Question

Methyl isocyanate, \(\mathrm{CH}_{3} \mathrm{NCO},\) is used to make certain pesticides. In December \(1984,\) water leaked into a tank containing this substance at a chemical plant to produce a toxic cloud that killed thousands of people in Bhopal, India. Draw Lewis structures for this compound, showing formal charges.

Short answer

The Lewis structure of the Methyl Isocyanate, CH3NCO, shows that the Carbon and Nitrogen have a negative formal charge and Oxygen has a positive formal charge. The Lewis structure has Nitrogen at the center, Carbon on one side and Oxygen on the other.

Step-by-step solution

Count the Valence Electrons

The total number of valence electrons in CH3NCO is calculated by adding the valence electrons of each atom. Carbon (C) has 4, Hydrogen (H) has 1 (but, there are three Hydrogens, so multiply by 3), Nitrogen (N) has 5, and Oxygen (O) has 6. The atom of concern here is the group 'NCO', because the group 'CH3' is a common structure that will be added in the last step. So we only count the Valence electrons from NCO, i.e. \(4+5+6=15\) valence electrons.

Arrange the Atoms

Arrange the atoms with Nitrogen in the center as it has a higher atomic number, with one Carbon and one Oxygen each on either side.

Establish a Skeleton structure

Connect the atom with the least number of valence electrons (that is Carbon) to the central atom (which is Nitrogen) with a single bond that is represented as a straight line. Do the same with Nitrogen and Oxygen.

Fill the Octets

Try to make all atoms (except Hydrogen) satisfy the Octet rule by using the remaining electrons. This means they must have 8 electrons in their valence shell. Each bond has 2 electrons, and hence, to fulfil the octet rule, an atom needs to have 4 bonds. Single electron pairs, double or triple bonds can be used to accomplish this.

Calculate the Formal Charge

The formula for calculating the formal charge of an atom in a molecule is 'FC = Valence e- - Nonbonding e- - Bonding e- / 2'. Calculate the formal charge for each atom to verify the structure.

Complete the Structure

Finally, add the 'CH3' group and any lone pairs of electrons to complete the structure.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are critical in determining how an atom can chemically bond with others. In the construction of Lewis structures, we first need to count the valence electrons from each atom in the molecule.

For example, in methyl isocyanate (\( \text{CH}_3\text{NCO} \)), we calculate valence electrons by adding the contributions of Carbon (4 valence electrons), Hydrogen (1 each but we have three, so that's 3 in total), Nitrogen (5), and Oxygen (6). It's essential to get this initial count correct, as it forms the basis for constructing the Lewis structure and ensuring that each atom achieves a stable electron configuration.

Formal Charge

The formal charge on an atom is calculated to predict the most stable Lewis structure. It is given by the formula: FC = (Valence electrons) - (Nonbonding electrons) - (Bonding electrons / 2).

In the given solution for methyl isocyanate, after creating the skeletal structure and filling the octets, we can calculate the formal charge to confirm that the structure is stable and represents the actual distribution of electrons. The aim is to have the formal charges as close to zero as possible, which generally corresponds to the most stable structure. Formal charge is a crucial tool for assessing the correctness of a proposed Lewis structure.

Molecular Geometry

Molecular geometry refers to the 3-dimensional shape a molecule takes based on the spatial arrangement of its atoms. VSEPR theory (Valence Shell Electron Pair Repulsion) is often used to predict this structure, which is influenced by the electron pairs surrounding the central atoms.

In the case of methyl isocyanate, understanding molecular geometry would require us to visualize how the atoms are spaced out after drawing the Lewis structure. The shape of the molecule significantly affects its physical and chemical properties. Recognizing the molecular geometry is therefore vital for understanding the reactivity and interactions of the molecule.

Octet Rule

The octet rule is a guideline that atoms tend to combine in such a way that they each have eight electrons in their valence shell, obtaining a noble gas configuration. This is a fundamental principle used when drawing Lewis structures.

Hydrogen is an exception since it's stable with two electrons. When constructing the Lewis structure for methyl isocyanate, every step aims to ensure that Carbon, Nitrogen, and Oxygen fulfill the octet rule, which leads to a stable molecule. However, the rule is sometimes violated for molecules with elements that have d-orbitals, allowing for an expanded octet.