Biology and chemistry are intertwined at intimate levels, so in order to make sense of what’s happening as we progress, you’re going to need some basic chemistry info. In case you have no chem background or you’re just really forgetful, I’m writing a couple of recap articles on necessary concepts.
All atoms want to have eight electrons in their outermost shell—i.e., have eight valence electrons. To do this, two atoms sometimes “share” electrons to create the illusion that they both have eight. This is called a covalent bond. There are two different types, determined by the electronegativity of an atom: how much it wants to hog electrons.
1. Polar covalent bonds are formed when one atom is more electronegative than the other, so it pulls on the electron more strongly. This causes it to gain a partial negative charge, while the less electronegative atoms gain a partial positive charge. Water is a good example of this—see how the electronegativity “distorts” the bonds at an angle.
2. Non-polar covalent bonds are formed when the electronegativity of the atoms are equal, so their shared electrons are shared equally. In this case, neither atom has an induced charge.
These are formed when atoms the electronegativity of one atom is much stronger than the electronegativity of the other atom, so it’s not content with sharing—it just strips electrons from the other atom. The atoms then become ions: atoms that have lost or gained electrons. One is negatively charged (anion) and the other is positively charged (cation), and because they’re oppositely charged, they’re electrostatically attracted to each other and ionic bonds are formed. Note that the electron transfer isn’t the cause of the bond, but it does create the charge that the bond is based on.
These intermolecular attractions are weaker than the bonds described, created when a hydrogen atom that is covalently bonded to one electronegative atom is attracted to another electronegative atom. Essentially, it’s an attraction between hydrogen (which, remember, is slightly positive) and an electronegative atom (which is slightly negative because it shares “more” of the electrons).
It’s important to understand these bonds because they help explain how many biological molecules act. For example, water is polar and covalent, and this configuration is incredibly vital for life. When water freezes, the hydrogen bonds between its molecules create a rigid, lattice-like structure that separates out the atoms, thus making its solid form less dense than its liquid form. This is the reason that ice floats, and without this particular quirk of chemistry, it would be tough for marine organisms to survive. Usually, the ice that freezes first in cold conditions rises to the tops of lakes or oceans and forms an insulating layer, preventing the rest of the water from freezing and allowing organisms to survive the cold—if entire oceans froze every winter, life may have never evolved.