Using the VSEPR theory, predict the molecular structure of...

Using the VSEPR theory, predict the molecular structure of each of the following polyatomic ions.
a. sulfate ion, $\mathrm{SO}_{4}{ }^{2-}$
b. phosphate ion, $\mathrm{PO}_{4}^{3-}$
c. ammonium ion, $\mathrm{NH}_{4}^{+}$

Key Concepts

VSEPR Theory

A model used to predict the shapes of molecules and ions by considering the repulsive interactions between electron pairs in the valence shell of the central atom. It assumes that electron pairs, both bonding and lone pairs, will arrange themselves as far apart as possible to minimize these repulsive forces, which ultimately determines the molecular geometry.

Electron Domains

These are regions of electron density around a central atom, including both bonding pairs (shared electrons between atoms) and lone pairs (non-bonding electrons). The number of electron domains predicts the basic electron geometry of a molecule, which is fundamental to understanding its 3D structure.

Molecular Geometry

The observable arrangement of atoms in a molecule, determined by the positions of the bonding electron pairs while accounting for the presence of lone pairs. This geometry is a consequence of the electron domain geometry but may differ when lone pairs occupy more space than bonding pairs.

Tetrahedral Geometry

A specific molecular shape that arises when there are four electron domains around a central atom with no lone pairs present. The electron pairs are arranged at approximately 109.5° to each other, which minimizes repulsion and is characteristic of many polyatomic ions with four bonding groups.

Transcript

00:01 In this question, we are determining the molecular structure of the following polyatomic ions based on vespher theory.

00:07 First, we're looking at the sulfate ion, and our first step is to analyze how many electrons we are working with.

00:15 So sulfur has six valence electrons, and our four oxygens each has six valence electrons.

00:21 Since there's an overall negative two charge on our polyatomic ion, there are an extra two electrons here, because when you have two, extra electrons that's going to create a negative two charge as each electron has a negative charge of one.

00:38 This adds together to give us 32.

00:44 We are going to use sulfur as our central atom and we have four oxygens branching off and we have an overall two minus charge.

00:56 Each of our bonds here has two electrons.

01:00 We have four bonds so that's eight electrons meaning we have 24 left.

01:06 We are going to add our remaining electrons to our oxygens first.

01:14 So that would be 6, 12, 18, and 24.

01:24 Here we have four bonding pairs, meaning that the sulfate ion is tetrahedral because there are no lone pairs.

01:35 Next we are looking at our phosphate ion.

01:38 So phosphorus has five valence electrons.

01:41 And our four oxygens have six valence electrons each, and an overall charge of negative three indicates that there are three excess electrons to give a negative three charge.

01:56 This is going to add to 32...

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