Question

A hydrogen bond could form between:

1. A partially negative nitrogen from one molecule and the hydrogen of a water molecule

2. The hydrogens of two methane molecules.

3. A calcium cation and the hydrogen of a water molecule.

4. The oxygen of one water molecule and the oxygen of another water molecule

Please select the correct one

Answer 1

If Hydrogen (H) atom is in a covalent compound (e.g. water) with electronegative atoms such as Nitrogen (N) or Oxygen (O), the electrons they share to form the covalent bond are pulled by electronegative atoms (N or O) due to their greater electronegativity. As a result, the shared electrons are closer to the electronegative atoms than to the H atom(s) which in turn effectively make H and the other electronegative atom(s) partially positively ( represented ) and partially negatively ( represented ) charged, respectively. This partially positively charged H atom now is attracted to another partially negatively charged atom and forms a weak electrostatic bond with that atom; this bond is termed H bond.

Now,coming to the question,

The correct answer is 1. The H atom in water is partially positively charged, as discussed above. This partially positively charged atom binds through weak electrostatic interaction called H bond to the partially negatively charged N atom, as depicted below:

Why other options are incorrect:

2. In methane (CH4), the H atoms are all bound to a carbon (C) atom, which is not a very electronegative atom. Therefore, electrons remain shared equally and no partial charges are formed which is a prerequisite for H bonding, as we discussed above.

3. H atom of an water molecule is partially positive and and a positive cation will only repel it.

4. The O atom in water (H2O) is partially negative. So, two partially negative charges will only repel each other , so they cannot form a bond.