Chapter 1: Problem 6

Construct Lewis structures for the following neutral molecules: *(a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{Be}\) (c) \(\mathrm{SiH}_{4}\) (d) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (e) \(\mathrm{HOCH}_{3}\) (f) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{NH}_{2}\)

Short Answer

Expert verified

The Lewis structures are determined by arranging atoms to satisfy valence requirements and distributing electrons to complete octets.

Step by step solution

Calculate the Total Number of Valence Electrons

Add the total number of valence electrons for each molecule. For each element, the number of valence electrons is equal to the group number in the periodic table.

Sketch the Basic Skeleton Structure

For each molecule, arrange the atoms in a way that the least electronegative atom (usually the one written first) is in the center, with the other atoms around it.

Distribute Electrons Around the Outer Atoms

Place electrons around the outer atoms (those attached to the central atom) to satisfy the octet rule (or duet rule for hydrogen).

Place Remaining Electrons on the Central Atom

Any remaining electrons should be placed on the central atom to satisfy its octet.

Check for Octet Rule Completion

Ensure that all atoms (except hydrogen, which should have 2 electrons) are surrounded by 8 electrons. If there are atoms that do not have 8 electrons, rearrange electrons to form double or triple bonds if necessary.

Example (a) \(\mathrm{BF}_{3}\)

1. Total valence electrons = 3 (B) + 3 × 7 (F) = 24 electrons.2. Skeleton structure: B in the center with three F atoms bonded to it.3. Distribute electrons: Each F gets 6 electrons to form an octet (18 electrons used).4. Remaining 6 electrons are placed around B giving it 6 electrons.5. Boron does not complete an octet but is stable with 6 electrons.\[\begin{array}{c@{}c@{}c@{}c@{}c} & \ \quad F \ & \ \quad | \ F―B―F \ & \ \quad | \ & \ \quad F & \ \end{array}\]

Example (b) \(\mathrm{H}_{2} \,\mathrm{Be}\)

1. Total valence electrons = 2 × 1 (H) + 2 (Be) = 4 electrons.2. Skeleton structure: Be in the center with two H atoms bonded to it.3. Distribute electrons: Each H gets 2 electrons (forming duet).4. Remaining electrons are placed on Be (2 left, giving 4 around Be).\[ H―Be―H \]

Example (c) \(\mathrm{SiH}_{4}\)

1. Total valence electrons = 4 (Si) + 4 × 1 (H) = 8 electrons.2. Skeleton structure: Si in the center with four H atoms bonded to it.3. Distribute electrons: Each H gets 2 electrons (forming duet).4. No remaining electrons.\[\begin{array}{c@{}c@{}c@{}c@{}c} & H \quad & \ & | \quad & \ H―Si―H & \ & | \quad & \ & H & \end{array}\]

Example (d) \(\mathrm{CH}_{2}\mathrm{Cl}_{2}\)

1. Total valence electrons = 4 (C) + 2 × 1 (H) + 2 × 7 (Cl) = 20 electrons.2. Skeleton structure: C in the center with two H and two Cl atoms bonded to it.3. Distribute electrons: Each H gets 2 electrons (forming duet) and each Cl gets 6 electrons for octet.4. Remaining 2 electrons go on central C.\[\begin{array}{c@{}c@{}c@{}c@{}c@{}c} H & – C – & H \ & | . . . & \ . . . Cl & & Cl & . . . \ & | . . . & \ e-‘s & & & \end{array}\]

Example (e) \(\mathrm{HOCH}_{3}\)

1. Total valence electrons = 1 (H) + 6 (O) + 4 (C) + 3 × 1 (H) = 14 electrons. 2. Skeleton structure: C in center bonded to O and 3 H atoms, O bonded to H3. Distribute electrons: Each H gets 2 electrons (forming duet), each O gets 4 electrons completing octet4. Remaining 2 electrons go on O\[\begin{array}{c@{}c@{}c@{}c@{}c} H & – O – & H & \ e-’s & ( ) & ( ) & e-’s & . . . H & ( ) & & \end{array}\]

Example (f) \(\mathrm{H}_{2} \,\mathrm{N}-N\mathrm{H}_{2}\)

1. Total valence electrons = 2 × 1 (H) + 2 × 5 (N) + 2 × 1 (H) = 14 electrons.2. Skeleton structure: N in center bonded to H atoms3. Distribute electrons: Each H gets 2 electrons (forming duet). 4. Remaining electrons are placed on N (4 electrons each)\[\begin{array}{c@{}c@{}c@{}c@{}c} H & – N – & N & – H & \ e-′s & & ( ) & ( ) & \end{array}\]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Valence Electrons

Valence electrons are the outermost electrons of an atom. They play a key role in chemical bonding. To determine the number of valence electrons, you look at the group number in the periodic table. For example:

Hydrogen (H) has 1 valence electron.
Carbon (C) has 4 valence electrons.
Fluorine (F) has 7 valence electrons.

These valence electrons are used to form bonds in molecules. In Lewis structures, dots represent valence electrons, showing how atoms share or transfer electrons to achieve stability.

Octet Rule

The octet rule is a guideline for drawing Lewis structures. It states that atoms tend to form bonds to have eight electrons in their valence shell. This gives them stability similar to noble gases. Here are some important points:

Hydrogen follows the 'duet rule', needing only 2 valence electrons.
Elements like Boron (B) and Beryllium (Be) may have fewer than 8 electrons but still be stable.
Octet rule helps predict the arrangement of electrons around atoms in a molecule.

For example, in \(\text{BF}_3\), Boron has 6 electrons around it and is stable despite not following the octet rule.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms in a molecule. The shape affects properties like reactivity and polarity. Here are some key shapes:

Linear: Example - \(\text{H}_2\text{Be}\). Beryllium in the middle with two bonds forming a straight line.
Tetrahedral: Example - \(\text{SiH}_4\). Silicon at the center with four hydrogen atoms, forming a tetrahedron.
Trigonal Planar: Example - \(\text{BF}_3\). Boron at the center with three fluorines forming a flat triangle.

Understanding molecular geometry helps predict how molecules interact and react.

Chemical Bonding

Chemical bonding involves the interaction of valence electrons between atoms to form a molecule. There are different types of bonds:

Covalent Bonds: Atoms share electrons. Example – \(\text{H}_2\), where each hydrogen shares an electron.
Ionic Bonds: Atoms transfer electrons. Example – \(\text{NaCl}\), where sodium gives an electron to chlorine.

In covalent bonds, electron sharing can form single, double, or triple bonds. For instance, in \(\text{H}_2\text{N}-\text{NH}_2\), nitrogen atoms share a pair of electrons, forming a double bond.

Skeletal Structure

The skeletal structure is a simple way to show how atoms are connected in molecules. Here is how you draw it:

Identify the central atom (usually the least electronegative).
Arrange surrounding atoms around the central atom.
Use single lines to represent bonds.

For instance, in \(\text{CH}_2\text{Cl}_2\), carbon is the central atom with hydrogen and chlorine atoms attached to it. This simplifies drawing and understanding complex molecules.

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