The polarity of water molecules
The key to understanding water’s chemical behavior is its molecular structure. A water molecule consists of two hydrogen atoms bonded to an oxygen atom, and its overall structure is bent. This is because the oxygen atom, in addition to forming bonds with the hydrogen atoms, also carries two pairs of unshared electrons. All of the electron pairs—shared and unshared—repel each other.
The most stable arrangement is the one that puts them farthest apart from each other: a tetrahedron, with the start text, O, end text, minus, start a text, H, end text bonds forming two out of the four “legs”. The lone pairs are slightly more repulsive than the bond electrons, so the angle between the start text, O, end text, minus, start a text, H, end text bonds is slightly less than the 109° of a perfect tetrahedron, around 104.5°, squared.
Because oxygen is more electronegative—electron-greedy—than hydrogen, the start text, O, end text atom hogs electrons and keeps them away from the start text, H, end text atoms. This gives the oxygen end of the water molecule a partial negative charge, while the hydrogen end has a partial positive charge. Water is classified as a polar molecule because of its polar covalent bonds and its bent shape start superscript, 2, comma, 3, end superscript.
Hydrogen bonding of water molecules
Thanks to their polarity, water molecules happily attract each other. The plus end of one—a hydrogen atom—associates with the minus end of another—an oxygen atom.
These attractions are an example of hydrogen bonds, weak interactions that form between hydrogen with a partial positive charge and a more electronegative atom, such as oxygen. The hydrogen atoms involved in hydrogen bonding must be attached to electronegative atoms, such as start text, O, end text, start a text, N, end text, or start a text, F, end text.
Water molecules are also attracted to other polar molecules and to ions. A charged or polar substance that interacts with and dissolves in water is said to be hydrophilic: hydro means “water,” and philic means “loving.” In contrast, nonpolar molecules like oils and fats do not interact well with water. They separate from it rather than dissolve in it and are called hydrophobic: phobic means “fearing.” You may have noticed this as a not-so-handy feature of oil and vinegar salad dressings. Vinegar is basically just water with a bit of acid.