Question

Each of the following molecules contains at least one multiple (double or triple) covalent bond. Give a plausible Lewis structure for (a) \(\mathrm{OCS} ;\) (b) \(\mathrm{CH}_{3} \mathrm{CHO}\) (c) \(\mathrm{F}_{2} \mathrm{CO} ;\) (d) \(\mathrm{Cl}_{2} \mathrm{SO} ;\) (e) \(\mathrm{C}_{2} \mathrm{H}_{2}\).

Short answer

Lewis Structures are: (a) OCS = O = C = S, (b) CH3CHO = H - C - H - C = O - H , (c) F2CO = F - C = O - F , (d) Cl2SO = Cl - S = O - Cl , (e) C2H2 = H - C ≡ C - H.

Step-by-step solution

– Calculate Total Valence Electrons

Calculate total valence electrons in each molecule using the periodic table. Here's the total number of valence electrons: (a) OCS : \(1*6 (O) + 1*4 (C) + 1*6 (S) = 16\) (b) CH3CHO : \(3 * 1(H) + 1 * 4 (C) + 1 * 6 (O) + 1 * 4 (C) + 1 *1 (H) = 14\) (c) F2CO : \(2 * 7 (F) + 1 * 6 (O) + 1 * 4 (C) =24\) (d) Cl2SO: \(2 * 7 (Cl) + 1 * 6 (O) + 1 * 6(S) = 26\) (e) C2H2: \(2 * 4(C) + 2 * 1 (H) = 10\)

– Arrange Atoms and Place Electrons

The central atom is the one with the least willingness to attract electrons or the least electronegative. In each molecule, distribute the electrons and arrange the atoms in such a way that each atom satisfies the octet rule, keeping in mind the multiple bonds.

– Draw Lewis Structure

(a) OCS: O = C = S, as C is central atom with double bonds with both O and S. (b) CH3CHO: H - C - H - C = O with another H bond with the first C atom. (c) F2CO: F - C = O - F with double bond between C and O. (d) Cl2SO: Cl - S = O - Cl with double bond between S and O. (e) C2H2: H - C ≡ C - H, where C atoms are connected with triple bond. The generated structures fulfill the octet rule, and it's assumed all molecules are neutrally charged.

Key concepts

Valence Electrons

Every atom in the periodic table comes with its own set of valence electrons that are used to form chemical bonds. Valence electrons are the outermost electrons in an atom and determine an atom's bonding potential.
They are key players in chemical reactions, as they are the ones that atoms will either share, lose, or gain to form stable molecules.
To find the number of valence electrons, one can refer to the group number in the periodic table for main-group elements.

• For example, oxygen (O) is in group 16 and has 6 valence electrons.
• Carbon (C) is in group 14, so it has 4 valence electrons.
• Sulfur (S) and chlorine (Cl) also have 6 and 7 valence electrons respectively, being in groups 16 and 17.

Knowing the count of valence electrons for each atom in a molecule is crucial for drawing its Lewis structure.
It helps in determining how the atoms are linked together through bonds, which can be either single, double, or triple bonds.

Covalent Bonds

Covalent bonds form when two atoms share one or more pairs of valence electrons to achieve a more stable arrangement.
These bonds are common in organic compounds and are essential for creating the molecules found in living organisms. Covalent bonds can be single, double, or triple bonds:

• In a single covalent bond, two atoms share one pair of electrons.
• In a double bond, as seen between carbon and oxygen in OCS and CH3CHO, four electrons are shared between two atoms.
• Triple bonds involve sharing six electrons, exemplified by the carbon-carbon bond in C2H2.

Forming covalent bonds allows atoms to come close enough to share valence electrons, satisfying each atom's desire to mimic the electron configuration of the nearest noble gas.
This results in a stable molecule with electrons being shared rather than transferred, as is the case with ionic bonds.

Octet Rule

The Octet Rule is a guiding principle useful for understanding how atoms bond by looking for electron configurations similar to the noble gases.
These configurations are very stable and are often characterized by eight electrons in the outer electron shell.
When drawing Lewis structures, this rule helps predict the arrangements of atoms and bonds in molecules. There are a few key insights:

• Atoms aim to have 8 valence electrons through sharing, gaining, or losing electrons.
• Hydrogen is an exception, as it only aims to fill its shell with 2 electrons, achieving the configuration of helium.
• For molecules like CH3CHO and C2H2, the atoms share electrons such that each achieves an ideal and stable electron configuration.
• This explains the use of multiple bonds, like double or triple bonds, to ensure atoms satisfy the octet rule.

While the octet rule is not universal (beyond the first row elements), it is a useful tool for predicting and explaining molecule stability and structure.
Understanding this concept makes it easier to predict molecular geometry and properties.