Download - Electron Configurations II
Electron Configurations II
F. Schifano, Department of ScienceBayonne High School, Bayonne NJ
Key Skills:
Describe electrons in detail
Write electron configurations
Use electron configurations
What are Valence Electrons?
The valence electrons of an element are the outermost electrons.
Only s and p electrons in an atom’s highest energy level count as valence electrons.
Determining the # of Valence e-
Write the configuration
Identify the highest energy
level
Add up s and p electrons at that
level only
Electron-Dot (Lewis) Diagrams Represent Valence Graphically
Each side represents a sub-orbital.
s-orbitals get both electrons first. Each p sub-orbital gets one e-
before any p-sub-orbital gets two. This is called Hund’s Rule.
Writing Electron-Dot Diagrams
Determine valence #
Write chemical symbol
Draw dots to represent the
valence e
Practice
Determine the number of valence electrons in each of the following elements. Then draw their dot diagrams:
CaFBrSrNO
Why are Valence Electrons Special?
Formation of Covalent Bonds
When molecules get close to each other, the valence electrons are attracted to the nucleus of the other atom.
Valence Electrons Form Bonds
If trading or sharing electrons would make both atoms more stable, this temporary attraction becomes a full-fledged chemical bond.
Valence Electrons Predict Chemical Behavior
If two elements have the same number of valence electrons, they will react in a very similar way. They need similar
changes in their electron configuration to become stable.
Practice
Which ones should have similar properties and react in similar ways?
CaFBrSrNO
The Holy Grail: Valence = 8
The most stable configuration of electrons most atoms can have is valence = 8, also known as the octet configuration.
In chemical reactions, most atoms just take the simplest path to valence = 8.
Noble Gases Are Already Stable
Neon, argon, krypton, radon, and xenon are all called noble gases.
They already have valence=8 . They don’t benefit from reacting, so they don’t react!
Same Column = Same Valence
The periodic table is designed in just such a way that elements with the same valence fall into the same vertical column.
Four Families
Alkali MetalsGroup I
s1
Lose 1 e-
Alkaline Earth Metals
Group 2s2
Lose 2 e-
HalogensGroup 17
s2p5
Gain 1 e-
Noble GasesGroup 18
s2p6
No Reaction
Special Cases
Hydrogen becomes stable at valence = 0 or at valence = 2. It can give away an electron, becoming
valence =0 (H+) or it can gain an electron, becoming valence =2 (H-).
Helium is already stable at valence =2. Like the other noble gases, helium is
already stable and doesn’t react. Boron is stable at valence =6.
Writing Configurations Using the Periodic Table
An element’s position on the periodic table tells you the last electron that filled its orbitals: Period (horizontal) = energy level Block = orbital shape Box number = #e- in the orbital.
Aufbau principle tells you all the rest!
Writing Configurations Using the Periodic Table
Sulfur is in the fourth box of the third row, in the p-block. Its last electron is
3p4. Aufbau principle
says everything under 3p4 must be filled, so: 1s2 2s2 2p6 3s2 3p4.
D-Block and F-Block Exceptions
If an element is in the d-block its last orbital will really be one energy level down from the row it’s in. Last electron in chromium (Cr) = 3d4,
NOT 4d4
If an element is in the f-block its last orbital will really be two energy levels down from the row it’s in. Last electron in plutonium (Pu) = 5f5,
NOT 6f5
Noble Gas Notation
Longer configurations are a chore. Noble gases can be used as starting
points for longer configurations.
Using Noble Gas Notation
Find target element on the Periodic Table
Choose a noble gas core
Write only electrons
between core and target
Noble Gas Notation
Cr =1s2 2s2 2p6 3s2 3p6 4s2 3d4 Ar = 1s2 2s2 2p6 3s2 3p6
____________________________
So we can write the configuration of Cr as [Ar] plus the difference:
[Ar] 4s2 3d4
Questions
What are valence electrons?