Question

Draw a Lewis structure for each molecule and determine its molecular geometry. What kind of intermolecular forces are present in each substance? (a) \(\mathrm{BCl}_{3}\) (remember that \(\mathrm{B}\) is a frequent exception to the octet rule) (b) \(\mathrm{HCOH}\) (carbon is central; each \(\mathrm{H}\) and \(\mathrm{O}\) bonded directly to \(C\) ) (c) \(C S_{2}\) (d) \(\mathrm{NCl}_{3}\)

Short answer

BCl3 has a trigonal planar geometry and exhibits London dispersion forces. HCOH has a bent geometry around the oxygen and exhibits hydrogen bonding, dipole-dipole, and London dispersion forces. CS2 has a linear geometry with London dispersion forces. NCl3 has a trigonal pyramidal geometry and exhibits London dispersion forces, dipole-dipole interactions, and possibly weak hydrogen bonding.

Step-by-step solution

Lewis Structure of BCl3

For BCl3, start by placing B (boron) in the center since it is less electronegative than Cl (chlorine). Boron has 3 valence electrons, and each chlorine atom has 7 valence electrons, for a total of 3 + (3x7) = 24 valence electrons. Distribute the electrons to form three B-Cl single bonds. Each chlorine atom will have three lone pairs to fulfill their octet, while boron will have only six electrons; it does not follow the octet rule in this case.

Molecular Geometry of BCl3

BCl3 has three bonding pairs and no lone pairs on the boron atom, which corresponds to a trigonal planar geometry according to the VSEPR model.

Intermolecular Forces in BCl3

BCl3, being a non-polar molecule with no permanent dipole moment, exhibits London dispersion forces.

Lewis Structure of HCOH

For HCOH, place C (carbon) in the center with O (oxygen) and H (hydrogen) bonded to it. Carbon has 4 valence electrons, oxygen has 6, and each hydrogen has 1, for a total of 4 + 6 + (2x1) = 12 valence electrons. Form a double bond between C and O to fulfil their octets and place the remaining two electrons as lone pairs on the oxygen. Each hydrogen will have a single bond to carbon.

Molecular Geometry of HCOH

The central carbon atom in HCOH is bonded to two other atoms and has no lone pairs, forming a bent molecular geometry around the oxygen due to the two lone pairs on the oxygen.

Intermolecular Forces in HCOH

HCOH has a polar bond (C-O) and can form hydrogen bonds because of the hydrogen directly bonded to oxygen. It also exhibits London dispersion forces and dipole-dipole interactions.

Lewis Structure of CS2

For CS2, place the C (carbon) in the center with two sulfur (S) atoms bonded to it. Carbon has 4 valence electrons, and each sulfur has 6, for a total of 4 + (2x6) = 16 valence electrons. Form double bonds between C and each S to account for all electrons and fulfill their octets.

Molecular Geometry of CS2

CS2 has two double bonds and no lone pairs on the central carbon atom, resulting in a linear molecular geometry according to the VSEPR model.

Intermolecular Forces in CS2

CS2 is a non-polar molecule and thus has only London dispersion forces as its intermolecular force.

Lewis Structure of NCl3

For NCl3, place N (nitrogen) in the center with three Cl (chlorine) atoms bonded to it. Nitrogen has 5 valence electrons, and each chlorine atom has 7, for a total of 5 + (3x7) = 26 valence electrons. Distribute the electrons to form three N-Cl single bonds and one lone pair on the nitrogen atom.

Molecular Geometry of NCl3

NCl3 has three bonding pairs and one lone pair on the nitrogen atom, which corresponds to a trigonal pyramidal geometry according to the VSEPR model.

Intermolecular Forces in NCl3

NCl3 is a polar molecule due to the presence of a lone pair on nitrogen, causing a permanent dipole. It exhibits London dispersion forces, dipole-dipole interactions, and possibly weak hydrogen bonding (because of the highly electronegative chlorine atoms).

Key concepts

Valence Electrons

In order to master Lewis structures, it's crucial to start with the basics—valence electrons, which are the electrons in the outermost shell of an atom that determine its ability to bond with other atoms.
These electrons are paramount since they participate in chemical reactions and bond formation. For example, boron (B) has 3 valence electrons, while chlorine (Cl) has 7. Similarly, carbon (C) has 4, and oxygen (O) and sulfur (S) each have 6. The number of valence electrons directly influences how atoms bond together to form molecules.

Finding Valence Electrons

To find the number of valence electrons for an element, you can look at its group number on the periodic table. For instance, group 1 elements have one valence electron, while group 17 elements have seven.
Understanding the count of valence electrons helps us predict how atoms will combine to achieve a stable octet or, in the case of hydrogen, a duet. This concept of completing the outermost shell is fundamental in Lewis structures, guiding the placement of dots that represent electrons around an element's symbol.

VSEPR Model

The Valence Shell Electron Pair Repulsion (VSEPR) model is a tool to predict the three-dimensional molecular geometry of a molecule based on the repulsion between electron pairs in an atom’s valence shell.
This model is based on the idea that electron pairs will arrange themselves as far apart as possible to minimize repulsion.

Understanding Molecular Shapes

In our exercise examples, BCl₃ is trigonal planar due to three bonding pairs, and HCOH has a bent structure because the lone pairs on oxygen push the bonding pairs away. CS₂ is linear because of the double bonds to sulfur, and NCl₃ has a trigonal pyramidal geometry due to the presence of a lone pair.
Mastering VSEPR is straightforward: count the number of bonding pairs and lone pairs, then use the VSEPR chart to determine the shape. Remember, symmetry matters, and molecules strive for the lowest possible energy state, which often results in the most symmetric arrangement that valence electrons allow.

Intermolecular Forces

Intermolecular forces play a significant role in determining a substance's state of matter, boiling and melting points, viscosity, and surface tension. They are the attractions between molecules that affect the physical properties of substances.
Polar molecules have dipole-dipole interactions due to asymmetrical charge distribution, such as in HCOH, which also can form hydrogen bonds, one of the strongest types of intermolecular forces.

Types of Intermolecular Forces

London dispersion forces occur in all molecular substances, resulting from temporary dipoles induced when electron densities fluctuate. They are the only type of intermolecular force present in nonpolar molecules like BCl₃ and CS₂. Dipole-dipole interactions occur between polar molecules, while hydrogen bonds, a special type of dipole-dipole interaction, can occur in molecules like HCOH where hydrogen is directly bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.
Understanding these forces is key not only for predicting the physical properties of substances but also for comprehending how molecules interact with one another in various states of matter.