After the quantum revolution started, many different ideas came about. Through countless ex-

periments and theories, it was eventually determined that in order to fully describe an electron

around an atom, four quantum numbers were needed. These quantum numbers are basically the

electrons zip code. The four quantum numbers are:

n, l, ml, ms Each of these quantum numbers can have different values according to the rules listed below:

Shells: n = 1, 2, 3. (integer value starting at 1)

n tells us the basic energy level and distance from the nucleus. The higher the value of n, the

higher the energy and the further out the electron is from the nucleus. It is important to realize

that the energy levels get successively closer together the further you go out from the nucleus.

This will play an important role later in the concepts.

Quantum Numbers

n=1

n=2 n=3

n=4

n=5

Subshell: l = 0.. (n 1) (from 0 up to the value of n-1)

The subshell tells us the shape of the electron cloud or how many nodal planes are in the elec-

tron cloud. Each subshell has a number, shape, and a letter associated with it. The shape of the

cloud represents the probability of finding an electron in that space around the atom:

l = 0 s subshells 0 nodal planes For an s orbital, l = 0, and has the general shape of a sphere. At successive energy levels, the spheres get bigger and bigger but the shape is still the same.

l = 1 p subshells 1 nodal plane For a p orbital, l = 1, and has the general shape of a dumbbell. It has 1 nodal plane, which is a space of zero probability of finding the electron:

l = 2 d subshells 2 nodal planes For a d orbital, l = 2, and has the general shape of a cloverleaf. It has 2 nodal planes, which bisect the probability twice, leading to the general four-leaf clover shape:

l = 3 f subshells 3 nodal planes For an f orbital, l = 3, and the shapes get quite complex. Imagine bisecting each of the clo-verleafs above through the middle again:

http://pubs.usgs.gov/

of/2005/1219/

Orbital: ml= -l . 0 . +l (integer values from l to +l)

The orbitals tells us the orientation of the electron cloud or how it is arranged around a central

point (the nucleus) in an x, y, z plane. The number of orientations is based upon the subshell or

l value.

l = 0 then according to the rules above, ml= 0 only Since ml= can only have a single value (0 in this case) there is only one orientation the electron cloud can have around the nucleus. This makes total sense, though, because the

shape of an s orbital is a sphere. No matter what way you turn it, it is still a sphere;

there is no difference.

l = 1 then according to the rules above, ml= -1 0 +1 Since ml= has three possible values (-1, 0, +1) that means there are three possible orien-tations of the orbital around the nucleus. These are in the x, y, and z planes respec-

tively:

l = 2 then according to the rules above, ml= -2 -1 0 +1 +2 Since ml= has five possible values (-2, -1, 0, +1, +2) that means there are five possible orientations of the orbital around the nucleus. These get pretty complicated:

l = 3 then according to the rules above, ml= -3 -2 -1 0 +1 +2 +3 Since ml= has seven possible values that means there are seven possible orientations of the orbital around the nucleus. These are simply insane:

Spin: ms = + 1/2 or 1/2

The last thing we need to fully describe an electron is to know which way it is spinning.

Against an arbitrarily chosen plane, the electron is either spinning clockwise or counter-clockwise. These are labeled as either +1/2 or 1/2. The reason it is 1/2 is well beyond the scope of this class. Just go with it.

http://hcper71011.blogspot.com/2011/02/quantum-numbers.html

Summary:

Name Letter Value Significance

Shell n 1, 2, 3. Tells energy and distance from nucleus

Subshell l 0, 1. (n-1) Tells shape of cloud (# of nodal planes) l = 0 s Spherical

l = 1 p Dumb bell

l = 2 d Cloverleaf

l = 3 f Split cloverleaf

Orbital ml -l...0+l Tells orientation of cloud around nucleus

Spin ms +1/2 or 1/2 Tells which way electron is spinning

So what do all these numbers mean? What do they have anything to do with Chemistry? They

seem pretty darn arbitrary and confusing. Where is this going?

Before I answer that, let me bring up one more concept that will help. There is another rule

called the Pauli Exclusion Principle that says that no 2 electrons in the same atom can have the

exact same 4 quantum numbers. As long as at least one of the numbers is different, it is a legal

arrangement. Lets start at the first energy level, n = 1 and see where it takes us.

n = 1 l = 0 (0 is the only possibility due to the previous rules)

n = 1 l = 0 ml = 0 (because l = 0, ml can only be zero

n = 1 l = 0 ml = 0 ms = +1/2 or 1/2 (ms can only ever be +1/2 or 1/2)

Now lets examine what we have. The Pauli Exclusion Principle says that as long as we can write a unique set of 4 numbers, we have an electron. How many electrons do we have here at

the first energy level?

