Polarity

An important concept that you will encounter in biochemistry is the hydrophobic and hydrophilic nature of different compounds. A compound is hydrophobic if it cannot dissolve in water, and hydrophilic if it can dissolve in water. You may already know from experience that fats and oils are hydrophobic, which makes them float on top of water. If you combine water and ethanol together however, ethanol will dissolve in the water, hence it is hydrophilic. So why do some compounds dissolve in water and so do not? It has to do with the polarity of their chemical bonds. Bonds that are ionic are polar in nature (has plus and minus charges). This allows them to interact with water, which is also polar.

Electronegativity

With that said, let's talk about electronegativity. This is simply the tendency of an atom to attract electrons to itself. The stronger the atom attracts electrons (high electron density), the more electronegative it is. The electronegativity of different elements has been worked out by the famous scientist, Linus Pauling. Pauling's Periodic Table of electronegativity shows that electronegativity increases from left to right and decreases down each group. Hence, group 1 elements have the lowest electronegativity, while group 7 has the highest. The electronegativity values for group 8 are not provided since they have a full valence shell and do not react with other elements.

When two or more elements are bonded, the electrons are not always evenly distributed between them. Instead, the electrons will shift more to the element that is more electronegative. This creates a separation of charge that we refer to as dipole moment. This dipole moment creates the plus and minus charge that enables the compound to form chemical bonds with water.

A difference in electronegativity of elements between chemical bonds will determine if the compound is pure covalent, polar covalent, or ionic. In general: