Just Enough Chemistry for Biology: Covalent Bonding

Chemical Reactions and Molecules

We know about valence shells. We understand the ability to fiddle with the electronsNegatively charged subatomic particles found in atoms. in them. Now, we have to talk about bonds.  There are different types of bonds, but they all involve interactions with electrons.  Atoms and moleculesGroups of atoms bonded together. can share electrons or they can just outright donate electrons to other atomsThe smallest units of matter that retain the properties of an element. or molecules.  Conversely, they can also accept them.  I mean, you can have one without the other.  When bonds are formed, they trap energyThe capacity to do work or cause change. in them.  When bonds are broken, that energy then becomes available for use. 

I have listed the 3 types of bonds we are concerned with in this class from strongest to weakest. Think of the picture of a waterThe universal solvent essential for life. molecule as the headRounded proximal end that fits into the acetabulum of the hip bone. of Mickey Mouse.  You have the face, the oxygen, and the ears, the hydrogens.   

One molecule of water has two covalent bonds that join each hydrogen atom to the oxygen atom. A covalent bondStrong chemical bonds formed by the sharing of electrons between atoms. consists of a pair of shared electrons. It’s like hand holding instead of just standing next to each other.

An oxygen atom has a greater affinity or attraction for electrons than a hydrogen atom does. This makes sense considering that oxygen has more protons than hydrogen.  When a hydrogen atom and an oxygen atom share a pair of electrons in a chemical bond, the electrons spend more time around the oxygen nucleusThe control center of the cell that contains DNA and directs cellular activities.. The chemical bond means the electrons are mostly near the oxygen nucleus. This happens because of the oxygen nucleus’s larger positive charge. We can think of this like an electron party around the oxygen nucleus. It’s apparently the place to be if you’re an electron.

Covalent Bonds

A covalent bond involves a pair of shared electrons. It’s like hand holding instead of just standing next to each other.  There no movementA fundamental property of life involving motion of the body or its parts. of electrons from an atom to another.  The sharing creates this like super-valence shell that surrounds both atoms.  This super valence shell provides each atom with the correct number of electrons to fill its outer shell.  Take a look at oxygen here.  The red dots here are electrons in the valence shell.  This box over here on the right are the shared electrons.  When two oxygen atoms come together, they share two pairs of electrons to make a double covalent bond.  This is what we call molecular oxygen.  This is also what you are taking in from the air right now: not oxygen atoms, but oxygen molecules.  On the bottom of this picture we have water or H2O.  The hydrogen atoms in water are bonded with single covalent bonds to the oxygen atom.  This single covalent bond is weaker than the double.  The more electrons you share, the stronger the bond. 

Polar Covalent Bonds

This unequal sharing of electrons creates two separate areas of electrical charge. Around the oxygen atom, or Mickey Mouse’s chin, the molecule is partially negatively charged. The electrons are there more of the time. Around the hydrogen atoms, or the ears, the molecule is partially positively charged because the electrons spend less time there.

This type of bond between the atoms of a water molecule is called a polar covalent bond. Water is a polar molecule. It is a molecule with two “poles”—one slightly negative and the other slightly positive. Many students associate polar molecules with their charged poles to the Earth’s magnetically charged north and south poles. They also relate them to the positive and negative poles of a magnet. 

A polar substance has both positively and negatively charged poles. This is in contrast to a nonpolar molecule, which has no charge.

Students seems to have a common misconception here about polar and nonpolar.  They sometimes think polar means positive and nonpolar means negative.  Polar means charged and thus represents BOTH positive and negative.  This polarity results from that unequal sharing of the electrons among the atoms.  Nonpolar molecules share their electrons equally, and have no charge, like a block of wood or like Princess Leia here.  Kidding, this is carbon dioxide, which is a nonpolar molecule like oxygen.

Nonpolar Covalent Bonds

The participants in covalent bonding can share the electron equally or unequally.  On the left here we have carbon dioxide, which is shaped like Princess Leia head with the braid buns.  Carbon is equally sharing its electrons with the oxygens.  We can also see two gray connecting lines between the carbon and the oxygens. These lines indicate double covalent bonds.  When electrons are shared equally, such as in carbon dioxide, we call this a nonpolar covalent bond.  It is nonpolar because no one area of this molecule has any type of charge, no positive or negative areas. 

On the right is water, which does not equally share its electrons.  Now, Princess Leia’s braid buns are on the top/sides of her head.  Actually, now we have Mickey Mouse.  An oxygen atom has a greater affinity or attraction for electrons than a hydrogen atom does. This makes sense considering that oxygen has more protons than hydrogen.  When a hydrogen atom and an oxygen atom share a pair of electrons, they form a chemical bond. The electrons spend more time around the oxygen nucleus. The electrons spend more time around the oxygen nucleus than the hydrogen nucleus because there are more protons in oxygen. We can think of this as an electron party around the oxygen nucleus. Apparently, it is the place to be if you’re an electron.

The significance here will attack us continually in this course.  Nonpolar molecules versus polar is an important concept because as the saying goes, like dissolves like.  This means that polar substances will dissolve in other polar substances and nonpolar in nonpolar.  I guess we could also say that oil a nonpolar substance and water don’t mix.  You are a water-based organismA living individual made up of cells.. Water is the basis of your blood. This can affect the solubility of things in your blood.  For example, estrogen, testosterone and other important hormones are nonpolar or hydrophobic.  How do they travel in your blood then?  Attached to a protein that creates a hydrophilic complex.  Just one note…if you want to get an oil stain out of your cotton shirt, use a nonpolar substance. It will lift out the oil.  Baby powder is nonpolar and does the trick wonderfully.  I know, because I am a messy eater.