Name: _________________________________
Biology I
Covalent Bonding #2: Everyone Needs A Soulmate
10.08.07
In 1916 Gilbert Lewis discovered the reason why elements (one type of atom) combine to form
compounds (made of at least two different types of atoms) or ions (we'll cover them later).
This is his story…
Introduction:
Lets start with a summary of energy levels. Electrons (e-) swarm around the nucleus of an
atom in a random fashion - like moths around a light - yet there is some order to the chaos.
Though e- have no predetermined two-dimensional orbit (like planets), they settle into an
assortment of constant distances (called shells or levels) from the nucleus - a set of distances
that is determined by the energy of the e-. The higher the energy, the further the distance.
The best analogy I can come up with is crazed, rabid mice on a flight of wooden stairs. The
nucleus (represented by an unattainable bowl of food) is at the bottom of the stairs with e(represented by the crazed mice) settling on the tops of the steps (the energy levels).
Obviously the mice want to get to the food, yet they can only get closer in discrete steps.
Either the mouse is on the step or it's off - no sticking to the face of the step allowed! The
higher their position on the stairs, the more crazed they are and the higher their energy. In
addition, you don’t know where on the step the crazed mouse is because, well, it's crazed.
That part is random. Yet you do know that it is definitely on that specific step. That is
predetermined.
Now comes the interesting part. The steps come in different sizes, getting bigger as you go
up. The first step can only hold 2 mice (e-), the second 8, and the third 18. However, we only
have 18 mice, so lets pretend that step #3 holds only 8 mice (2+8+8=18) and is the same size
as step #2 (hey, this is my worksheet and I can make any rule I want!). Of course the mice
want to be as close as possible to the bowl and they fill the lower steps before moving up to the
higher ones. This is what it looks like:
Enough talk of crazed mice - there's science to be learned!
The Octet Rule:
Gilbert Lewis noticed that the atoms in compounds tend to have the e- configuration of
noble gases and called this tendency the octet rule (octet = eight). Look at your periodic table.
Now look up the atomic numbers (atomic number = number of protons = number of electrons)
of helium, neon, and argon. Notice that each of them completely fills its outermost energy
level, making them very stable and non-reactive. Remember: only the outermost level counts once a level is filled ignore it and move on to the next highest level. Most of the other elements
yearn for the eternal bliss of stability and actively search out their soulmate. By soulmate, I
mean an atom (or collection of atoms) willing to share e- so that it can have a full outer shell.
This sharing of e- is called a covalent bond.
A word of warning is in order. Hydrogen can fill its outermost shell by gaining only 1 e-,
ignoring the octet rule. This is because the innermost shell can only hold 2 e-, not 8 e- like
levels 2 and 3. One way it can do this is the following:
Notice the dots? These represent e- in the outermost, or valence, level. Whenever we
represent e- this way we create a Lewis Structure to illustrate the bonding that occurs between
atoms. Hydrogen is not alone in this ability to fall in love with itself and form covalent bonds.
The other diatomic elements (F, Cl, Br, O, N, and I) can also do it. They look like this:
Notice O and N? O forms a double covalent bond with itself (this is not always true, but for our
purposes ignore this statement), while N forms a triple covalent bond! All is fair in the name of
love, right? Sometimes atoms really fall for each other and share more than one e-. Double
bonds require sharing two e- each, and triple bonds require sharing three e- each (duh!).
Remember, though, that each atom must share the same number of e- as its partner for it to
be considered a covalent bond!
Compounds:
Diatomic elements are simple enough, right? Now comes the tough stuff. I already
mentioned that when 2 or more elements combine to form a stable molecule a compound is
formed. Let's figure out how.
Water is always a good place to start. Two atoms of H bond to one atom of O in the
following manner:
The key is to set it up so that each atom is satiated and content with a full allotment of 8 e- in
its outermost shell (except H, which desires only 2 e- in its outermost shell). Let's do ammonia.
Three atoms of H bond to one atom of N in the following manner:
Easy, huh? Before I cut you loose lets do one more. Carbon dioxide is formed when two atoms
of O bond to one atom of C in the following manner:
Notice that two double bonds are required to accomplish the task.
Your Turn:
We finally arrive at the worksheet proper. On a separate piece of paper write Lewis
Structures for the following compounds, writing down the chemical formula as well as the name
of the compound:
SiO2
Silicon dioxide
MgCl2
Magnesium chloride (trick question – think back to last week’s worksheet)
HCl
Hydrochloric acid
CH4
Methane
PCl3
Phosphorous trichloride
C2H6
Ethane
H2SO4
Sulfuric acid
HNO3
Nitric acid
H2O2
Hydrogen peroxide
HCN
Hydrogen cyanide
C2H6O
Ethanol (Hint: alcohols always have an "O-H" at one end of the molecule)
C6H8O6
Extra Credit: Ascorbic acid (vitamin C)