are 2 N atoms) H - 1 valence electron (there are 5 H atoms) 1 less electron due to the +1 charge Total valence electrons: (2 × 5) + (5 × 1) - 1 = 14 Step 2: Arrange the atoms and distribute the electrons. Place a single bond between the two N atoms and surround each the two N atoms with 3 H atoms connected through single bonds. There are 4 more electrons left. Put a lone pair on each of the N atoms. Lewis Structure: H - N - H ↔ H - N - H (with one lone pair on each N atom) Step 3: Determine the molecular geometry using the VSEPR theory. For each nitrogen atom in the molecule, there are 3 bonded atoms and 1 lone pair. The molecular geometry for each N is trigonal pyramidal.

(d) \(\mathrm{NO}_{3}^{-}\) Lewis structure and geometry

Step 1: Find the total number of valence electrons. N - 5 valence electrons O - 6 valence electrons (there are 3 O atoms) 1 extra electron due to the -1 charge Total valence electrons: 5 + (3 × 6) + 1 = 24 Step 2: Arrange the atoms and distribute the electrons. Place N as the central atom and the 3 O atoms around it, connected with single bonds. To fulfill the octet rule for the N atom, add a double bond with one of the O atoms. Then, place the remaining electrons around the O atoms as lone pairs. Lewis Structure: O - N = O ↔ O - (with 3 lone pairs on the single bonded O atoms and 2 lone pairs on the double bonded O atom) Step 3: Determine the molecular geometry using the VSEPR theory. The central N atom has 3 bonded atoms and no lone pairs. The molecular geometry is trigonal planar.

Key concepts

Valence Electrons

Valence electrons are the electrons present in the outermost shell of an atom. These electrons play a pivotal role in the chemical bonding and properties of elements because they are most likely to interact with other atoms. For instance, when determining the Lewis structure of a molecule, like in the example of \(\mathrm{HNO}_{2}\), the first step involves counting the total number of valence electrons from hydrogen (H), nitrogen (N), and oxygen (O) to understand how the atoms bond together.

Specifically, hydrogen has 1 valence electron, nitrogen has 5, and oxygen has 6. It's crucial to remember that for main group elements, the group number in the periodic table equally gives the number of valence electrons. This background aids in predicting molecule structures and their chemical behavior.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory offers insights into molecule shapes. It states that electron pairs around a central atom will organize themselves as far apart as possible to minimize repulsion. This helps us predict the molecular geometry, like the trigonal pyramidal shape in the \(\mathrm{HNO}_{2}\) example.

The VSEPR theory assigns a 3D molecular structure based on the number of bonding pairs of electrons (connect atoms) and lone pairs (non-bonding pairs) associated with a central atom. For example, the AX3E notation for \(\mathrm{HNO}_{2}\) suggests that there are three regions of electron density (bonded atoms) and one lone pair surrounding the central atom, leading to the described geometry. This theory helps identify the spatial distribution of atoms which is essential in understanding how molecules interact.

Octet Rule

The octet rule is fundamental in understanding chemical bonding and molecule stability. It states that atoms tend to form bonds in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas.

In constructing Lewis structures, achieving an octet becomes the main goal for most elements. For instance, when drawing the Lewis structure for the nitrate ion \(\mathrm{NO}_{3}^{-}\), the octet rule guides us to ensure that nitrogen and oxygen atoms have complete valence shells. However, it's important to recognize exceptions to the rule, such as hydrogen (which only requires 2 electrons) and expanded octet situations with elements beyond the second period of the periodic table.