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Orbital Diagrams & Electron Configurations for Atoms and Ions, Lecture notes of Chemistry

Orbital diagrams represent the arrangement of electrons in orbitals. The aufbau principle: The atom is “built up” by progressively adding electrons. Electrons ...

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Orbital Diagrams
& Electron Configurations
for Atoms and Ions
Section 3.5
The Periodic Table is like a MAP
that describes the arrangement of
electrons within their orbitals.
Orbital diagrams represent the
arrangement of electrons in orbitals.
boxes or lines represent each orbital
arrows within boxes represent the electrons
max two per box
opposite direction (represents opposite spin)
1s
e.g., Hydrogen
(Z=1)
1s
e.g., Helium
(Z=2)
The aufbau principle:
The atom is “built up” by
progressively adding
electrons.
Electrons will fill the lowest
available energy levels first,
before filling higher levels.
Within a principal energy level,
s < p < d < f
Example: 3s fills before 3p
But… the (n+1)s orbitals always
fill before the ndorbitals.
Example: 5s fills before 4d
Order for orbital filling
pf3
pf4
pf5

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Orbital Diagrams

& Electron Configurations

for Atoms and Ions

Section 3.

The Periodic Table is like a MAP that describes the arrangement of electrons within their orbitals.

Orbital diagrams represent the

arrangement of electrons in orbitals.

  • boxes or lines represent each orbital
  • arrows within boxes represent the electrons
    • max two per box
    • opposite direction (represents opposite spin)

1s

e.g., Hydrogen (Z=1) 1s

e.g., Helium (Z=2)

The aufbau principle :

  • The atom is “built up” by progressively adding electrons.
  • Electrons will fill the lowest available energy levels first, before filling higher levels.

Within a principal energy level, s < p < d < f

Example: 3s fills before 3p

But… the (n+1) s orbitals always fill before the n d orbitals.

Example: 5s fills before 4d

Order for orbital filling

Practice!

Draw an orbital diagram for beryllium (Z=4)

1s

Guidelines for drawing orbital diagrams

  1. Fill orbitals in order of increasing energy.
  2. Orbitals of the same energy level must be filled before moving onto the next energy level.
  3. Place electrons singly into orbitals within a sublevel before any pairing occurs. Hund’s rule: To achieve the lowest possible energy, electrons will spread out singly before pairing up within an orbital.

N

nitrogen 14.

1s 2s 2p

O

oxygen 16.

1s 2s 2p

Orbital diagrams for ions

  • anion (negative charge): ADD appropriate number of electrons
  • cation (positive): REMOVE appropriate number of electrons

1s 2s 2p

Example: Oxide ion, O 2-

Before beginning, ask yourself: a) How many e -^ in a neutral oxygen atom? b) How many e -^ do I add/remove? c) What’s the new total number of e -?

Practice!

Draw orbital diagrams for:

a) Sulfur (Z=16)

b) Iron (Z=26)

c) Aluminum (Z=13)

d) Nitride ion (Z=7)

Tip: Use lines instead of boxes to represent the orbitals.

Two exceptions to the pattern: Cr and Cu

4s 3d

[Ar]

Chromium:

Possible explanation: Filled and half-filled sub-shells have lower energy than unfilled ones  more stable

  • An element’s location on the periodic table reflects the location of its outermost electrons.

Learning Checkpoint

An electron configuration is a shorthand notation that provides the same information as the orbital diagram.

B

boron 10.

Guidelines for writing electron configurations

  1. Write out subshells that have electrons in them.
  2. Use superscripts to indicate how many electrons are in each subshell.

Practice!

Write electron configurations for:

a) Sulfur (Z=16)

b) Iron (Z=26)

c) Aluminum (Z=13)

d) Gold (Z=79)

For simplicity, electron configurations can be abbreviated :

Gold: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f^14 5d 9

= [Xe] 6s 2 4f^14 5d^9

Nearest preceding noble gas

All the “extra” electrons

Elements in a group:

  • same valence electron configuration
    • common properties: chemical , physical

Electron configurations can explain periodic properties.

Electron configurations of ions

Why do atoms form ions?

Na: 1s 2 2s 2 2p 6 3s 1

Na +:

Mg: 1s 2 2s 2 2p 6 3s 2

Al: 1s 2 2s 2 2p 6 3s 2 3p 1

1s 2 2s 2 2p 6

1s 2 2s 2 2p 6

All of these ions are ISOELECTRONIC with Neon.

They have the same electron configuration as Ne.

Na +, Mg 2+, and Al 3+^ are isoelectronic with neon.

What other ions would have

the same electron configuration?

F: 1s 2 2s 2 2p 5

F -^ :

O: 1s 2 2s 2 2p 4

1s 2 2s 2 2p 6

Multivalent metals can have more

than one ionic charge.

  • “Transition Metals”
  • d-block elements
  • examples?
  • more orbitals = more possibilities

Pb: [Xe] 6s 2 4f 14 5d^10 6p^2

Pb4+^ : [Xe] 6s 0 4f 14 5d^10 6p^0

Lose 4e -

Pb2+^ : [Xe] 6s 2 4f 14 5d^10 6p^0

Lose 2e -

MO ORBITALS MO PROBLEMS

Summary

  • Orbital diagrams use boxes or lines to visually represent the orbitals of atoms. Arrows are used to represent electrons.
  • Electron configurations convey the same information, but in a more condensed form.

1s 2 2s 2 2p 6