Electron ConfigurationHistory of Modern Atomic Theory

Energy Levels and Sublevels

The Energy Levels

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History of Modern Atomic Theory

Niels Bohr noticed that electrons could only gain certain amounts of energy This implied that there were energy levels in the electron cloud Electrons could jump from one energy level to another, but could not exist

between levels. The levels were still pictured as circular orbits. This model of the atom was called the Bohr model and agree with

experimental data for hydrogen atoms. It failed to explain other atoms though.

History of Modern Atomic Theory

In the modern description of the electrons in atoms, the quantum mechanical model, the electron does not travel in a circular orbit, instead it exists somewhere within a region of space called an orbital We don’t know it exact position only the probability of

finding it We should no longer picture the electron as tiny particle

whizzing around the nucleus The propeller blade has the same probability of being

anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller.

Energy Levels and Sublevels Remember that e- are what hold atoms together to form

molecules. Understanding more about how they are arranged in the atom will help us better understand chemical reactions

The electron cloud is divided into energy levels Theoretically there are an infinite number of levels, but even the

biggest atoms only use seven The energy levels closest to the nucleus are lower in energy

Electrons always try to achieve the lowest energy state possible; this is called the Ground State.

This tendency is known as the Aufbau Principle (which means to “Build up”) When we start to place electrons into energy levels we begin with the lowest levels and

build our way up as needed.

Each energy level contains one or more Sublevels The first energy level has only one sublevel (called the s sublevel) The second energy level has two sublevels (called s and p sublevels) The third energy level has three sublevels (called s, p, and d sublevels) The fourth energy level has four sublevels (called s, p, d, and f sublevels)

Energy Levels and Sublevels Sublevels

Each sublevel is divided into Orbitals which are 3 dimensional regions of space where a pair of e- is most likely to reside

The s sublevel (the first sublevel in every energy level) has one orbital The p sublevel (the second sublevel in energy levels 2-7) has three orbitals The d sublevel (the third sublevel in energy levels 3-7) has five orbitals The f sublevel (the forth sublevel in energy levels 4-7) has seven orbitals

Each orbital can house two electrons

The Apartment Building Analogy: You can think of the electron cloud as an apartment building The energy levels are the same as the floors The sublevels are the same as the apartments The orbitals are the same as a bunk bed for two people Each person represents an orbital

s

s

s

s

p

p

p

d

d

f

1

2

3

4

1 sublevel1 orbital2e-

2 sublevels1+3=4 orbitals2 + 6 = 8e-

3 sublevels1+3+5=9 orbitals2 + 6 + 10 = 18e-

4 sublevels1+3+5+7=16 orbitals2 + 6 + 10 + 14 = 32e-

Comprehension Check

Knowing that each orbital can hold 2 e-, how many electrons can the following sublevels hold? s sublevel

1 orbital x 2 e- = 2 e-

p sublevel 3 orbitals x 2 e- = 6 e-

d sublevel 5 orbitals x 2 e- = 10 e-

f sublevel 7 orbitals x 2 e- = 14 e-

Comprehension Check

How many electrons can each energy level hold? 1st Energy Level

2nd Energy Level

3rd Energy Level

4th Energy Level

Answers to Comprehension Check

How many electrons can each energy level hold? 1st Energy Level

2 (2)

2nd Energy Level 8 (2+6)

3rd Energy Level 18 (2+6+10)

4th Energy Level 32 (2+6+10+14)

Energy Levels and sublevels

Fill out the following table using what you know about electrons and energy levels

Energy Level

SublevelsNumber of

OrbitalsTotal Number of

Electrons

1

2

3

4

s

s p

s p d

s p d f

1

1 + 3 = 4

1 + 3 + 5 = 9

1 + 3 + 5 + 7 = 16

2

2 + 6 = 8

2 + 6 +10 = 18

2 + 6 +10 +14 = 32

The First Energy Level

The first energy level is the lowest energy level in all atoms This level only has an s sublevel and can only hold two electrons

because it has only 1 orbital

The Second Energy Level

Second lowest energy level Has as s sublevel and a p sublevel The 2s sublevel has one orbital and can therefore hold 2 electrons The 2p sublevel has 3 orbitals and can therefore hold up to 6 electrons

The Third Energy Level

The third energy level has three sublevels s, p, and d The 3s and 3p sublevels have the same number of orbitals as 2s

and 2p The 3d orbital has 5 orbitals and

Can therefore hold up to 10 electrons

The Fourth & Higher Energy Levels

The 4th energy level has 4 sublevels: s, p, d, and f The 4s, 4p, and 4d are all the same shape, except bigger than lower

energy levels The 4f sublevel has 5 orbitals

Electron Arrangement in Atoms

There are three rules to help us understand where electrons are in the electron cloud Aufbau principle – electrons will occupy the lowest energy level

possible Pauli Exclusion Principle – only two electrons may occupy an orbital

and they must have opposite spin (clockwise and counterclockwise) Hund’s Rule – when filling a sublevel such as 3p, e- will go to an

empty orbital prior to pairing up (spread out before pairing up)

Writing Electron Configurations

Electron Configurations tell you the ground state location of each electron in an atom

Example 1: What is the Electron Configuration of nitrogen First of all, how many electron does a nitrogen atom have?

7

Example 2: What is the Electron Configuration of silicon First of all, how many electron does a silicon atom have?

14

1s2 2s2 2p3

1s2 2s2 2p6 3s2 3p2

# of e-

Energy LevelSublevel

The Energy of Orbitals

Higher sublevels do not necessarily have a higher energy The order from lowest to highest energy is as follows

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7sLowest Energy Highest Energy

A Trick

1s

2s 2p

3s 3p

4s

3d

4p

5s

4d

5p

6s

4f

5d

6p

7s

5f

6f6d

7p 7f7d

Fill the sublevels in the order that the arrows pass through, beginning with the top arrow.

Comprehension Check

Write the electron configurations for the following elementsBe, Mg, S, Ar, Sc, Fe, Ge, Kr, Tc

Comprehension Check

Write the electron configurations for the following elements Be

1s2 2s2

Mg 1s2 2s2 2p6 3s2

S 1s2 2s2 2p6 3s2 3p4

Ar 1s2 2s2 2p6 3s2 3p6

Comprehension Check

Write the electron configuration for the following elements. Sc

1s2 2s2 2p6 3s2 3p6 4s2 3d1

Fe 1s2 2s2 2p6 3s2 3p6 4s2 3d6

Ge 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2

Kr 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

Tc 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 5d10

Congratulations, you can now write long electron configurations!