Significance

n = 1 l = 0 ml = 0 ms = +1/2

n = 1 l = 0 ml = 0 ms = -1/2

We have 2 sets of 4 quantum numbers here (1, 0, 0, +1/2) and (1, 0, 0, -1/2) so the first

energy level has 2 electrons in it.

First energy level = 2 electrons

Im sure youre still lost but stick with me here. Lets examine the 2nd energy level, n = 2.

n = 2 n = 2 l = 0 l = 1 n = 2 n = 2 l = 0 l = 1 ml = 0 ml = -1, 0, +1

n = 2 n = 2 l = 0 l = 1 ml = 0 ml = -1, 0, +1 ms = +1/2 or 1/2 ms = +1/2 or 1/2

Now how many sets of 4 quantum numbers can we get here?

If n = 2, then l could = 0 or l could = 1

If n = 2 and l = 0, then ml = 0

If n = 2 and l = 1, then ml = -1, 0, +1

ms = +1/2 or 1/2 always

n 2

l 0

ml 0

ms +1/2

n 2

l 0

ml 0

ms -1/2

n 2

l 1

ml -1

ms +1/2

n 2

l 1

ml -1

ms -1/2

n 2

l 1

ml 0

ms +1/2

n 2

l 1

ml 0

ms -1/2

n 2

l 1

ml +1

ms +1/2

n 2

l 1

ml +1

ms -1/2

There are 2

electrons here

There are 6

electrons here

Second energy level

A group of 2 electrons

A group of 6 electrons

Total of 8 electrons

Do you get it yet? Probably not so lets go one more level...

You still probably dont see where this is going but bear with me for one more level.

n = 3 n = 3 n = 3 l = 0 l = 1 l = 2 n = 3 n = 3 n = 3 l = 0 l = 1 l = 2 ml = 0 ml = -1, 0, +1 ml = -2, -1, 0, +1, +2

n = 3 n = 3 n = 3 l = 0 l = 1 l = 2 ml = 0 ml = -1, 0, +1 ml = -2, -1, 0, +1, +2 ms = +1/2 or 1/2 ms = +1/2 or 1/2 ms = +1/2 or 1/2

Now how many sets of 4 quantum numbers can we get here?

n 3

l 0

ml 0

ms +1/2

n 3

l 0

ml 0

ms -1/2

n 3

l 1

ml -1

ms +1/2

n 3

l 1

ml -1

ms -1/2

n 3

l 1

ml 0

ms +1/2

n 3

l 1

ml 0

ms -1/2

n 3

l 1

ml +1

ms +1/2

n 3

l 1

ml +1

ms -1/2

There are 2

electrons here

There are 6

electrons here

n 3

l 2

ml -2

ms +1/2

n 3

l 2

ml -2

ms -1/2

n 3

l 2

ml -1

ms +1/2

n 3

l 2

ml -1

ms -1/2

n 3

l 2

ml +1

ms +1/2

n 3

l 2

ml +1

ms -1/2

n 3

l 2

ml +2

ms +1/2

n 3

l 2

ml +2

ms -1/2

n 3

l 2

ml 0

ms +1/2

There are 10

electrons here

Third energy level

A group of 2 electrons

A group of 6 electrons

A group of 10 electrons

Total of 18 electrons

I know, I know. What does this have to do with ANYTHING???!!!! GET TO THE POINT!

Lets recap what we learned in the past three pages:

1st Energy Level: 2 electrons

2nd Energy Level: 2 and 6 electrons to make 8 total

3rd Energy Level: 2 and 6 and 10 to make 18 total

Now lets look at the Periodic Table of Elements.

But instead of looking at the whole thing, lets look at it row by row; or more to the point, en-ergy level by energy level:

1

2

3

4

5

6

7

1

1st Energy Level: 2 elements

Does this sound familiar? It should. If we look at the listing above, we see the same thing.

Does this continue?

2

2nd Energy Level: 2 elements and 6 elements

Holy cow! Our quantum model predicted exactly what we see here.

1

2

3

4

5

6

7

1

2

What is the pattern here? If we continue the trend and apply it to the Periodic table we see that

the table is broken into the groups we see here. Groups of 2, 6, 10, and 14:

1st Energy Level: 2

2nd Energy Level: 2 and 6

3rd Energy Level: 2 and 6 and 10

4th Energy Level: 2 and 6 and 10 and 14

Group of 2 Group of 6

Group of 10

Group of 14

These are sections of the Periodic Table that contain elements that behave similarly to elements

in the rest of the section. Thus, we are going to designate each section with a letter to signify

what part of the table it is from. This is just like our baseball ticket that told us where to sit!

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