Electron Configuration

Chemistry

Learning Objectives

• TLW express the arrangement of electrons in atoms through electron configurations (TEKS 6.E)

Agenda

• Electron Configuration Overview• Aufbau Principle and Diagram• Pauli Exclusion Principle• Hund’s Rule• Writing Electron Configurations

– Full, Noble Gas, For Ions

• Orbital (Box) Diagrams• Exceptions• Group and Individual Practice

Electron Configurations

The way electrons are arranged in atoms:

1. Every element has a specific electron configuration – NO two are alike

2. Very important = scientists know how an atom will react and how to manipulate that element

Break into Lab Teams – each with an Atom Building Game as we Inquire about e-

config

Take your Periodic Table to add info

Three Rules for Electron Configuration

• Aufbau Principle

• Pauli Exclusion Principle

• Hund’s Rule

1. Aufbau Principle

a. Electrons enter the lowest energy level first

b. Orbitals do overlap – so Aufbau’s principle plays a very important part in writing electron configuration

Aufbau’s principle

c. so, 1s would fill first; followed by 2s, then 2p etc

2. Pauli Exclusion Principle

a. at most there is only 2 electronsper orbital

b. Different spin for each

c. there are four orbitals =s, p, d, f

1s 2s 2p

3. Hund’s Rule

a. electrons do not pair up until they have to

b. the empty bus seat

RightWrong

C. Writing electron configuration

1. Remember the 3 rules: Aufbau’s, Pauli’s and Hund

2. Orbitals DO NOT fill in a nice neat order; orbitals overlap

3. Must fill lowest energy FIRST (Aufbau’s)

4. 1st energy level is s; 2nd is s & p; 3rd is s, p & d; 4th is s, p, d & f

5. This is the order electrons fill up in going from lowest to highest energy level per Aufbau Principle

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 6f 7d 7f

Oxygen

8 e-

6. Orbital Diagram (Box Diagrams)

• Electron Configuration

1s2 2s2 2p4

1s 2s 2p

D. Practice

Let’s determine the electron configuration for Phosphorus

A total of 15 electrons

Incr

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nerg

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

Aufbau diagram

• The first two electrons go into the 1s orbital

• Notice the opposite spins

• only 13 more to go...

Incr

easi

ng e

nerg

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1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next 2 electrons go into the 2s orbital

• only 11 more...Incr

easi

ng e

nerg

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1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next 6 electrons go into the 2p orbital

• only 5 more...

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next 2 electrons go into the 3s orbital

• only 3 more...

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The last three electrons go into the 3p orbitals.

• They each go into separate shapes

• 3 unpaired electrons

• = 1s22s22p63s23p3

The easy way to remember

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2

• 2 electrons

Fill from the bottom up following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2

• 4 electrons

Fill from the bottom up following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2 2p6 3s2

• 12 electrons

Fill from the bottom up following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2 2p6 3s2 3p6 4s2

• 20 electrons

Fill from the bottom up following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2 2p6 3s2

3p6 4s2 3d10 4p6

5s2

• 38 electrons

Fill from the bottom up following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2 2p6 3s2

3p6 4s2 3d10 4p6

5s2 4d10 5p6 6s2

• 56 electrons

Fill from the bottom up following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2 2p6 3s2

3p6 4s2 3d10 4p6

5s2 4d10 5p6 6s2

4f14 5d10 6p6 7s2

• 88 electrons

Aufbau PrincipleFill from the bottom up (lowest energy

levels) first following the arrows

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

• 1s2 2s2 2p6 3s2

3p6 4s2 3d10 4p6

5s2 4d10 5p6 6s2

4f14 5d10 6p6 7s2

5f14 6d10 7p6 • 108 electrons

S 16e-

Valence Electrons 6

Core Electrons

1s2 2s2 2p6 3s2 3p4

Practice Again

Practice one more time

Cl 17 e-

1s2

2s2 2p6

3s2 at this point we need 5 more

3p5

So Cl is 1s22s22p63s23p5

with 7 valence electrons

Shorthand Writing e- Configs

• Noble Gas Notation

• Noble Gas Notation: 

C = [He]2s22p2

Br = ?

