Download The Quantum Mechanical Model of the Atom

The Quantum Mechanical Model of the Atom
Quantum Theory is used to explain the chemical behavior of atoms.
Heisenberg’s Uncertainty Principle: It is impossible to determine both the position and the
momentum of a moving object. You can’t know both where a particle is at any given moment
and how it is moving in order to predict where it will be in the future.
Ex) To locate a set of keys in a dark room you use a flashlight and can see when the light
reflects off of the keys and in to your eyes. To locate an electron you can strike it with another
particle and see where it bounces off. The problem is that the electron is so small that the
collision would move it in an unpredictable way.
We can get around this problem by discussing the locations of electrons in terms of probability.
Let’s review what we know so far about electrons:
1. Electrons can only have specific amounts of energy.
2. Electrons exhibit wavelike behavior.
3. It is impossible to know the exact position and location of an electron at any given
instant.
The quantum mechanical model has been developed to explain the properties of an atom while
incorporating these ideas. It says that an electron travels like a wave, and has quantized its
energy. We can visualize the location of electrons in terms of probability.
The probability of finding an electron in various locations around the nucleus can be pictured as
a cloud of negative charges. The cloud is the densest in the regions where the electrons have the
highest probability of being. Conversely, the lowest electron density is the region where the
electrons are least likely to be.
Electrons can be described by a set of four numbers called Quantum Numbers.
The first number is called the principle quantum number and it describes the major energy level
that the electron is in. This number is the same as the number of the energy level that contains
the electron, so if the electron is in the second energy from the nucleus, it has a principle
quantum number of 2. Each energy level also contains as many sublevels as the number of the
level. For example, level 3 contains 3 sublevels.
There are 4 types of sublevels that contain orbitals.
S Sublevel is spherical shaped. There is only one way a sphere can be arranged, so there is only
1 orbital in an S sublevel.
P Sublevel is dumbbell shaped. They can be arranged at right angles to each other in the X, Y,
and Z planes, so the P sublevel contains 3 orbitals.
D and F Sublevels are more complicated. D has 5 orbitals and F has 7.
Each orbital can hold 2 electrons.
All of these factors that are taken into consideration make up what’s called the Electron
Configuration. Here is a table to make better sense out of some of the components we have just
talked about:
Principal
Energy Level (n)
Type of
Sublevel
1
s
s
p
s
p
d
s
p
d
f
2
3
4
Number of
Orbitals in a
Sublevel
1
1
3
1
3
5
1
3
5
7
Total Orbitals
per Energy
Level
1
Maximum
Number of
Electrons
2
4
8
9
18
16
32
If the principal energy level is n, can you figure out a formula to calculate the maximum
2n2
number of electrons per level?
Electron configurations are written in a string of letters and numbers that are used to refer to
the P.E. Level, the sublevel, and the # of electrons present on that sublevel. This is like an
address, where you might have a house number, a street and a town, for instance.
1s2
P.E. Level
# of electrons
Sublevel
The current version on Periodic Table denotes # of P.E. levels by dashes which separate the #
of electrons on each level.

You are going to learn to write the full configuration though. The first step is to figure out the
total number of electrons that you have, and then to fill in all of the sublevels starting with the
one with the lowest energy and working your way up, in order. (The Aufbau Principle)
Here is a quick chart that you can draw to remember the order in which the energy levels and
sublevels are filled:
1
s
2
s
p
3
s
p
d
f
4
s
p
d
f
5
s
p
d
f
Follow each arrow to the end and then start at the beginning of the next arrow. Remember that
each s holds 2 electrons, each p holds 6 total (2 is each of the 3 orbitals), each d holds 10 total
and each f holds 14 total.
Here’s an example: 1s22s22p63s23p4
Can you tell me what element this is? 16e = 16p = Sulfur
Okay, let’s write the electron configuration of carbon. Carbon has 6 electrons.
How about sodium?
1s22s22p2
1s22s22p63s1
What would be the new way of writing this with just the principle energy levels?
Count how
many electrons are in each energy level and separate them by dashes: 2-8-1
Remember, each energy level has a maximum number of electrons that it can hold. You can
never have more than 2 in the first level or 8 in the second level.