Question

Write a Lewis structure for the orthosilicate anion, \(\mathrm{SiO}_{4}{ }^{4-}\), and deduce the formal charges and oxidation numbers of the atoms. Use the VSEPR model to predict the shape of the ion.

Short answer

To represent the orthosilicate anion, draw Si at the center with four O atoms bonded. Each O has a formal charge of +1, Si has a formal charge of 0, and oxidation numbers are +4 for Si and -2 for O respectively. The VSEPR model predicts a tetrahedral shape for the \(\mathrm{SiO}_{4}^{4-}\) ion.

Step-by-step solution

Counting Total Valence Electrons

Determine the total number of valence electrons available for the orthosilicate anion, \(\mathrm{SiO}_{4}{ }^{4-}\). Silicon has 4 valence electrons and each oxygen atom has 6. Since the molecule has a -4 charge, we add 4 more electrons. Total valence electrons: \(4 \times 6 + 4 + 4 = 32\).

Drawing the Skeleton Structure

Place the least electronegative atom, silicon (Si), in the center with the four oxygen (O) atoms surrounding it. Draw single bonds between the central Si atom and each O atom.

Distributing Electrons

Place the remaining valence electrons around the oxygen atoms while making sure that each oxygen atom has 8 electrons in its outer shell. After forming the single bonds, there are \(32 - 4 \times 2 = 24\) valence electrons left, which are distributed amongst the four oxygen atoms to satisfy the octet rule.

Double-check Octet Rule

Ensure that each oxygen has a full octet. In this case, all oxygen atoms will already have 8 electrons after step 3. The central Si atom will also have a full octet because it has 4 single bonds to O atoms.

Calculating Formal Charges

Calculate the formal charge for each atom. For oxygen (with 6 valence electrons), the formal charge is \(6 - (2 + \frac{6}{2}) = 6 - 5 = +1\). For silicon (with 4 valence electrons), the formal charge is \(4 - (4 + \frac{0}{2}) = 0\). Since there are four such oxygens with a +1 formal charge, and the ion is a -4 charge in total, this satisfies the overall charge of the anion.

Determining Oxidation Numbers

The oxidation number of silicon is typically +4 when bonded to four atoms. Each oxygen has an oxidation number of -2 when it is in a compound (excluding peroxides and superoxides).

Predicting Molecular Geometry Using VSEPR

According to the VSEPR theory, a molecule with four bonding pairs and zero lone pairs on the central atom will have a tetrahedral shape. Thus, the shape of the \(\mathrm{SiO}_{4}^{4-}\) ion is tetrahedral.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in forming chemical bonds. In building the Lewis structure for the orthosilicate anion \(\mathrm{SiO}_{4}^{4-}\), the first step is to count the total number of valence electrons. Silicon, the central atom, contributes four valence electrons, and each oxygen atom contributes six. Since the anion carries a -4 charge, we add four additional electrons, bringing the total to 32 valence electrons. Understanding the distribution of these electrons is key in predicting molecular structures and bonding patterns.

When putting the electrons around oxygen atoms, it's important to ensure that each atom follows the octet rule, meaning that it has eight electrons in its outer shell. This concept is foundational for predicting how atoms bond and the stability of resulting molecules.

VSEPR Model

The Valence Shell Electron Pair Repulsion (VSEPR) model is a principle that helps predict the shape of molecules based on electron pair repulsions. The model states that due to the repulsions between electron pairs in the valence shell, molecular shapes adjust so that these electron pairs are as far apart as possible.

Applying the VSEPR model to the orthosilicate anion, we consider silicon as the central atom surrounded by four bonding pairs of electrons, with no lone pairs. According to VSEPR, this results in a tetrahedral geometry, as the electron pairs maximize their distance from one another in three-dimensional space.

Formal Charges

Calculating formal charges is an essential step in drawing a Lewis structure and assessing the stability of the molecule or ion. The formal charge is the hypothetical charge an atom would have if all bonding electrons were shared equally. It is calculated by taking the number of valence electrons of an atom, subtracting the number of non-bonding electrons, and subtracting half the number of bonding electrons.

In our example, the oxygen atoms in \(\mathrm{SiO}_{4}^{4-}\) would have a formal charge of +1, since they have 6 valence electrons, and are surrounded by 5 shared electrons in bonds. Silicon, which has four valence electrons and is bonded to four oxygen atoms without any non-bonding electrons, would carry a formal charge of 0. These formal charges verify the stability of the Lewis structure and are consistent with the total ionic charge.

Oxidation Numbers

The oxidation number is a concept used in chemistry to keep track of electron transfer in redox reactions. It represents the hypothetical charge an atom would have if the compound was composed of ions. In ionic bonding, the metal (silicon in this case) typically has a positive oxidation number equal to its loss of electrons, and the nonmetal (oxygen) has a negative oxidation number equivalent to its gain of electrons.

For the \(\mathrm{SiO}_{4}^{4-}\) ion, silicon has its typical oxidation number of +4, indicating it has 'lost' four electrons to oxygen atoms. Each oxygen atom has an oxidation number of -2 assuming full ionic character. Keeping track of these numbers helps in understanding redox properties of substances and in balancing chemical reactions.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It's influenced by the electron pairs, both bonding and non-bonding, surrounding the central atom in a molecule. By using the VSEPR model, we can predict the geometry of molecules based on the repulsion between these electron pairs to determine the shape that provides the most stability.

For the orthosilicate anion \(\mathrm{SiO}_{4}^{4-}\), with no lone pairs and four bonding pairs around the silicon atom, a tetrahedral structure is preferred. This shape is characterized by bond angles of approximately 109.5 degrees between each pair of oxygen atoms, optimizing the spatial distribution and minimizing electron pair repulsions. Understanding molecular geometry is fundamental for predicting the properties and reactivity of molecules.