Electron Configuration
When electrons swarm around the nucleus of an atom, the result may look like just a blur of movement like
watching a propeller spin. But there is a definite order in that electron cloud. In fact, electrons appear to
follow some very specific rules of organization in the atom. You may already know from experience at airports,
that flight attendants load the plane with passengers in a specific order. Notice that it isn’t simply back row
first like on school busses. First class and those needing assistance board first. Then they board coach from
back to front. Atoms have their own seemingly strange rules as well.
If you look at some periodic tables, they include a strange notation that is meant to signify the rules of
electron organization. For example, the electron configuration for Oxygen looks like this:
1 s2 2s2 2p4
Let’s use the airplane analogy to help you understand the notation and learn how to “load” electrons in an
atom. Then you’ll do some in your lab groups for practice.
Say you’re boarding Atomic Airlines Flight # 8. Your ticket says “Row: 2 Seat : p3”; you wait patiently for
your row number to be called. The flight attendant calls Row 1, S-seats and two people board. Then they
call Row 2, S-seats only and two more people board. When they finally call Row 2, P -seats, you and
three other people board. When you get to your seat, you notice that Flight #8 only has the 3 rows and there
are 2 more empty seats available in row 2p. You ask the attendant why they didn’t load those seats and she
says they are vacant now, but the airline prefers to fill all rows and if two more passengers show up, they
would gladly board them. She tells you the AA boarding rules:
AA
3s
1 2
2s
1 2
AA
3p
1 2
3 4
1 2
2p
3 4
1 2
1s
5 6
1. Passengers board by row number in
order 1,2,3...
2. “S” seats board first, “P”seats board
next.
5 6
3. Limit of 2 “s” seats and 6 “p” per row.
In your lab group, try a virtual airplane boarding
using AA rules. You are boarding AA Flight # 12
with 12 passengers. Indicate the number of
passengers in each row with a superscript: for
example, 3 people in row 2p is written 2p3.
For atoms, the passengers are electrons, row numbers are energy levels and the “s” and “p” are
subdivisions of the energy levels. Electrons load the energy levels from lowest to highest. We can picture
how the electrons are organized using an energy level diagram. Each energy level is drawn as a circular orbit
corresponding to the energy level 1,2,3 etc. AA Flight #8 is Oxygen with a total of 8 electrons. AA flight #12 is
Magnesium with 12 electrons.
IMPORTANT: The circles are NOT orbit paths. They are
symbols for the energy level of electrons. Electrons closest to
the nucleus have the lowest energy.
+8
Vacancies = 2
Oxygen: 1s2 2s2 2p4
Try loading the following atoms in your lab groups. Draw energy
level diagrams (bullseye) and use the s and p notation. For each
one, count the number of vacancies in the last energy level and
write it down. Finally, look for patterns between vacancies and
placement in the periodic table: Li, Be, C, F, Na, Si, S, Cl.
Electron
Vacancy
+12
Vacancies = 6
Magnesium: 1s2 2s2 2p6 3s2
Chemical Bonds Part 1
You should be comfortable with these words and their meanings before proceeding: electron, proton, ion,
energy level, vacancy If you cannot remember what they mean, read your notes first.
Chemical bonds are really an “agreement” among atoms on how to deal with electrons and electron
vacancies in energy levels. They deal with them in one of three ways:
1. Give electrons away.
2. Take electrons from another atom
3. Agree to share a group of electrons.
Recall from “Electron Configuration” that Atomic Airlines likes to fill all seats in all rows and will gladly
board passengers that want to fly. Atoms are similar. They “want” to fill all vacancies in each energy level
with electrons. In fact the desire to fill energy levels is often stronger than the desire to remain electrically
neutral (equal numbers of protons and electrons) and that is why there are so many naturally occurring ions.
Ionic Bonds
Electron
Na+ 1
Cl -1
Attractive Coulomb force
Ionic [Eye-On-Ick] Bonds are what you get when two atoms choose 1 and 2 from
the list above; giving and taking electrons. Take for example Na (Sodium) and Cl
(Chlorine). Look at your electron configurations for these atoms from the
previous lab. Notice that Na has two choices: 1) Find 7 electrons to fill its outer
energy level 2) Get rid of 1 electron and be left with its No. 2 energy level filled.
