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?