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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
- Fill orbitals in order of increasing energy.
- Orbitals of the same energy level must be filled before moving onto the next energy level.
- 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
- Write out subshells that have electrons in them.
- 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
