AP Biology

## Polarity of the water molecule A water molecule is made of one oxygen atom joined to two hydrogen atoms by **polar covalent bonds**. Oxygen attracts the shared electrons more strongly than hydrogen does, so the oxygen end of the molecule carries a slight negative charge ($\delta^-$) and each hydrogen carries a slight positive charge ($\delta^+$). The molecule is also bent rather than straight, so these charges do not cancel and the molecule as a whole is **polar**. Opposite partial charges on neighboring molecules attract one another. The attraction between the $\delta^+$ hydrogen of one water molecule and the $\delta^-$ oxygen of another is a **hydrogen bond**. A single hydrogen bond is weak, much weaker than a covalent bond, but every water molecule can form up to four of them, and in liquid water they break and re-form constantly. The large number of them gives water most of the properties that matter for living things. Hydrogen bonds are not special to water. They also form between and within biological molecules wherever a hydrogen attached to oxygen or nitrogen sits near another oxygen or nitrogen. They hold the two strands of DNA together and help fold proteins into shape. ## Water as a solvent Polar molecules and ions dissolve in water because water molecules cluster around them, with $\delta^-$ oxygens facing positive charges and $\delta^+$ hydrogens facing negative charges. Substances that dissolve this way are **hydrophilic**. Nonpolar substances, such as the…