Question

Origins of Life The discovery of polyatomic organic molecules such as cyanamide (H \(_{2} \mathrm{NCN}\) ) in interstellar space has led some scientists to believe that the molecules from which life began on Earth may have come from space. Draw Lewis structures for cyanamide and select the preferred structure on the basis of formal charges.

Short answer

Based on the step-by-step solution provided, draw the Lewis structures for cyanamide (H2NCN) and select the preferred structure. The preferred Lewis structure for cyanamide (H2NCN) is: ``` H - N = C = N \ H ``` This structure is preferred because it has formal charges of 0 on all the atoms, making it more stable than other possible structures.

Step-by-step solution

Determine the total number of valence electrons.

To determine the total number of valence electrons in the cyanamide molecule, we look at the number of valence electrons each individual atom possesses. Here is the breakdown: - Hydrogen (H): 1 valence electron - Nitrogen (N): 5 valence electrons - Carbon (C): 4 valence electrons In the cyanamide (H2NCN) molecule, there are two hydrogen atoms, one nitrogen atom, and one carbon atom. So, the total number of valence electrons is: (2 × 1) + 1 × 5 + 1 × 4 = 2 + 5 + 4 = 11 valence electrons.

Draw the skeleton structure for the molecule.

Based on the molecular formula, we can start by placing one nitrogen atom in the center, then bond it with two hydrogen atoms and one carbon atom. The carbon atom will then form a triple bond with another nitrogen atom to satisfy its valence electrons requirement. The skeletal structure will look like this: ``` H - N - C ≡ N \ H ```

Distribute the electrons around the atoms.

Now, we will distribute the remaining electrons around the atoms to complete the octet or duet requirement: - Nitrogen (central) has 3 bonds; thus, it uses 3 × 2 = 6 valence electrons, leaving it with 5 - 6 = -1 electrons left to have an octet arrangement. - Carbon has a triple bond with another nitrogen atom, utilizing 3 × 2 = 6 valence electrons, and one bond with the central nitrogen atom, using 1 × 2 = 2 valence electrons, making a total of 6 + 2 = 8 valence electrons, satisfying the octet. - Nitrogen (end) has a triple bond with a carbon atom, utilizing 3 × 2 = 6 valence electrons, leaving it with 5 - 6 = -1 electrons left to maintain the octet arrangement.

Calculate the formal charges for each atom.

To calculate the formal charges of each atom, we can use the formula: Formal Charge = Valence Electrons - Non-bonding Electrons - 1/2 Bonding Electrons Here's the calculation for each atom in our current structure: - Hydrogen (each): Formal Charge = 1 - 0 - 1/2(2) = 0 - Central Nitrogen: Formal Charge = 5 - 0 - 1/2(6) = +1 - Carbon: Formal Charge = 4 - 0 - 1/2(8) = 0 - End Nitrogen: Formal Charge = 5 - 0 - 1/2(6) = -1 Now, we can observe that the current structure has a formal charge of +1 on the central nitrogen and -1 on the end nitrogen. To make the molecule more stable, we should try to get the formal charges as close to zero as possible.

Adjust the structure to minimize the formal charges.

To minimize the formal charges, we will change the triple bond between carbon and nitrogen to a double bond and move one lone pair from the end nitrogen to the central nitrogen. ``` H - N = C = N \ H ``` Now, let's recalculate the formal charges: - Central Nitrogen: Formal Charge = 5 - 2 - 1/2(6) = 0 - Carbon: Formal Charge = 4 - 0 - 1/2(6) = 0 - End Nitrogen: Formal Charge = 5 - 4 - 1/2(4) = 0

Choose the preferred Lewis structure.

Comparing the two structures, the second structure has formal charges of 0 on all the atoms, making it more stable than the first structure. Therefore, the preferred Lewis structure for cyanamide (H2NCN) is: ``` H - N = C = N \ H ```

Key concepts

Valence Electrons

Understanding valence electrons is crucial when drawing Lewis structures. Valence electrons are the outermost electrons in an atom and they determine the atom's ability to form bonds. These electrons are found in the outermost shell and are responsible for chemical properties. In Lewis structure formation, we focus on these electrons because:

• They are the ones involved in bonding.
• They help us understand the connectivity and interaction among atoms.

To correctly depict a molecule's structure, we count the total number of valence electrons, considering each atom's contribution. For cyanamide (H\(_{2}\)NCN), we calculated:

• Hydrogen: 1 valence electron per atom, totaling 2.
• Nitrogen: 5 valence electrons.
• Carbon: 4 valence electrons.

This sums up to 11 valence electrons to use in the Lewis structure, helping ensure all atoms form the necessary bonds while meeting rules like the octet rule.

Formal Charge

When drawing Lewis structures, understanding the formal charge helps us predict the most stable arrangement of atoms in a molecule. The formal charge is determined using the formula: \[\text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{1}{2} \text{Bonding Electrons}\]This charge indicates how electrons are distributed and helps in identifying potential adjustments needed to achieve a stable and chemically correct structure.For cyanamide, we initially computed that:

• Each hydrogen had a formal charge of 0.
• The central nitrogen had a formal charge of +1.
• Carbon had a formal charge of 0.
• The end nitrogen had a formal charge of -1.

To achieve a more stable molecule, ideally, the formal charges should be as close to zero as possible for each atom. By adjusting the bonds, for instance, from a triple bond to a double bond, we can alter the electron distribution, resulting in a formal charge of 0 on all atoms, suggesting a more stable molecule.

Octet Rule

The octet rule is a fundamental guideline in chemistry, explaining how atoms achieve stability through their electronic configurations. This rule posits that atoms gain, lose, or share electrons to reach a total of eight electrons in their outer shell, mimicking the electron configuration of noble gases. While drawing Lewis structures, fulfilling the octet rule ensures each atom achieves this stable electronic state.

• Hydrogen is an exception, striving for a duet - a total of two electrons.
• Carbon, nitrogen, and other main-group elements aim for an octet.

During the construction of cyanamide’s Lewis structure, we ensured that carbon and nitrogen atoms obeyed this rule by forming appropriate numbers of bonds. Initially, carbon formed a triple bond with nitrogen and an additional bond with another nitrogen to complete its octet. Adjusting these bonds to satisfy the octet without disrupting other constraints like formal charge optimization helped confirm the stability and correctness of the Lewis structure. Ultimately, a stable arrangement satisfied both the octet rule and the reduction of formal charges to zero.