Electron Configuration - Describes the distribution of electrons in atomic orbitals according to specific rules.

Electron Diagram - Illustrates the distribution of electrons in atomic orbitals according to specific rules.

Aufbau Principle

Electrons are added one at a time, starting with the lowest energy orbital until all the electrons have been accounted for.

		Relative Energies of Atomic Orbitals
Wolfgang Pauli

Pauli Exclusion Principle

• An orbital can hold a maximum of 2 electrons.
• 2 electrons in the same orbital must have opposite spins.
• An electron is "paired" if it is sharing an orbital with another electron with an opposite spin.
• An electron is "unpaired" if it is alone in an orbital

A H atom has one electron which is unpaired in the lowest energy orbital, 1s.

H:  1s¹    

A He atom has two electrons;  the second one fills the 1s orbital with a spin opposite that of the first electron.  These two electrons are now paired.

He:  1s²    

^{A Li atom has 3 electrons;  the third one is added to the next lowest energy orbital, 2s and is unpaired.}

Li:  1s²2s¹    

A Be atom has 4 electrons;  the fourth one is fills the 2s orbital with a spin opposite that of the first electron in the orbital.  These two electrons are now paired.

Be:  1s²2s²     

A B atom has 5 electrons;  the fifth one is added unpaired to the next lowest energy orbital, 2p.

B:  1s²2s²2p¹    

The p subshells have 3 orbitals, each which can hold two electrons.  Orbitals within the same subshell (e.g.  2p_x, 2p_y, and 2p_z) have identical sizes and shapes, and only their orientation differs.  Therefore, these orbitals are called degenerate orbitals because they have the same energy.

Hund's Rule

Electrons are added to degenerate, equal energy, orbitals so that a maximum number of unpaired electrons results.

B:  1s22s22p1		O:  1s22s22p4
C:  1s22s22p2		F:  1s22s22p5
N:  1s22s22p3		Ne:  1s22s22p6

An atom is diamagnetic if all of its electrons are paired.

e.g.  Of the atoms listed directly above, only Ne is diamagnetic.  Atoms ending with xs², xp⁶, xd¹⁰, and xf¹⁴ are diamagnetic.

An atom is paramagnetic if it has one or more unpaired electrons

e.g.  B, C, N, O, and F above are all paramagnetic because they all have at least one unpaired electron.

Electron configurations are often abbreviated by naming the last element with a filled shell (e.g.  He and Ne) in brackets and listing only the orbitals after the filled shell.  This outermost shell is called the valence shell and the electron in the valence shell are called valence electrons.

Li:  1s²2s¹       shorthand    Li:  [He] 2s¹    

Na:  1s²2s²2p⁶3s¹      shorthand    Na:  [Ne] 3s¹    

Atomic Number		Symbol		Electron Configuration		Electron Diagram
1		H		1s1
2		He		1s2
3		Li		[He] 2s1
4		Be		[He] 2s2
5		B		[He] 2s22p1
6		C		[He] 2s22p2
7		N		[He] 2s22p3
8		O		[He] 2s22p4
9		F		[He] 2s22p5
10		Ne		[He] 2s22p6
11		Na		[Ne] 3s1
12		Mg		[Ne] 3s2
13		Al		[Ne] 3s23p1
14		Si		[Ne] 3s23p2
15		P		[Ne] 3s23p3
16		S		[Ne] 3s23p4
17		Cl		[Ne] 3s23p5
18		Ar		[Ne] 3s23p6
19		K		[Ar] 4s1
20		Ca		[Ar] 4s2
21		Sc		[Ar] 4s23d1
22		Ti		[Ar] 4s23d2
23		V		[Ar] 4s23d3
24		Cr		[Ar] 4s13d5
25		Mn		[Ar] 4s23d5
26		Fe		[Ar] 4s23d6
27		Co		[Ar] 4s23d7
28		Ni		[Ar] 4s23d8
29		Cu		[Ar] 4s13d10
30		Zn		[Ar] 4s23d10
31		Ga		[Ar] 4s23d104p1
32		Ge		[Ar] 4s23d104p2
33		As		[Ar] 4s23d104p3
34		Se		[Ar] 4s23d104p4
35		Br		[Ar] 4s23d104p5
36		Kr		[Ar] 4s23d104p6

Exceptions to the predicted electron configurations

Two elements of the first 40 elements have electron configurations different from what would be normally predicted.

Predicted:  Cr:  [Ar] 4s²3d⁴    

Actual:       Cr:  [Ar] 4s¹3d⁵    

^Predicted:  Cu:  [Ar] 4s²3d⁹    

Actual:       Cu:  [Ar] 4s¹3d¹⁰    

Relationship between electron configurations and the periodic table

• The shell number of the s and p orbitals is the same as the period, or row, in which they are located.
  • e.g.  the 2s orbital is in the 2nd period and the 3p orbitals are in the 3rd period.
• The shell number of the d orbitals is one number lower than the period in which they are located.
  • e.g.  the 3d orbitals are in the 4th period.
• The shell number of the f orbitals is one two numbers lower than the period in which they are located.  To consolidate space, the atoms with f-orbital electrons (Lanthanide and Actinide series) are listed separate from the rest of the periodic table.
  • e.g.  the 4f orbitals are considered to be in the 6th period.

Next:  "Development of Elements and the Periodic Table"