Intermolecular and Intramolecular Forces - College Chemistry

is an ionic compound because it is composed of a metal and a nonmetal.

is a molecular compound because it consists of a nonmetal connected to another nonmetal.

Xenon is an atomic element because its elemental form consists of one atom.

In order to answer this question correctly, you must remember the different types of intermolecular forces and their effects.

only contains London dispersion forces. Since it is a smaller molecule compared to the others, it cannot have the highest boiling point.

also only contains London dispersion forces. However, since it is a bigger molecule, it will have a higher boiling point than .

While contains both London dispersion forces and dipole-dipole interactions, it lacks hydrogen boding as the fluorine atom is attached directly to the second carbon.

has the highest boiling point because it contains London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

A hydrogen bond refers to the attraction between a hydrogen attached to an electronegative atom of one molecule and an electronegative atom of another molecule. The atoms which commonly form hydrogen bonds are oxygen, nitrogen, and fluorine, which are very electronegative. When a hydrogen bond forms between hydrogen and one of these three atoms, hydrogen gains a partial positive charge while the electronegative atom gains a partial negative charge. Carbon is not a very electronegative atom and thus cannot form a hydrogen bond.

London dispersion forces are the weakest of the three. Every molecule is composed of electrons, which are free to move around. This can create a temporary charge, on any molecule, at any time. When two molecules come close together, their varying charges can orient such that one end of a molecule may be slightly positive, while the end of a nearby molecule may be slightly negative. This leads to a slight attraction between the two molecules, called London dispersion forces until they move around again. This is a weak, temporary force.

Dipole-dipole interactions develop when polar compounds line up and are attracted to each other. These forces are stronger than London dispersion forces due to the permanency of the dipoles, but weaker than hydrogen bonds.

Hydrogen bonds are the strongest force of the three. The name refers to the attractive force between the hydrogen attached to one electronegative atom (usually oxygen, nitrogen, or fluorine) and an electronegative atom of a different molecule. The electronegative atom gains a partial negative charge, while the hydrogen gains a partial positive charge. These forces are responsible for many of the qualities of water.

Drawing the Lewis Diagrams for these molecules reveals that the C-N bond in is a triple covalent bond, whereas the C-N bonds in and are double covalent bonds and the C-N bond in is a single covalent bond. Triple covalent bonds between two given atoms are always stronger than double bonds between these same two atoms, and similarly double bonds are even stronger than single bonds. Bond length is inversely related to bond strength; therefore a shorter bond is a stronger bond, and triple covalent bonds are shorter than either double or single covalent bonds. Thus, the C-N bond in is the shortest in length.

Ne is the only element that does not exist as a diatomic molecule because it is a noble gas, meaning it has a stable resting valence electron configuration, and exists simply as an atomic molecule. By comparison, N, O, and Cl can all achieve stable states by forming a diatomic molecule. and are held together by a single covalent bond, or shared electron pair. is held together by sharing two electron pairs (two covalent bonds), and by sharing three pairs.

Generally, the stronger the intermolecular forces between molecules, the higher a compound's melting point. This trend is due to the fact that in melting, the distance between molecules increases, a process which is counteracted by any intermolecular forces pulling them together. The ion-ion interaction forces between ionic compounds - the attraction of positively charged and negatively charged ions - are the strongest. The greater strength of this intermolecular force is due to the greater separation of charge between species. In decreasing order of strength after that would be hydrogen bonding, dipole-dipole interaction, and dispersion forces. Thus, molecules undergoing ion-ion interactions will have the highest melting point, followed by those undergoing Hydrogen bonding, dipole-dipole interaction, and dispersion in that order.

is an asymmetrical molecule with polar covalent bonds. The dipoles of these three constituent bonds do not cancel each other out due to the trigonal pyramidal geometry of the molecule (see 3-dimensional Lewis diagram below), so the molecule has a net dipole (here, pointing down). Thus, the molecule will undergo dipole-dipole interactions with its neighbors. None of the other options would apply to this polarized molecule with asymmetrically oriented polar covalent bonds, which is not ionic, is not nonpolar, and does not possess the required acidic hydrogen needed for hydrogen bonding to occur.

Hydrazine is a polar compound that possesses the requirements for hydrogen bonding: an "acidic hydrogen" (a hydrogen bonded to a highly electronegative atom such as oxygen, nitrogen or fluorine) and the presence of a lone pair. Both Nitrogen atoms in hydrazine have lone (unshared) electron pairs, and all four hydrogen atoms are "acidic," making hydrazine a candidate for intermolecular hydrogen bonding.

Melting point is inversely related to the strength of intermolecular forces between molecules. In other words, the higher the attractive forces between molecules, the harder they will be to pull apart from a solid into a liquid state during melting. Thus, nonpolar compounds experiencing relatively weak intermolecular forces (dispersion/van der Waals forces) tend to have lower melting points than polar compounds experiencing dipole-dipole or hydrogen bonding interactions, and lower still than ionic compounds with strong attractions between positive and negative ionic species. Thus, in this list, the nonpolar compound has the lowest melting point, followed by the polar compound The ionic compound has the highest melting point.