Electron Configuration

And a Brief Introduction to the Quantum Model of the Atom

Bohr’s Atomic Model Electrons are located in specific

energy levels

n=1

n=2

Energy Levels and the Periodic Table

The main energy level number corresponds to a period in the periodic table of the same number

All the elements of that period use that energy level for their electrons

n=1 Period 1

n=2 Period 2

n=3 Period 3

etc.

Sublevels Sublevels are located within an energy level Each sublevel has a name

Energy Level Names of Sublevels

1st Energy Level; n=1

s

2nd Energy Level; n=2

s and p

3rd Energy Level; n=3

s, p, and d

4th Energy Level; n=4

s, p, d, and f

Subdivisions in the Periodic Table

The periodic table can be subdivided to show the sublevels

Orbitals Energy levels can have different sublevels Sublevels can have different orbitals Orbitals are located inside sublevels Different sublevels have different numbers

of orbitals

Sublevel# of Orbitals

Possible

s 1

p 3

d 5

f 7

Orbitals Only 2 electrons can fit in each orbital There are 2 electrons in an s orbital There are 2 electrons in a d orbital Since there are 5 d orbitals, there are a

total of 10 electrons in the d sublevel There is a slight variation in the energy of

electrons between sublevels, but electrons in orbitals of the same sublevel have the same energy

https://www.wisc-online.com/learn/natural-science/chemistry/gch904/the-structure-of-an-atom

Diagrams of s, p, and d

dx

y

px

s pz

py

dz2dx

2y

2

dxzdyz

Electron Configuration

Each element has a distinct electron configuration that can be written using the energy levels, sublevels, and orbitals that its electrons occupy

The electron configuration for hydrogen is

1s1Main energy level

sublevel

Indication Of Orbital

Electron Configuration

Unfortunately, some orbitals at higher energy levels fill before all orbitals at lower energy levels

This makes an atom more stable For example, 4s fills before 3d

Using the Periodic Table We can use and element’s position on the

periodic table to determine its electron configuration

The electron configuration for oxygen is:1s2 2s2 2p4

Exception

A d sublevel that is half full or full (i.e. 5 or 10 electrons) is more stable than the s sublevel of the next energy level

As a result, electron configurations rarely end in __ d 4 or __ d 9

An electron is taken from the previous s sublevel to change this to __ d 5 or __ d 10

Example: chromium

The electron configuration for chromium is

1s2 2s2 2p6 3s2 3p6 4s1 3d5

not

1s2 2s2 2p6 3s2 3p6 4s2 3d4

Abbreviated Electron Configuration

The abbreviated electron configuration only shows the number of electrons in each main energy level

The abbreviated electron configuration for oxygen is: 2 , 6

The abbreviated electron configuration for chromium is: 2 , 8 , 13 , 1

The Quantum Model of the Atom

Bohr’s Work

Bright Line Spectra

Electron Transitions

Properties of Light

Light as a Wave • Light can be thought of as electromagnetic

radiation having a particular wavelength and frequency

Light as a Particle• Albert Einstein proposed almost a century ago

that electromagnetic radiation can be viewed as a stream of particles known as photons

• A photon has a particular amount of energy associated with it

Back to Bohr• Neils Bohr wanted to determine exactly where

electrons were located in an atom• Studied gaseous hydrogen atoms and the

specific colours of light they produced when electricity was applied to them

• When Bohr focused the light through a prism, he observed lines of only certain specific colours

• These lines are known as a bright line spectrum

• Every element on the periodic table has its own unique bright line spectrum

Bright Line Spectra• Bohr concluded that electrons exist in specific

energy levels in the atom and that these energy levels are quantized (i.e. have a certain value of energy associated with them

• the energy levels where electrons are normally found are called ground states

• If an electron absorbs sufficient energy it moves to a higher energy level to produce an excited state

• When the electron releases the energy, it drops back to a lower energy level, and the energy is released in the form of electromagnetic radiation

The Electromagnetic Spectrum

• The wavelength of the emitted light indicates the difference in energy between the ground state and the excited state

• Each wavelength corresponds to a specific type of electromagnetic radiation, which may or may not be visible

Electron Transitions

Electron Transitions in a Hydrogen Atom

INVISIBLE

INVISIBLE

SEEN AS 4 DIFFERENT COLOURS

The Visible Light Spectrum

The Bright Line Spectrum for Hydrogen

• While many electron transitions are possible for a hydrogen atom, only four of them produce visible light

Inside the Hydrogen Atom

1. Electron jumps from n=1 to n=4

2. Electron jumps from n=4 to n=2.

3. Light is emitted (486nm)

Who Cares? (besides physicists and chemists)

• The absorption and emission of electromagnetic radiation is one of the most powerful tools used to probe molecular structure and chemical reactions

• It forms the basis of nuclear magnetic resonance imaging (NMR)

• It is intrinsic to many analytical techniques used to monitor manufacturing and the environment

• Trace materials (evidence from a crime scene, lead in paint, mercury in drinking water) can be identified