Chlorine has a similar decision: 1) Find 1 electron to fill its outer energy level 2)
Get rid of 7 electrons and be left with a filled No. 2 energy level. Which do you
think is the easier task for Na and Cl? Discuss it within your lab group.
Hopefully, you noticed that Na and Cl can easily help each other out. Na has an
electron to give and Cl needs one. In the exchange, Na becomes a +1 ion, Cl
becomes a -1 ion and the two ions are drawn together by an attractive coulomb force (remember that
from electricity unit). This is an IONIC BOND since it occurs between ions.
Ionic bonds occur only between Metals and Non-Metals. Whoa, wait! What am I talking about?!? Okay look
at the periodic table and you’ll notice a zig-zag line toward the right . Atoms on the right are non-metals
while those on the left are metals. So ionic bonds must include one representative from each side. Why?
Well, that is how the electron configurations work out. In general, metals have extra electrons to give away
and non-metals need them to fill energy levels.
IMPORTANT: Hydrogen is a switch hitter. Having only 1 electron in it’s first energy
level, it could either gain or give and so it often appears on both sides of the
periodic table.
Often, more than two atoms are involved in the exchange. For example, Li (Lithium)
has 1 electron to give but S (Sulfur) needs 2. To solve the problem, Li gets another Li
to give up its electron and then the two LI-1 ions form an ionic bond with the 1 S-2:
Li2S (VOILE!! A molecule!!)
S -2
Ionic bonds
Li+ 1
Li+ 1
Li2 S
Try these ionic bonds on your own. Draw a picture of the bond on paper. For each one,
label which atom gives, which takes, the charge on each ion, and write the final formula for the molecule:
Li and O
Mg and S
Be and P
Na and B
Chemical Bonds Part 2
You may have noticed from forming ionic bonds that the electrons in the outermost energy level play a part in
forming chemical bonds. That is no accident. The electrons furthest from the nucleus are easier to move.
Since the attractive coulomb force decreases as distance increases, the electrons farther away experience
less force and thus are easier to remove.
The outermost electrons are called VALENCE [VAY-LENTS] electrons. Since columns of atoms in the
periodic table share the same number of vacancies in their outermost energy levels, they also share chemical
“behaviors”: Chemically, they act alike. That is a very powerful understanding. Think about it! If you want to
know how Barium, for example, will react with Oxygen, you need only look at how the atoms in its column
react and you will get a pretty good picture of what will happen without even doing the experiment.
Covalent Bonds
H2O
O2
O OHO
H
shared electrons
orbit all atoms
Covalent [Co-vay-lent] bonds are the second type of chemical bond.
Look at the word “covalent” and you might guess where we’re going
with this. “Co” meaning to share, and “-valent” sounds like valence
which are the outermost electrons. So covalent means, “sharing the
outermost electrons”.
Sometimes, atoms choose plan 3 from the list of how to deal with
electrons. They don’t give up electrons but instead agree to share
them. The electrons literally orbit both atoms and since they move
so fast, both atoms “feel” as if their outermost energy level is filled.
Covalent bonds occur between non-metals only. Take oxygen gas (O2) for example. Both Oxygen atoms have
two vacancies in their outer energy level so they agree to share 4 electrons and form a covalent bond.
Covalent bonds are written using the Lewis dot notation. Each dot is a valence electron. Water is another
covalent bond. Two hydrogen atoms share their one electron with one Oxygen essentially filling all three
energy levels .
Try these covalent bonds using the Lewis dot notation. Use colored pencils to distinguish between atoms:
Nitrogen gas (N2), Carbon Dioxide gas (C02), Methane gas (CH4), Ethanol liquid (C2H 6O).
Metallic
Bonds
Metallic bonds occur only between....Metals! In metals, the valence electrons are only mildly attracted to the
nucleus of the atoms so they are free to move about from atom
to atom. In metals, the electrons are like a herd of underfed
dogs at a family picnic of slobs. As the humans stand still and
carelessly drop bits of food from their plates and mouths, the
dogs roam about randomly picking up snacks. In metals, the
valence electrons move about freely so the atoms exist as
positive ions. The electrons hold the ions together through a
shared coulomb force. That is why you can often bend or
reshape metals without destroying the bonds or chemical
properties of the metal.