Only nitrogen, oxygen, and fluorine can form hydrogen bonds since these three elements are very electronegative. Thus, their partial negative charge (and the presence of a lone pair of electrons) attracts a partially positive hydrogen atom. Carbon, on the other hand, is not very electronegative and thus cannot form hydrogen bonds.

Covalent bonds occur between atoms within (or intra) molecules. This means that covalent bonds are intramolecular bonds. Hydrogen bonds, on the other hand, occur between hydrogen atoms of one molecule and either a nitrogen, an oxygen, or a fluorine atom in another molecule. Since hydrogen bonds occur between molecules, they are classified as intermolecular bonds. There are some situations in which hydrogen bonds may be formed intramolecularly, but these are special cases.

Hydrogen bonds are found in primary structure of protein, as well as between the bases in DNA structure. Hydrogen bonds are only found between hydrogens attached to oxygen, nitrogen, or fluorine. They increase the attraction between water molecules, therefore are harder to break in large numbers, causing an increase in boiling point.

Oxygen is more electronegative than hydrogen. This results in the electrons to spend more time with the oxygen atom making it negatively chared and the hydrogen atoms to be positively charged. The charged water molecule is then able to form hydrogen bonds with polar molecules.

Hydrogen can form hydrogen bonds with nitrogen, oxygen, and fluorine. A water molecule consists of an oxygen atom bonded to two hydrogen atoms. Each hydrogen atom can form a hydrogen bond with a nitrogen, fluorine, or oxygen atom. Also, the oxygen, which has two lone pairs of electrons, can form two hydrogen bonds with hydrogen atoms. This sums to four hydrogen bonds per water molecule.

In a solution of water, the outside of a folded protein is going to be in direct contact with water. Therefore, polar or ionic attractions are most favored on the outside of a protein in a solution of water since these can form attractions. The only amino acid listed that can form either polar or ionic attractions is glutamic acid.

Bond strengths are measured in kilojoules/mole. Hydrogen bonds can have a strength of . Ionic bonds can have a strength of . This makes hydrogen bonds much weaker than ionic bonds.

Uracil is not a base in DNA, so A-U can be ruled out as an answer. Moreover, C-G and A-T are the correct base pairs, so the other combinations can be ruled out as well. C-G requires more energy to break because there are three hydrogen bonds between these bases, while A-T has only two hydrogen bonds holding them together.

An example of strong intermolecular bond is a hydrogen bond. A hydrogen bond occurs between a hydrogen atom on a molecule and an either nitrogen, oxygen, or fluorine atom on an adjacent molecule. The question states that there are bonds between molecule A and molecule B. We can assume that these are hydrogen bonds. This means that molecule A can have hydrogen atom and/or nitrogen, oxygen, or fluorine atoms. Similarly, molecule B can also have hydrogen atom and/or nitrogen, oxygen, or fluorine atoms.

Molecule C doesn’t form hydrogen bonds with either of the other two molecules. This means it cannot have hydrogen and/or nitrogen, oxygen, or fluorine atoms. This is because if molecule C had any of these atoms then it would interact with at least one of the other molecules and form hydrogen bonds (because molecule A and molecule B have these other atoms).

For a hydrogen bond to occur, the atom to which the hydrogen is bonded has to have a high relative Pauling electronegativity. Only oxygen, nitrogen, and fluorine, in the upper right corner of the periodic table, have these electronegativities. Neon, argon, and xenon are, of course, inert noble gases, although xenon is sometimes assigned a high electronegativity. This is due, however, to rare bonding events -- certainly not regular hydrogen bonding. Sodium, magnesium and aluminum often form positively charged ions, so they would not tend to attract a hydrogen nucleus.