Question

Predict the geometries of the following ions: (a) \(\mathrm{NH}_{4}^{+}\), (b) \(\mathrm{NH}_{2}^{-},(\mathrm{c}) \mathrm{CO}_{3}^{2-},\) (d) \(\mathrm{ICl}_{2}^{-},(\mathrm{e}) \mathrm{ICl}_{4}^{-}\)

Short answer

(a) Tetrahedral (b) Bent (c) Trigonal planar (d) Linear (e) Square planar

Step-by-step solution

Understanding the Problem

We need to predict the geometries of five different ions: \( \mathrm{NH}_{4}^{+} \), \( \mathrm{NH}_{2}^{-} \), \( \mathrm{CO}_{3}^{2-} \), \( \mathrm{ICl}_{2}^{-} \), and \( \mathrm{ICl}_{4}^{-} \). To do this, we'll use VSEPR theory to determine the electron pair geometry and the molecular shape.

Determine Electron Groups on \( \mathrm{NH}_{4}^{+} \)

\( \mathrm{NH}_{4}^{+} \) has one nitrogen as the central atom bonded to four hydrogen atoms. There are no lone pairs on the nitrogen in this case. This is an \( \mathrm{AX}_{4} \) system, which results in a tetrahedral geometry.

Determine Electron Groups on \( \mathrm{NH}_{2}^{-} \)

The \( \mathrm{NH}_{2}^{-} \) ion has a central nitrogen atom with three regions of electron density: two bonds to hydrogen atoms and one lone pair. This gives us an \( \mathrm{AX}_{2}E \) system, leading to a bent or angular shape.

Determine Electron Groups on \( \mathrm{CO}_{3}^{2-} \)

In \( \mathrm{CO}_{3}^{2-} \), carbon is bonded to three oxygens, and there are no lone pairs on carbon. The system can be seen as \( \mathrm{AX}_{3} \), resulting in a trigonal planar geometry, due to resonance distributing the negative charge equally.

Determine Electron Groups on \( \mathrm{ICl}_{2}^{-} \)

The \( \mathrm{ICl}_{2}^{-} \) ion has iodine as the central atom with two bond pairs and three lone pairs (\( \mathrm{AX}_{2}E_{3} \)). This results in a linear molecular shape due to the three lone pairs occupying equatorial positions in a trigonal bipyramidal structure.

Determine Electron Groups on \( \mathrm{ICl}_{4}^{-} \)

In \( \mathrm{ICl}_{4}^{-} \), iodine is the central atom bonded to four chlorine atoms with two lone pairs (\( \mathrm{AX}_{4}E_{2} \)). This leads to a square planar shape as the lone pairs are opposite each other, making the chlorines occupy the equatorial plane.

Key concepts

ionic geometry prediction

Ionic geometry prediction is an essential part of understanding the structure of molecules, especially when dealing with ions. Using VSEPR (Valence Shell Electron Pair Repulsion) theory, we can predict how ions will shape up based on the number of electron pairs around the central atom.

VSEPR theory helps us assess the repulsion between electron pairs, both bonding and non-bonding. The aim is to position these pairs as far apart as possible, minimizing repulsion and leading to a specific geometry. This involves:

• Identifying the central atom in the ion.
• Counting the number of electron pairs, including bonds and lone pairs, around the central atom.
• Using the arrangement of these areas of electron density (regions) to determine the ion's shape.

Understanding ionic geometry helps to predict physical and chemical properties. For instance, the ammonium ion \(\mathrm{NH}_{4}^{+}\) has four bonding pairs and adopts a tetrahedral geometry, which influences its interactions and behavior in solutions.

electron pair geometry

Electron pair geometry is a term that describes how electron pairs, including both bonding pairs and lone pairs, are arranged around a central atom. This concept is crucial in predicting the overall shape of the molecule or ion based on the electron pair distribution.

Let's break it down with examples:

• In \(\mathrm{NH}_{4}^{+}\), the nitrogen atom is bonded to four hydrogen atoms (an \(\mathrm{AX}_{4}\) system). There are no lone pairs, pushing the bonds to orient themselves in a tetrahedral shape.
• The \(\mathrm{NH}_{2}^{-}\) has two hydrogen atoms bonded to a central nitrogen and one lone pair (\

molecular shape

Molecular shape is often determined after understanding the geometry of electron pairs around an atom. While electron pair geometry considers all electron densities including lone pairs, molecular shape focuses only on the placement of atoms in space, as perceived after excluding the lone pairs.

Here's how it differs in certain ions:

• For \(\mathrm{NH}_{2}^{-}\), although the electron pair geometry is trigonal planar including the lone pair, the molecular shape considers only the actual hydrogen atoms present, leading to a bent or angular shape.
• The \(\mathrm{ICl}_{2}^{-}\), despite having a trigonal bipyramidal electron pair geometry due to lone pairs, presents a \'linear\' actual molecular shape when considering only the bond pairs.

Helping to distinguish these shapes can provide deeper insights into molecular interactions, reactivity, and function in chemical processes.

tetrahedral geometry

Tetrahedral geometry is one of the fundamental molecular shapes as predicted by VSEPR theory. It arises when there are four regions of electron density around a central atom, all of which are bonding pairs.

A classic example of a tetrahedral geometry is found in the ammonium ion \(\mathrm{NH}_{4}^{+}\). This ion has four hydrogen atoms symmetrically arranged around a central nitrogen atom. The structure is highly stable and is characterized by angles of \(109.5^\circ\), which minimizes repulsion between electron pairs.

Below are characteristics of tetrahedral geometry:

• Four bonding pairs and no lone pairs on the central atom.
• 3D symmetrical shape ensuring even distribution of atoms.
• Commonly found in molecules like methane (\(\mathrm{CH}_4\)).

Understanding tetrahedral geometry allows chemists to predict molecule behavior, such as reactivity and interaction with other molecules in various environments.