Problem 85 - Chemistry The Science in Context

Question

Hydrogen isocyanide (HNC) has the same elemental composition as hydrogen cyanide (HCN), but the H atom in HNC is bonded to the nitrogen atom. Draw a Lewis structure for HNC and assign formal charges to each atom. How do the formal charges on the atoms differ in the Lewis structures for HCN and HNC?

Step-by-Step Solution

Answer

Answer: In the Lewis structure of hydrogen cyanide (HCN), the formal charges for hydrogen, carbon, and nitrogen are all 0. In the Lewis structure of hydrogen isocyanide (HNC), the formal charges for hydrogen, nitrogen, and carbon are 0, -2, and +2, respectively. So, HCN has no formal charges, while HNC has a -2 charge on nitrogen and a +2 charge on carbon.

Step 1: Determine the total number of valence electrons

Find the total number of valence electrons for all the atoms in the HNC molecule. Hydrogen has 1 valence electron, nitrogen has 5, and carbon has 4. Thus, the total number of valence electrons in HNC is 1+5+4=10.

Step 2: Draw a skeleton structure for HNC

First, connect the nitrogen atom to the hydrogen atom with a single bond, and then connect the nitrogen atom to the carbon atom with a single bond. The skeleton structure for HNC looks like this: H-N-C.

Step 3: Distribute the remaining valence electrons

We have already used 4 valence electrons to form the two single bonds. There are 6 more valence electrons left to distribute (10−4). Add 3 lone pairs to the nitrogen atom (6 electrons) to complete its octet. The Lewis structure for HNC is now: H-N≡C.

Step 4: Calculate the formal charges for each atom in HNC

Formal charge = (Number of valence electrons in the free atom) - (Number of electrons assigned to the atom in the molecule) - For Hydrogen: Formal charge = 1 - (1 bonding electron) = 0 - For Nitrogen: Formal charge = 5 - (1 bonding electron + 6 electrons in the triple bond) = 5 - 7 = -2 - For Carbon: Formal charge = 4 - (6 electrons in the triple bond) = 4 - 6 = +2 The formal charges for the atoms in HNC are 0, -2, and +2 for hydrogen, nitrogen, and carbon, respectively.

Step 5: Compare the formal charges in the Lewis structures for HCN and HNC

In HCN, the Lewis structure is H-C≡N. The formal charges for hydrogen, carbon, and nitrogen are 0, 0, and 0, respectively. In HNC, as we calculated, the formal charges for hydrogen, nitrogen, and carbon are 0, -2, and +2, respectively. Thus, the formal charges in the Lewis structures for HCN and HNC differ as follows: HCN has no formal charges, while HNC has a -2 charge on nitrogen and a +2 charge on carbon.

Key Concepts

Formal Charges

Formal charges are important for understanding how electrons are distributed around atoms within a molecule. They help predict the most stable structure and identify any charges on atoms. To calculate the formal charge for an atom, subtract the number of electrons assigned to the atom from the number of valence electrons the atom has in its free state.
For example, in hydrogen isocyanide (HNC), the nitrogen atom has a formal charge of -2. This is because nitrogen initially has 5 valence electrons, but it uses 7 electrons in the molecule due to its participation in bonding and lone pairs.

• Hydrogen's formal charge is 0, as it donates 1 electron and is involved in 1 bond.
• Nitrogen's formal charge is calculated by subtracting the number of electrons involved in bonds and lone pairs from the number of initial valence electrons.
• Carbon's formal charge in HNC becomes +2 because it has 4 valence electrons and shares 6 electrons in a triple bond.

It's crucial to compare with HCN, where all atoms have a formal charge of 0 due to a balanced distribution of electrons.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in forming chemical bonds. They determine the chemical reactivity of an element. The ability to draw Lewis structures depends on understanding how these electrons are shared or transferred.
To find the total number of valence electrons in a molecule like HNC, add up the valence electrons for each atom:

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

This gives HNC a total of 10 valence electrons. During bond formation, some of these electrons are shared, creating bonds and potentially leaving other valence electrons as lone pairs. Knowing the count of valence electrons helps in drawing the Lewis structure and predicting molecular behavior.

Molecular Geometry

Molecular geometry refers to the 3D arrangement of atoms in a molecule. While the Lewis structure gives an idea of connectivity, geometry provides insight into the molecule's shape, affecting its physical and chemical properties.
In HNC, nitrogen forms a triple bond with carbon and a single bond with hydrogen. This creates a linear molecular geometry. Linear molecules have bond angles of approximately 180 degrees and are often determined by the presence of triple bonds or certain chemical arrangements. Observing this geometry is essential to understand the molecule's polarity, reactivity, and how it might interact with other substances.