Question

Draw a Lewis structure for each of the following polyatomic ions and determine their geometries: a. \(\mathrm{NO}_{2}^{-}\) b. \(\mathrm{NO}_{3}^{-}\) c. \(\mathrm{NH}_{4}^{+}\)

Short answer

The geometries are: \(\text{NO}_2^-: \text{bent}\), \(\text{NO}_3^-: \text{trigonal planar}\), \(\text{NH}_4^+: \text{tetrahedral}\).

Step-by-step solution

Calculate the total number of valence electrons for \(\text{NO}_2^-\)

Nitrogen (N) has 5 valence electrons and each Oxygen (O) has 6 valence electrons. An additional electron is added because of the negative charge. \[5 + 6 + 6 + 1 = 18 \text{ electrons}\]

Draw the skeleton structure for \(\text{NO}_2^-\)

Place the nitrogen atom in the center with single bonds to each oxygen atom.

Complete the octets for the oxygen atoms for \(\text{NO}_2^-\)

Each oxygen needs 8 electrons. Distribute the remaining 12 valence electrons around the oxygens to complete their octets.

Place remaining electrons on the nitrogen atom and create double bonds for \(\text{NO}_2^-\)

Place remaining electrons on N to complete its octet. Because there are only 4 left, we form one double bond with one of the O atoms.

Determine the geometry for \(\text{NO}_2^-\)

With 2 bonding domains and one lone pair on the central atom, the geometry is bent.

Calculate the total number of valence electrons for \(\text{NO}_3^-\)

Nitrogen has 5 valence electrons, each oxygen has 6, and add one extra electron for the negative charge. \[5 + 6 + 6 + 6 + 1 = 24 \text{ electrons}\]

Draw the skeleton structure for \(\text{NO}_3^-\)

Place nitrogen in the center with single bonds to each of the three oxygen atoms.

Complete the octets for the oxygen atoms for \(\text{NO}_3^-\)

Distribute the remaining 18 valence electrons around the three oxygens so each has an octet.

Place remaining electrons on the nitrogen atom and create double bonds for \(\text{NO}_3^-\)

Place remaining electrons on N to complete its octet. Form one double bond with one of the O atoms.

Determine the geometry for \(\text{NO}_3^-\)

With 3 bonding domains and no lone pairs on the central atom, the geometry is trigonal planar.

Calculate the total number of valence electrons for \(\text{NH}_4^+\)

Nitrogen has 5 valence electrons and each hydrogen has 1 valence electron. Subtract one electron for the positive charge. \[5 + 4 \times 1 - 1 = 8 \text{ electrons}\]

Draw the skeleton structure for \(\text{NH}_4^+\)

Place nitrogen in the center with single bonds to each of the four hydrogen atoms.

Check octets for \(\text{NH}_4^+\)

Each hydrogen has 2 electrons and nitrogen has 8 electrons, so both elements follow the octet rule (in nitrogen's case, the expanded version to accommodate extra bonds).

Determine the geometry for \(\text{NH}_4^+\)

With 4 bonding domains and no lone pairs on the central atom, the geometry is tetrahedral.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how atoms bond with each other.
For example, Nitrogen (N) has five valence electrons and each Oxygen (O) has six valence electrons. In an ion like \(\text{NO}_2^-\), an extra electron is added due to the negative charge, giving us a total of 18 valence electrons.

• Valence electrons are used to form bonds between atoms in a molecule or ion.
• Valence electrons can be shared (covalent bonds) or transferred (ionic bonds).
• When drawing Lewis structures, you always start by calculating the total number of valence electrons in the molecule or ion.

This ensures that you have the right number of electrons to distribute within the structure.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms in a molecule. Understanding molecular geometry helps predict the shape and properties of the molecule.
For instance, \(\text{NO}_2^{-}\) has a bent geometry due to having two bonding domains and one lone pair of electrons on the central atom. This arrangement is derived from the VSEPR (Valence Shell Electron Pair Repulsion) theory which states that electron pairs will arrange themselves to minimize repulsion.

• The geometry affects physical and chemical properties like boiling point, melting point, and reactivity.
• Different geometries include linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.
• For \(\text{NH}_4^+\), with four bonding domains and no lone pairs, the geometry is tetrahedral.

Understanding these shapes can make it easier to predict molecule behavior in different chemical reactions.

Polyatomic Ions

Polyatomic ions are ions that consist of more than one atom. These atoms are typically covalently bonded to each other, but as a whole, the group of atoms carries a charge.
Examples from the exercise include \(\text{NO}_2^-\), \(\text{NO}_3^-\), and \(\text{NH}_4^+\).

• Factors such as the number of atoms and the type of bonds within the polyatomic ion impacts its overall stability and reactivity.
• In practice, to draw polyatomic ions' Lewis structures, calculate the total number of valence electrons by considering all atoms involved and adding or subtracting electrons based on the ion's charge.
• Polyatomic ions can appear in both ionic and covalent compounds, contributing significantly to the compound's properties.

For instance, \(\text{NH}_4^+\) is a common polyatomic ion forming compounds such as ammonium chloride.

Electron Distribution

Electron distribution refers to how electrons are allocated among the atoms in a molecule or ion.
For a stable structure, electrons need to be distributed to fulfill atom's octet requirements.
In \(\text{NO}_2^-\), for example, the 18 valence electrons are distributed so Oxygen atoms complete their octet first and remaining electrons are placed on Nitrogen.

• Start by placing the bonding pairs of electrons between atoms, then place any leftover electrons to complete octets on outer atoms first.
• Form double bonds if necessary to fulfill octet rule requirements, as seen in \(\text{NO}_3^-\).
• Extended octets are possible for elements in period 3 or below, accommodating more than 8 electrons as in \(\text{PF}_5\).

Electron distribution is essential for accurately depicting the stable form of a molecule or ion.

Octet Rule

The octet rule is a guiding principle in chemistry that states atoms tend to bond in such a way that each atom has eight electrons in its valence shell.
In the Lewis structures we drew earlier, notice that every atom follows this rule, including Nitrogen in \(\text{NO}_2^-\), \(\text{NO}_3^-\), and \(\text{NH}_4^+\).

• Completing the octet provides atoms with the same electron configuration of a noble gas, which is highly stable.
• Hydrogen is an exception, as it follows the duet rule (only 2 electrons in its valence shell).
• The octet rule can be expanded for elements in the third period or beyond, due to the availability of d-orbitals.

The octet rule is foundational in predicting and explaining the bonding behavior in molecules.