Question

Draw a Lewis structure for the following molecules: (a) Nitrous acid, \(\mathrm{HNO}_{2}(\mathrm{H}\) is bonded to an \(\mathrm{O}\) atom \()\) (b) Sulfur trioxide, \(\mathrm{SO}_{3}\) (c) Ethanal, \(\mathrm{CH}_{3} \mathrm{CHO}\)

Short answer

Nitrous acid: H-O-N=O. Sulfur trioxide: O=S=O with double bonds. Ethanal: CH₃-CHO with C=O bond.

Step-by-step solution

Determine Total Valence Electrons

For each molecule, tally the total number of valence electrons available, accounting for all atoms in the molecule. - **Nitrous acid** (HNO₂): - **H**: 1 valence electron - **N**: 5 valence electrons - **O** (2 atoms): 6 valence electrons each Total = 1 + 5 + (2 * 6) = **18 valence electrons** - **Sulfur trioxide** (SO₃): - **S**: 6 valence electrons - **O** (3 atoms): 6 valence electrons each Total = 6 + (3 * 6) = **24 valence electrons** - **Ethanal** (CH₃CHO): - **C** (2 atoms): 4 valence electrons each - **H** (4 atoms): 1 valence electron each - **O**: 6 valence electrons Total = (2 * 4) + (4 * 1) + 6 = **20 valence electrons**.

Arrange Atoms and Connect with Single Bonds

Place the central atom(s) and surround with other atoms. Start by connecting atoms with single bonds. - **Nitrous acid** (HNO₂): - Central atom: Nitrogen (N) - Connect N to one O and then to another O, with H bonded to one of the O atoms: H-O-N-O - **Sulfur trioxide** (SO₃): - Central atom: Sulfur (S) - Connect S to each O: O-S-OO - **Ethanal** (CH₃CHO): - Chain of atoms begins with C, connect one C to another, then to an O, and branch a H off each C: CH₃-CH=O (indicating single bonds and one double bond).

Distribute Remaining Electrons to Fulfill Octet/Pair

Distribute the rest of the valence electrons to satisfy the octet rule (8 electrons per atom, except for H, which requires 2). - **Nitrous acid** (HNO₂): - Oxygen (O) needs 8 electrons; place remaining electrons around it. Ensure one oxygen forms a double bond with N to fulfill octet. - **Sulfur trioxide** (SO₃): - Distribute remaining electrons around each oxygen. Since S can expand the octet, create double bonds with oxygen to stabilize. - **Ethanal** (CH₃CHO): - Place remaining electrons on oxygen. Double bond the carbonyl group (C=O) to satisfy octet for C and O.

Adjust If Necessary for Stability and Lone Pairs

Ensure all atoms have complete valence shells. Adjust bonds to satisfy octet rule where needed. Look for formation of double bonds if required. - **Nitrous acid**: Form the structure H-O-N=O with one oxygen singly bonded to N (carrying H) and one doubly bonded. - **Sulfur trioxide**: Double bond S with each O atom, forming O=S=O symmetry. - **Ethanal**: Structure = H-C(H)-C(=O)-H, ensuring oxygen double bonds to its carbon.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons in an atom. They play a crucial role in chemical bonding. For a molecule, finding the total number of valence electrons is the first step in drawing the Lewis structure. Valence electrons determine how atoms interact and bond with each other. To calculate the total, you add up the valence electrons from each atom in the molecule. This versatility is essential because it guides the subsequent steps in arranging atoms and drawing bonds.

• The number of valence electrons for an element is often equal to its group number in the periodic table.
• Elements strive to achieve a stable electronic configuration, typically like that of the nearest noble gas.

In molecules like Nitrous acid (HNO₂), Sulfur trioxide (SO₃), and Ethanal (CH₃CHO), recognizing the total valence electrons provides a foundation to predict how they form bonds.

Octet Rule

The octet rule is a principle that atoms tend to bond in such a way that they each achieve an octet of electrons in their valence shell. This stable configuration is reminiscent of the noble gases. Generally, atoms form bonds to reach a full set of eight valence electrons.

• Hydrogen is an exception; it seeks only two electrons.
• Some elements like sulfur can expand beyond an octet, especially when in the third period or beyond.

In drawing Lewis structures, fulfilling the octet rule for each atom is a key focus. For example, in Ethanal, carbon and oxygen need to achieve an octet, leading to the formation of double bonds between carbon and oxygen.

Chemical Bonds

Chemical bonds are connections between atoms due to the sharing or transfer of valence electrons. The primary types evident in Lewis structures are covalent bonds, where electrons are shared between atoms.

• Single bonds involve one pair of shared electrons, like the bonds between hydrogen and carbon in Ethanal.
• Double bonds mean two pairs of electrons are shared, such as between carbon and oxygen in the carbonyl group of CH₃CHO.

These bonds are depicted as lines in Lewis structures. Stronger coverage of these bonds ensures atoms can satisfy the octet rule and reach stable configurations. When constructing molecules like Sulfur trioxide, multiple double bonds may form to achieve stability.

Molecular Geometry

Molecular geometry describes the 3D arrangement of atoms in a molecule. Understanding geometry is essential for predicting reactivity and properties.

• The "shape" of a molecule influences its physical properties and interactions.
• VSEPR (Valence Shell Electron Pair Repulsion) theory is often used to predict molecular shapes based on minimizing electron-pair repulsions.

Nitrous acid, for example, has a bent shape due to the geometry influenced by the lone pairs on oxygen. In contrast, the symmetrical double bonds in sulfur trioxide result in a trigonal planar shape. These geometries help explain how molecules like Ethanal interact with others.