Problem 87 - Chemistry The Science in Context

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.

Step-by-Step Solution

Answer

Answer: The preferred Lewis structure for cyanamide based on formal charges is H-N=C=N-H (structure 1).

Step 1: Calculate the total number of valence electrons

Calculating the total number of valence electrons helps us determine how electrons can be distributed in our Lewis structures. To do this, determine the number of valence electrons in each atom and add them up. In cyanamide, we have the following atoms: - Hydrogen (H): 1 valence electron (2 atoms) - Nitrogen (N): 5 valence electrons (2 atoms) - Carbon (C): 4 valence electrons (1 atom) Total valence electrons = 2(1) + 2(5) + 4 = 2 + 10 + 4 = 16 valence electrons.

Step 2: Draw possible Lewis structures for cyanamide

Using the total number of 16 valence electrons, we can create possible Lewis structures for cyanamide by connecting the atoms with single, double, or triple bonds while respecting the octet rule. Possible structures are as follows: Structure 1: H-N=C=N-H Structure 2: H-N-C≡N-H

Step 3: Calculate the formal charges for each structure

The formal charge of an atom can be calculated using the following formula: Formal charge = (Valence electrons) - (Non-bonding electrons + 1/2(shared electrons)) For structure 1: Formal charge of N1 = 5 - (2 + 1/2(6)) = 0 Formal charge of C = 4 - (0 + 1/2(8)) = 0 Formal charge of N2 = 5 - (4 + 1/2(2)) = 0 For structure 2: Formal charge of N1 = 5 - (4 + 1/2(4)) = +1 Formal charge of C = 4 - (0 + 1/2(8)) = 0 Formal charge of N2 = 5 - (2 + 1/2(6)) = -1

Step 4: Select the preferred structure based on formal charges

The most stable or preferred structure is the one with formal charges closer to zero. Comparing the two structures, we can see that the formal charges in structure 1 are all zero, while in structure 2, the N1 and N2 atoms have formal charges of +1 and -1, respectively. Hence, the preferred Lewis structure for cyanamide based on formal charges is: H-N=C=N-H (structure 1)

Key Concepts

Valence Electrons

Valence electrons play a crucial role in drawing Lewis structures. These are the electrons found in the outermost shell of an atom. They are key players when it comes to chemical bonding, as they are the ones involved in forming bonds between atoms. To count valence electrons, simply look at an atom's position in the periodic table. For example, carbon has 4 valence electrons because it sits in group 4. Nitrogen, being in group 5, has 5 valence electrons, while hydrogen, found in group 1, has just 1.

When drawing Lewis structures, it is important to first sum up the total number of valence electrons. For cyanamide (H \(_2\)NCN), adding the contributions from each atom gives us a total of 16 valence electrons:

• 2 from the two hydrogen atoms (1 each)
• 10 from the two nitrogen atoms (5 each)
• 4 from the carbon atom

Once you know the total number, you are ready to start arranging the electrons into a structure that reflects how these atoms are bonded.

Formal Charge

Formal charge is really helpful when deciding which Lewis structure is more likely to be correct. It lets you see the distribution of charge across the molecule. A good rule of thumb is that the preferred structure is the one with formal charges as close to zero as possible. This means a more stable configuration.

The formula to calculate the formal charge of an atom is:\[\text{Formal charge} = \left( \text{Valence electrons} \right) - \left( \text{Non-bonding electrons} + \frac{1}{2}\times \text{shared electrons} \right)\]
For example, in a possible cyanamide structure, if nitrogen is double-bonded to carbon and has a lone pair, the formal charge is calculated to ensure total balance. By systematically assigning these charges and adding them, you can easily compare different possibilities. The structure where each atom approaches a formal charge of zero or where charges are optimally reduced is generally the better choice, as it tends to yield a stable molecule.

Octet Rule

The octet rule is a guiding principle in valence electron theory that helps predict how atoms will bond in a molecule. Atoms tend to share, lose, or gain electrons until they have 8, mimicking the electron configuration of a noble gas, which is known for its stability.

Here's how the octet rule works for cyanamide:

• Hydrogen atoms are satisfied with just 2 electrons, adhering to a simplified version of the octet rule (the duet rule).
• Carbon likes to make four bonds, achieving an octet by sharing its four valence electrons with others.
• Nitrogen in cyanamide strives to have 8 electrons around it through sharing electrons with carbon and itself.

This rule must be respected when drawing Lewis structures. Although there are exceptions, especially involving elements beyond the second period, Lewis structures aim to follow the octet rule as closely as possible. It maintains stability, giving us clues on how atoms connect in molecules like cyanamide, where a preferred arrangement satisfies the octet requirement in the most stable configuration.