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Orbital Diagrams and Electron Configuration

• Drawing orbital diagrams gives information not only about the orbitals that are/have been filled but also about the number of unpaired electrons.

• Orbital diagrams can be cumbersome!!

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Electron Configuration

• A short-hand notation is commonly used in place of orbital diagrams to describe the electron configuration of an atom.

• Electron configuration:– a particular arrangement of electrons in the

orbitals of an atom

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Another way of expressing electron distribution:

Electron Configurations• Know relative energies of orbitals• Pauli exclusion principle

Distribution of electrons among the various orbitals =

Electron configuration

eg: Carbon: 1s2 2s22p2

6 electrons

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Electron Configurations• Each component

consists of – A number denoting

the energy level (n),

– A letter denoting the type of orbital (l),

- A superscript denoting the number of electrons in those orbitals.

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Electron Configuration

• The electron configuration tells the number of electrons found in each subshell using superscripts

• If there are three electrons in a 2p subshell, we would write:

2p3

where the superscript (3) indicates the number of electrons in that subshell

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Electron Configuration

• The orbital diagram for an O atom:

1s 2s 2p 3s

The electron configuration for an O atom:

1s22s22p4

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Orbital Diagrams & Electron Configurations

The orbital diagram for potassium.

Z = 19 so there are 19 electrons

And number of subshells (s,p,d..) and orbitals per energy level (n)

3p 4s1s 2s 2p 3s

Electron configuration of K: 1s2 2s22p6 3s23p6 4s1

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Electron Configuration

To determine the electron configuration of an atom (or ion) without first writing the orbital diagram:– determine the number of electrons present– add electrons to each subshell in the

correct filling order until all electrons have been added

• use the “diagonal” diagram to help determine the filling order

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Electron Configuration

Example: Write the electron configuration of a Mn atom (Z = 25).

1s2 2s22p6 3s23p6 4s2 3d5

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Electron Configuration can be written for ions as well

Example: Write the electron configuration of an O2- ion (Z = 8).

1s22s22p6

An O2- ion has 8 protons and 10 electrons

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Electron Configuration

Write the electron configuration of a krypton atom (Z = 36).

1s22s22p63s23p64s23d104p6

This is the Kr “core” [Kr]

• The noble gas “core” can be used to write the electron configuration of an element using

core notation:

noble gas “core” + valence electrons

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Core notation

To write the electron configuration using the core notation:• Find the Noble Gas that comes before the atom.• Determine how many additional electrons must be

added beyond what that noble gas has.

(= Atomic number of atom minus atomic number of noble gas)

• Determine the period that element is in. (This determines the value of n of the s subshell to start with when adding extra electrons)

• Add electrons starting in that “n” subshell

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Electron Configuration

Write the core electron configuration of Sr (Z = 38).

Previous noble gas: Kr (Z = 36)Extra electrons: 38 (e of Sr) - 36 = 2Period number of Sr: 5So: Kr core plus 2 extra e- starting in 5s

[Kr] 5s2

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Electron Configuration

Write the core electron configuration of Br (Z = 35).

Previous noble gas: Ar (Z = 18) Extra electrons: 35 - 18 = 17Period number: 4So: Ar core plus 17 extra e- starting with 4s

[Ar] 4s23d104p5

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Isoelectronic Series

• When atoms ionize, they form ions with the same number of electrons as the nearest

(in atomic number) noble gas.

Na = 1s22s22p63s1 = [Ne]3s1

Na+ = 1s22s22p6 = [Ne]

Cl = 1s22s22p63s23p5 = [Ne]3s23p5

Cl- = 1s22s22p63s23p6 = [Ar]

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Isoelectronic Series

• N (7 e-): 1s22s22p3

• O (8 e-): 1s22s22p4

• F (9 e-): 1s22s22p5

N3- (10 e-): 1s22s22p6 = [Ne]

O2- (10 e-): 1s22s22p6 = [Ne]

F- (10 e-): 1s22s22p6 = [Ne]

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Isoelectronic Series

• Na (11 e-): 1s22s22p63s1

• Mg (12 e-): 1s22s22p63s2

• Al (13 e-): 1s22s22p63s23p1

Na+ (10 e-): 1s22s22p6 = [Ne]

Mg2+ (10 e-): 1s22s22p6 = [Ne]

Al3+ (10 e-): 1s22s22p6 = [Ne]

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H

Li Be

Na Mg

Rb

Cs

Fr Ra

Ba

Sr ITeSbSnInCdAgPdRhRuTcMoNbZrY

La

Ac Rf

Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At

Db Sg Bh Hs Mt

LuYbTmErHoDyTbGdEuSmPmNdPrCe

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

F

ClSPSiAl

B C N O

1A

2A

3B 4B 5B 6B 7B 8B 8B 8B 1B 2B

3A 4A 5A 6A 7A

CaK Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Xe

Rn

Ar

Ne

He

8AIons of the highlighted elements are

isoelectronic with Ne.

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Isoelectronic Series

• Isoelectronic: having the same number of electrons

• N3-, O2-, F-, Ne, Na+, Mg2+, and Al3+ form an isoelectronic series.– A group of atoms or ions that all contain the

same number of electrons

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Isoelectronic Series

• Examples of isoelectronic series:– N3-, O2-, F-, Ne, Na+, Mg2+, Al3+

– Se2-, Br-, Kr, Rb+, Sr2+, Y3+

– Also: Cr, Fe2+, and Co3+

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Sizes of Ions - Trends

• In an isoelectronic series, ions have the same number of electrons.

• Ionic size decreases with an increasing nuclear charge.