Electron
Configurations
What is an e- configuration ?
• We want to be able to create an econfiguration in order to know where
electrons will be found. This will help us
with bonding.
• It is also helpful in seeing how atoms are
stable and how they create compounds.
Nature likes stability !!
• A common idea throughout Science and
not just Chemistry is that nature likes to be
in the most stable situation possible.
• When given a choice, things will adjust to
be in the most stable situation possible.
• For example, an object falls when we let
go of it because it wants to be in a lower,
more stable situation.
• Sports gives us many examples.
More Stability notes
• The lower center of gravity
is going to result in a higher
stability.
• In sports like soccer and
basketball, the players on
defense want to get low so
that they can change
directions easily. Notice the
player in blue playing
defense.
More notes on stability
• In US football, the linemen get low so that
they are more stable and don’t get knocked
down by the linemen on the other team.
In car racing, the
cars are built low to
the ground so they
are more stable
around curves.
How do we apply this idea ?
• What we need to know is the order of the
orbitals from lowest energy to highest
energy.
• If we know this, we can start placing the
electrons where they need to be.
• If only we had an easy way, electron
configuration life would be so much easier.
• Yes, minions, wait for it….
The Diagonal Rule
Your best friend when doing e- configurations
1s
2s
3s
4s
5s
6s
7s
2p
3p
4p
5p
6p
7p
3d
4d 4f
5d 5f
6d 6f
7d
The arrows start with 1s
which is the orbital with
lowest energy. We
simply want to follow the
arrows and that gives us
the order of orbitals
from lowest energy to
highest energy.
EUREKA !!
• The chart can be made as big as
possible, however this size is OK for
any atom currently known which is up to
about element 118.
• If a larger chart is needed (perhaps for
some hypothetical atom in extra credit),
there is a larger sheet on the class
website.
• www.scramlinged.com
Electron Configuration
• ELECTRONS ALWAYS GO INTO THE
ORBITAL WITH THE LOWEST
POSSIBLE ENERGY !!
• Refer to the diagonal rule when doing this.
• Remember our little phrase.
s
1
p
3
d
5
f
7
Aufbau Principle
• Aufbau is German for ‘to build up’
• Electrons are added one at a time to the
lowest energy orbitals available until all the
electrons of the atom have been
accounted for.
• The number of electrons in a neutral atom
equals the atomic number of the element.
Pauli Exclusion Principle
• An orbital can hold a maximum of 2
electrons, but they must be of opposite
spins.
• A lone electron is unpaired while 2
opposite electrons together is paired.
Hund’s Rule
• Electrons are first placed into 2p orbitals
keeping them unpaired for as long as
possible.
• In other words, to be more stable, an
electron will enter an empty orbital before
it will go into an orbital with an electron
already in it.
Let’s get started # 1
• With your diagonal rule by your side, let’s
start with a small atom – Nitrogen.
• We first determine how many electrons we
need. Nitrogen has an atomic number of 7
(# of protons) and since this is an atom,
the # of electrons = the # of protons. So
we need 7 electrons.
• The diagonal rule says we start with the
orbital with the lowest energy; 1s
Let’s get started # 1
• Following the diagonal rule gives us the
following electron configuration for Nitrogen.
•
•
•
•
1s2
2s2
2p3
For a final result of:
1s2 2s2 2p3
Example # 2
•
•
•
•
Now try Nickel.
We see it has 28 electrons.
1s2 2s2 2p6 3s2 3p6 4s2 3d8
However, we need to rearrange the
orbitals for reasons we will take later.
• We rearrange in order of Principal
Quantum Number as follows:
• 1s2 2s2 2p6 3s2 3p6 3d8 4s2
Example # 3
• Osmium has 76 electrons.
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
4d10 5p6 6s2 4f14 5d6
• Now let’s rearrange
• 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10
4f14 5s2 5p6 5d6 6s2
Exceptions to the Aufbau Principle
• Not every element exactly follows the
aufbau principle. For example:
Chromium and Copper
Through experimentation, it has been
found that sets of orbitals that are either
filled or half-filled are more stable and as
we learned, NATURE LOVES TO BE
AS STABLE AS POSSIBLE.
• Let’s look at Chromium now, shall we ?
Chromium
• 1s2 2s2 2p6 3s2 3p6 3d4 4s2 is what we
would expect, however it looks like this:
• 1s2 2s2 2p6 3s2 3p6 3d5 4s1
• Notice that the d – orbitals which has 5
orbitals and can hold 10 electrons is half-filled
so it is more stable.
• It is just how it looks – an electron from the s –
orbital moves to the d – orbitals.
• On a personal note, I am very fond of chromium since my
artificial knee is made of chromium stainless steel. It feels
great - thanks for asking !
EXTRA CREDIT
• The first three people who gives me the
correct electron configuration for the
hypothetical atom that has 350 electrons
will receive a bounty of extra credit points.
• However, I will not inform anyone of how
many I have received so be forewarned.
• Speaking of Bounty, who was the captain of the HMS Bounty
which was subjected to a mutiny in the year 1789 ? Who led
the mutiny ? More extra credit for the first person who gives
me the answers on a piece of paper with their name on it.
Electron configurations of ions
• There are two types of ions.
• CATIONS are positive ions like Sodium
(Na+1) and Magnesium (Mg+2).
• ANIONS are negative ions like Bromide
(Br -1) and Sulfide (S-2).
• For both, we start by writing out the econfiguration of the atom, ignoring the
charge (for now).
e- configurations of cations
• Let’s look at the Barium ion (Ba+2).
• It looks like this:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2
Rearranged:
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2
• Since it has a charge of + 2 we need to
REMOVE 2 electrons. We get:
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6
e- configurations of cations
• Let’s look at the Iron ion (Fe+3).
• It looks like this:
1s2 2s2 2p6 3s2 3p6 4s2 3d6
Rearranged:
1s2 2s2 2p6 3s2 3p6 3d6 4s2
• Since it has a charge of + 3 we need
to REMOVE 3 electrons. We get:
1s2 2s2 2p6 3s2 3p6 3d5
e- configurations of anions
• Now let’s look at Bromide ion (Br-1).
• Here is the electron configuration of the
atom after it is rearranged.
• 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5
• Since it is a -1 charge we need to add an
electron.
• So the new configuration is:
– 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
• Anions are easier to do than cations.