Question

Which of the following is not correctly paired with its dominant type of intermolecular forces? 1. SiH4, instantaneous dipoles 2. C6H6 (benzene), instantaneous dipoles 3. NH3, hydrogen bonding 4. HBr, hydrogen bonding 5. CaO, ionic forces

Answer 1

Answer:

Option 4. There's no hydrogen bonding between HBr molecules at all.

Explanation:

SiH₄

SiH₄ molecules are tetrahedral and symmetric. Dipoles due to the polar Si-H bonds balance each other. SiH₄ molecules are nonpolar. Only instantaneous dipoles are possible between those molecules.

C₆H₆ Benzene

Similar to SiH₄, benzene is symmetric. Dipoles due to the weakly polar C-H bonds balance each other. Benzene molecules are nonpolar. Only instantaneous dipoles are possible between those molecules.

NH₃

There are two conditions for hydrogen bonding to take place:

• H atoms are directly bonded to a highly electronegative element: Nitrogen, Oxygen, or Fluorine.
• There is at least one lone pair of electrons nearby.

Consider the Lewis structure of NH₃. There are three H atoms in each NH₃ molecule. Each of the three H atoms is bonded directly to the N atom with a highly polar N-H bond.  Also, there is a lone pair of electrons on the N atom. Hydrogen bonding will take place between NH₃ molecules.

NH₃ is a relatively small molecule. As a result, hydrogen bonding will be the dominant type of intermolecular force between NH₃ molecules.

HBr

There are three lone pairs on the Br atom in each HBr molecule. However, no H atom is connected to any one of the three highly electronegative elements: N, O, or F. The Br atom isn't electronegative enough for the H atom to form hydrogen bonding. HBr molecules are polar. As a result, the dominant type of intermolecular forces between HBr molecules will be dipole-dipole interactions (A.k.a. permanent dipole.)

CaO

Calcium is a group 2 metal. Oxygen is one of the three most electronegative nonmetal. (Again, the most electronegative elements are: Nitrogen, Oxygen, and Fluorine.) As a main group metal, Ca atoms tend to lose electrons and form positive ions. Oxygen will gain those electrons to form a negative ion. As a result, CaO will be an ionic compound full of Ca²⁺ and O²⁻ ions. Forces between ions with opposite charges are called ionic bonds.