Group Practice• Using the Build-an-Atom Game determine the

electron configurations for the 36 elements in Lab C-1, Section 4 Table

E. Exceptions to the rules

1. Orbitals fill in order

2. Lowest energy to highest energy

3. Adding electrons can change the energy of an orbital

4. Half filled orbitals have a lower energy

5. Half filled orbitals make an atom more stable

6. this stability can change the order in which electrons fill up an orbital

7. so if you have 4s2 and 3d9

it is better to have 4s1 and 3d10

8. Because a full or half is more stable than a 7, 8, 9

9. BUT only in certain elements

10. Example

Titanium - 22 electrons =

1s22s22p63s23p64s23d2

Vanadium - 23 electrons =

1s22s22p63s23p64s23d3

Chromium - 24 electrons =

1s22s22p63s23p64s23d4

But this is wrong for chromium!!

We expect chromium to be

1s22s22p63s23p64s23d4

BUT it is really:

1s22s22p63s23p64s13d5

Why?

This gives two half orbitals

Which means more stable atom

And slightly lower energy

11. Another Exception CopperCopper has 29 electrons so

we expect: 1s22s22p63s23p64s23d9

But the actual configuration is:1s22s22p63s23p64s13d10

This gives one filled orbital and one half filled orbital.Remember these exceptions: d4, d9

12. Writing e- Configs for Ions

• For anions – add arrows to boxes and add to orbitals to written format– Example – O2-

1s2 2s2 2p6

• For cations – remove arrows from boxes and subtract from orbitals in written format– Example – Li1+

– 1s2 2s1

1s 2s 2p

Group Practice1. What is the electron configuration

for Strontium?

Sr has 38 electrons

Electron configuration is

1s22s22p63s23p64s23d104p65s2

7s 7p 7d 7f 6s 6p 6d 6f5s 5p 5d 5f4s 4p 4d 4f3s 3p 3d2s 2p1s

1s2 2s2 2p6 3s2 3p6 4s2

3d10 4p6 5s2

2. What is the electron configuration of Cadmium?

Cd has 48 electrons

Incr

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

Aufbau diagram - page 3672

1s2

2

2s2

6

2p6

2

3s2

6

3p6

2

4s2

10

3d10

6

4p6

2

5s2

At this point we have 38

Need 10 more10

4d10

1s22s22p63s23p64s23d104p65s24d10

3. What element has an electron configuration of:

1s22s22p63s23p64s23d104p65s24d105p3

Answer: 2+2+6+2+6+2+10+6+2+10+3

The element with 51 electrons

Antimony

4. Determine electron configuration of Oxygen?

8 electrons

1s2

2s2

2p4

Answer: 1s22s22p4

5. How many valence electrons does oxygen have?

if the electron configuration is

1s22s22p4

then is has 6 valence electrons

Electron Configuration Shorthand

• You can write an element’s electron configuration beginning with the closest Nobel Gas

• For example – Rather than write Strontium as 1s22s22p63s23p64s23d104p65s2 you could just write [Kr]5s2

Summary1. Fill lower energy levels 1st (Aufbau Principle)

2. Remember 2 electrons per orbit (Pauli Exclusion Principle)

3. Electrons don’t pair up until they must - Fill “up” arrows 1st (Hund’s Rule)

4. Energy Levels = Periods = Rows

1st energy level has an s orbital 2nd has s and p = 8

3rd has s, p & d = 18 4th has s, p, d & f = 32

5. The order of electrons

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d

6. Exceptions – chromium and copper (shift an electron between 4s and 3d orbitals for stability)

7. Can use shorthand by starting with closest Nobel Gas and building from there – ex. Strontium [Kr]5s2

8. Count superscripts to determine the element

9. Valence electrons determined by counting electrons in highest energy level

10.Can also show ions – add or subtract appropriate number of electrons

Individual Practice

• Atom Building Game – Lab C-1 “How is the Periodic Table Organized?”

• Guided Practice from Chemistry book• Worksheets on Aufbau Principle, Pauli

Exclusion Principle, Hund’s Rule, Orbital Diagrams, Writing Electron Configurations

Practice Set #1

Practice Set #2

Practice Set #3