Kaplan MCAT Sample Question
Which of the following is the best explanation of the phenomenon of hydrogen bonding?
- Hydrogen has a strong affinity for holding onto valence electrons.
- Hydrogen can only hold two valence electrons.
- Electronegative atoms disproportionately carry shared pairs when bonded to hydrogen.
- Hydrogen bonds have ionic character.
The answer is:
C. Electronegative atoms disproportionately carry shared pairs when bonded to hydrogen.
Answer choice (C) correctly describes the underlying forces of hydrogen bonding. Electronegative atoms bound to hydrogen dispropotionately pull covalently bonded electrons toward themselves, which leaves hydrogen with a partial positive character. That partial positive charge is attracted to nearby negative or partial-negative charge, such as those on other electronegative atoms. (A) is not true; hydrogen has little electronegativity and does not hold its valence electrons closely. (B) is a true statement, but does not describe the cause of hydrogen bonding. (D) is not correct; although these bonds are highly polarized, they are not ionic.