1.1 What is Chemistry?

5.3. Electron Arrangement in Atoms http://www.ck12.org

Quantum Numbers

We use a series of specific numbers, calledquantum numbers, to describe the location of an electron in an
associated atom. An electron in an atom or ion has four quantum numbers to describe its state. Think of them
as important variables in an equation which describes the three-dimensional position of electrons in a given atom.

Principal Quantum Number

The principal quantum number, signified by (n), is the main energy level occupied by the electron. Electrons in
orbitals with higher principal quantum numbers are, on average, further from the nucleus and higher in energy.
Possible values for the principal quantum number include any positive whole number (e.g., 1, 2, 3, 4, 5, 6, ...). As
we will see, the principal quantum number is related to the row in which an element appears on the periodic table.

Angular Momentum Quantum Number

The angular momentum quantum number, signified by (l), describes the general shape of the region occupied by an
electron. The possible value(s) ofldepend on the value of the principal quantum number n. The angular momentum
quantum number can be any whole number between zero and (n-1). For example, if n = 2,lcould be either 0 or 1.

Magnetic Quantum Number

The magnetic quantum number, signified by (ml), describes the orientation of an orbital in space. For a given value
of the angular momentum quantum numberl, there are (2l+ 1) possible values forml, which are determined as
follows:

• l, (-l+1) ... 0 ... (+l–1), +l

For example:

If n = 2

Thenl= 0 or 1

forl= 0,ml= 0

forl= 1,ml= -1, 0, or +1

TheTable5.1 shows the possible magnetic quantum number values (ml) for the corresponding angular momentum
quantum number (l).

TABLE5.1: Relationships among Values of n,

n Possible Values

ofl

Subshell Desig-

nation

Possible Values

ofml

Number of Or-

bitals in Sub-

shell

Total Number of

Orbitals in Shell

1 0 1s 0 1 1

2 0

1

2s

2p

0

1, 0, -1

1

3

4

3 0

1

2

3s

3p

3d

0

1, 0, -1

2, 1, 0, -1, -2

1

3

5

9