Atomic Structure :  Electron Configurations

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.

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.  2px, 2py, and 2pz) have identical sizes and shapes, and only their orientation differs.  Therefore, these orbitals are called degenerate orbitals because they have the same energy.

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

 1s 1s 2 s 2 p 3 s 3 p 4 s 3 d 4 p 5 s 4 d 5 p 6 s 5 d 6 p 7 s 6 d 7 p

 4 f 5 f

• 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"