Question

Draw an electron-dot structure for carbon disulfide, \(\mathrm{CS}_{2}\), showing lone pairs and identifying any multiple bonds.

Short answer

CS2 has two double bonds between carbon and each sulfur, with no lone pairs on carbon and 4 lone electrons on each sulfur.

Step-by-step solution

Determine Total Valence Electrons

First, calculate the total number of valence electrons in carbon disulfide (\( \text{CS}_2 \)). Carbon has 4 valence electrons, and each sulfur atom has 6 valence electrons. Therefore, the total number of valence electrons is: \[ 4 + (2 \times 6) = 16 \] electrons.

Arrange Atoms and Form Initial Bonds

Place carbon in the center as it is less electronegative, and arrange sulfur atoms on either side. Connect each sulfur atom to the carbon atom with a single bond. This uses 2 electrons for each bond, totaling 4 electrons. We still have \( 16 - 4 = 12 \) electrons remaining.

Distribute Remaining Electrons to Complete Octets

Start filling the remaining 12 electrons around each sulfur atom to complete their octet. After placing 6 electrons (3 pairs of lone electrons) around each sulfur atom, each sulfur atom will have 8 electrons around it when considering the single bond.

Check and Adjust by Forming Multiple Bonds

Carbon currently only has 4 electrons from the single bonds. To give carbon a full octet, convert a lone pair of electrons on each sulfur atom into a pi bond shared with carbon, forming double bonds. This results in double bonds for both S=C connections. Now, carbon also has an octet configuration with 8 valence electrons.

Finalize and Verify Electron Count

Now, we have two double bonds between carbon and each sulfur, with carbon having no lone pairs and each sulfur having 4 remaining lone electrons. Check: Carbon (4), each sulfur (6), and double bonds (4x2) = total of 16 electrons, which matches the initial count.

Key concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a crucial role in chemical bonding and determine how an atom interacts with others. In the case of carbon disulfide, we first calculate the number of valence electrons present:

• Carbon, a group 14 element, has 4 valence electrons.
• Each sulfur atom, being in group 16, contributes 6 valence electrons.

Adding these gives us a total of 16 valence electrons, which must be represented in the electron-dot structure. Knowing the valence electron count helps us create bonds and fulfill the octet rule, which stabilizes the molecule. Without understanding valence electrons, predicting molecule shapes and reactivity would be nearly impossible.

Multiple Bonds

Multiple bonds occur when two atoms share more than one pair of electrons. They are not as flexible as single bonds but provide extra strength and stability to molecules. In the electron-dot structure of carbon disulfide ( CS_2 ), carbon forms double bonds with each sulfur atom:

• Initially, carbon bonds with each sulfur through a single bond, using 4 electrons in total.
• To satisfy the carbons octet, lone pairs from sulfur are converted into pi bonds, resulting in two double bonds (C=S).

Why do we need multiple bonds?
Multiple bonds are sometimes necessary to fulfill the octet rule for central atoms like carbon. They also influence the molecule's properties like bond length and energy. For instance, double bonds make CS_2 planar and determine its chemical behaviors.

Octet Rule

The octet rule is a chemical guideline which states that atoms tend to form bonds until they have eight electrons in their valence shell, resembling the electron configuration of a noble gas. Let's see how it applies to CS_2:

• Each sulfur atom naturally seeks to complete its octet, which is achieved by sharing electrons in bonds.
• Carbon starts with only 4 electrons from single bonds, not meeting the octet rule initially.

To satisfy this, sulfur atoms share doubly with carbon, moving CS_2 to a structure where every atom involves complete octets.
This rule not only helps predict the stability of molecules but also explains why atoms form specific types of bonds. While there are exceptions, the octet rule provides sound guidelines for drawing electron-dot structures and understanding molecule interactions.