Problem 104 - Introductory Chemistry Atoms First

Question

Hydrogen isothiocyanate has formula HNCS, and its atoms are connected in the order written. Draw dot diagrams showing all the valid resonance forms.

Step-by-Step Solution

Answer

The valid resonance forms for hydrogen isothiocyanate (HNCS) are: \(1. H: :N-C-S: :\) \\ \(2. H: :N=C-S: :\) \\ \(3. H: :N-C=S: :\)

Step 1: Determine total valence electrons

We first need to determine the total number of valence electrons in the HNCS molecule. Hydrogen has 1 valence electron, nitrogen has 5, carbon has 4, and sulfur has 6. Therefore, the total number of valence electrons is: 1 (hydrogen) + 5 (nitrogen) + 4 (carbon) + 6 (sulfur) = 16 valence electrons

Step 2: Choose a central atom

The hydrogen atom cannot be the central atom since it can only form one bond. Between nitrogen, carbon, and sulfur, carbon is the least electronegative, making it a suitable central atom. Therefore, the molecule will have a structure of H-N-C-S.

Step 3: Draw single bonds

Now, we will draw single bonds between the central carbon atom and each surrounding atom (nitrogen and sulfur). The hydrogen atom is only attached to the nitrogen atom. Each of these single bonds represents two electrons: H :\ N-C-S So far, we have utilized 6 of the 16 valence electrons.

Step 4: Complete the octet

We will now complete the octet for nitrogen and sulfur by adding lone pairs of electrons. Remember to exclude the hydrogen atom because it only requires two electrons to achieve an electron configuration similar to helium. H :: N-C-S: : Here, nitrogen has an octet, and sulfur has 6 electrons. 2 more electrons are needed to complete the octet. In this case, since sulfur can accommodate an expanded octet, we can form a double bond between carbon and sulfur.

Step 5: Check for resonance structures

Now that we have completed the octet rule for all atoms, let's check if there are any additional valid resonance structures. H :: N=C=S: : We can additionally create a double bond between nitrogen and carbon, forming another resonance structure: H :: N=C-S: : Summarizing, the valid resonance forms for HNCS are: \(1. H: :N-C-S: :\) \\ \(2. H: :N=C-S: :\) \\ \(3. H: :N-C=S: :\)

Key Concepts

Lewis dot diagrams

Lewis dot diagrams are a simple way to represent the valence electrons around atoms in a molecule. They help visualize the bonding between atoms and any lone pairs of electrons that may exist. In these diagrams, dots are used to symbolize electrons.

To draw a Lewis dot diagram:

• Write the chemical symbol of each atom.
• Place dots around the symbol to represent valence electrons.
• Arrange the dots to show the electrons involved in bonding and those that remain as lone pairs.

For our example, HNCS, the sequence of atoms determined by the bond order tells us how to layout the Lewis dot structure. Each atom tries to share or pair its valence electrons to form a stable configuration, mimicking having a complete set of electrons in their outermost layer.

Valence electrons

Valence electrons are the electrons located in the outermost shell of an atom. These electrons play a crucial role in chemical bonding, as they are the ones involved in the formation of bonds between atoms.

When determining the number of valence electrons in a molecule, like HNCS, each atom contributes its electrons, which need to be counted:

• Hydrogen has 1 valence electron.
• Nitrogen has 5 valence electrons.
• Carbon has 4 valence electrons.
• Sulfur has 6 valence electrons.

Altogether, the molecule contains 16 valence electrons. Valence electrons are crucial for satisfying the octet rule in bonding by either sharing or transferring between atoms.

Octet rule

The octet rule is a key concept in understanding chemical bonding. It states that atoms tend to form bonds so that each atom has eight electrons in its valence shell, achieving a stable electron configuration similar to noble gases.

In most molecules, except for hydrogen, atoms seek to satisfy the octet rule. Here's how it works for HNCS:

• Hydrogen needs only 2 electrons because it's following the duet rule (similar to helium).
• Nitrogen, carbon, and sulfur aim for 8 valence electrons. Nitrogen and sulfur have lone pairs, and through bonding, achieve their octet.

In some cases, like with sulfur, atoms can accommodate an expanded octet due to the availability of d-orbitals. This permits additional electrons, helpful in forming resonance structures.

Chemical bonding

Chemical bonding involves the joining of atoms to form molecules and compounds. This occurs through the interaction of valence electrons leading to different types of bonds.

In HNCS, chemical bonds can be identified by:

• Single bonds, representing two shared electrons between atoms.
• Double bonds, involving four shared electrons, as seen in some resonance structures.

The formation of these bonds is driven by the desire to fill each atom's valence shell, adhering to the octet rule. Bonds can be formed by sharing electrons (covalent bonds) or exchanging them, leading to the stability and formation of molecules. In resonance structures, chemical bonding configurations can shift, illustrating the double and single bond rearrangements among atoms.