Q106
General ScienceChemistry
Which particular bonding exists in $\mathrm{H_2O}$ and HF but does not exist in $\mathrm{H_2S}$?
- a.Covalent bonding
- b.Ionic bonding
- c.Hydrogen bonding
- d.Metallic bonding
Answer: (C) Hydrogen bonding
A hydrogen bond forms when hydrogen is attached to a small, highly electronegative atom: fluorine, oxygen or nitrogen. Water and HF therefore have hydrogen bonding between their molecules, but $\mathrm{H_2S}$ does not, because sulphur is larger and much less electronegative. This explains why water boils at $100^\circ$C while $\mathrm{H_2S}$ is a gas, why HF has a far higher boiling point than HCl, and why ice is lighter than water (an open cage-like structure). All three molecules have covalent bonds within the molecule, so A does not distinguish them, and none is ionic or metallic. Hydrogen bonds also hold together the two strands of DNA and give proteins their shape.