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

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

Explanation:

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.

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.

There are two conditions for hydrogen bonding to take place:

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.

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.)

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.

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."