Question

Write the electron dot formula and draw the structural formula for the silicate ion, \(\mathrm{SiO}_{3}^{2-},\) whose central atom is a semimetal.

Short answer

Draw the silicate ion with silicon at the center, single bonds to two oxygens and a double bond to one oxygen to minimize formal charges.

Step-by-step solution

Understanding the Silicate Ion

The silicate ion, \(\mathrm{SiO}_{3}^{2-}\), consists of a silicon (Si) atom as the central atom bonded to three oxygen (O) atoms. The entire ion carries a \(2^-\) charge. Silicon is in group 14 and has typically \(4\) valence electrons, and each oxygen typically has \(6\) valence electrons.

Determining Total Valence Electrons

Calculate the total number of valence electrons in the ion. Silicon provides \(4\) electrons, each oxygen provides \(6\) electrons (for three oxygens, that's \(3 imes 6 = 18\) electrons), and the \(2^-\) charge adds \(2\) more electrons. Thus, the total is \[4 + 18 + 2 = 24\] valence electrons.

Drawing the Electron Dot Formula

Distribute the valence electrons around the silicon and oxygen atoms to visualize the bonds. Start by drawing a single bond (2 electrons per bond) between silicon and each oxygen. After making three Si-O bonds, \(6\) of the \(24\) valence electrons are used. Distribute the remaining \(18\) electrons to fulfill the oxygen atoms' octets (remember to prioritize completing the octet rule for oxygen). Any additional electrons are placed on the central atom or considered as lone pairs.

Finding Formal Charges

To ensure each atom's formal charge leads to the most stable ion form, assign each oxygen atom \(2\) non-bonding lone pairs and calculate formal charges: \(\text{Formal charge} = \text{valence electrons} - (\text{non-bonding electrons} + \frac{1}{2}\text{bonding electrons})\). Adjust bond orders if needed to minimize charge.

Drawing the Structural Formula

Conclude with the structural formula: place the silicon in the center with an oxygen atom bonded to it, forming a triangle in planar geometry. If double bonds are formed to satisfy formal charge constraints, they must be depicted as double lines between Si and O. An example configuration would be a Si bonded to two oxygen atoms by single bonds and one by a double bond.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in bonding and chemical reactions. For the elements involved in the silicate ion,

• Silicon (Si) has 4 valence electrons, being in group 14 of the periodic table.
• Oxygen (O), found in group 16, carries 6 valence electrons.

In a compound like the \(\mathrm{SiO}_{3}^{2-}\), these valence electrons are shared or transferred between atoms to form bonds.
The total count of valence electrons is fundamental, as these electrons are involved in defining the structure of the ion. Here, by summing up the valence electrons from all component atoms and factoring in the ion's charge, we get a total of 24 valence electrons: 4 from Si, 18 from the three O atoms, and 2 additional electrons due to the ion's 2- charge.
Recognizing the valence electrons helps in predicting how atoms in a molecule will bond together, ensuring that each atom achieves a stable electron arrangement.

Formal Charge

Formal charge is a concept used to determine the apparent charge on each atom within a molecule or ion, ensuring stability and neutrality in structures. Calculating formal charges involves using the formula:\[\text{Formal Charge} = \text{Valence Electrons} - (\text{Non-bonding Electrons} + \frac{1}{2}\text{Bonding Electrons})\]This calculation helps to find the most stable arrangement of atoms in a molecule or ion.
For each oxygen in the \(\mathrm{SiO}_{3}^{2-}\) ion, practicing this involves assigning two lone pairs (4 non-bonding electrons) and determining the formal charge based on single or double bonds with silicon. The ideal structure has minimal formal charges, helping attain a stable configuration.
In the most stable structural formula, the sum of individual formal charges equals the charge of the molecule or ion. This practice not only aids in structural predictability but also validates the stability of the molecular geometry chosen.

Octet Rule

The octet rule is paramount in balancing atoms in compounds and ions. It states that atoms tend to adjust their electron arrangements to have 8 electrons in their valence shell, mimicking the electron configuration of noble gases.
This rule gives insight into the bonding patterns and electron distribution, especially for nonmetals like oxygen.

• Each oxygen atom in the \(\mathrm{SiO}_{3}^{2-}\) wants to achieve an octet, prompting bonding with silicon.
• Despite silicon's tendency to also follow the octet rule, it occasionally accommodates more than 8 electrons due to its central position and ability to expand its valence shell.

Adhering to the octet rule ensures that atoms within molecules like the silicate ion are in their most stable electron configuration. This rule simplifies predicting the bonding patterns, revealing why certain atoms form specific types and numbers of bonds.