ELECTRON CONFIGURATIONS

A B r ie f H is to ry o f A to m ic T h eo ry

W hile th ese m od e ls w o rk re a so na b ly w e llth e ir lim ita ton s h av e le d to m o re m o de rn the or ies

a s to th e n a tu re o f the a to m .

L im ita tio ns o f D a lton 's m od e lle d to th e T h om pson an d R u th e rfo rd

m o de ls o f the a to m .

T h e ir e xpe rim en ts led toD a lto n 's A to m ic T h eo ry

E a rly ch e m is tsp e rfo rm ed expe r im en ts

G re eks w e re th e f i r st to su gg e stth a t m a tte r is m a d e u p o f a to m s

Development of Atomic Models

Rutherford modelIn the early twentieth century, Rutherfordshowed that most of an atom's mass isconcentrated in a small, positively chargedregion called the nucleus.

Bohr modelAfter Rutherford's discovery, Bohr proposedthat electrons travel in definite orbits aroundthe nucleus.

Thomson modelIn the nineteenth century, Thomson describedthe atom as a ball of positive charge containinga number of electrons.

Quantum mechanical modelModern atomic theory described theelectronic structure of the atom as theprobability of finding electrons withincertain regions of space.

• At high temperatures or voltages, elements in the gaseous state emit light of different colors.

• When the light is passed through a prism or diffraction grating a line spectrum results.

10.2

visible light is part of the electromagnetic

spectrum

X-rays are part of the electromagnetic

spectrum

Infrared light is part of the

electromagnetic spectrum

The Electromagnetic Spectrum

Line spectrum of hydrogen. Each line corresponds to the wavelength of the energy emitted when the electron of a hydrogen atom, which has absorbed energy falls back to a lower principal energy level.

These colored lines indicate that light is being emitted only at certain wavelengths.

Each element has its own unique set of spectral emission lines that distinguish it from other elements.

10.3

Modern View

The atom is mostly empty space

Two regionsNucleus

protons and neutrons

Electron cloud region where you might find an

electron

• Instead of being located in orbits, the electrons are located in orbitals.

• An orbital is a region around the nucleus where there is a high probability of finding an electron.

Quantum Numbers

Principal Quantum NumberPrincipal Quantum Number ( nn )

Angular Momentum Quantum #Angular Momentum Quantum # ( ll )

Magnetic Quantum NumberMagnetic Quantum Number ( mmll )

Spin Quantum NumberSpin Quantum Number ( ms )

Four Quantum Numbers:Specify the “address” of each electron in

an atom

Quantum Numbers1. Principal Quantum NumberPrincipal Quantum Number ( nn )

Indicates the number of the energy level

As n increase, size of electron cloud increases.

Energy increases as n increases.

2n2 = maximum # of electrons possible in the energy level

Ex. if n=1, energy level 1, can only have 2 electrons Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

1s

2s

3s

Quantum Numbers

s p d f

2. Angular Momentum Quantum #Angular Momentum Quantum # ( ll )Describes the sublevel within each energy

level

# of sublevels = value of principal quantum number of that level Ex. n=1, has 1 sublevel

n=2, has 2 sublevels

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Quantum Numbers

s

p

d

f

2. Angular Momentum Quantum #Angular Momentum Quantum # ( ll )The lowest sublevel has been named s.

The second sublevel has been named p

The third sublevel has been named d

The fourth sublevel has been namde f

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Quantum Numbers

s

2. Angular Momentum Quantum #Angular Momentum Quantum # ( ll )

There is just one s sublevel , thus it has one orbital that can hold only 2 electrons.

Orbital: space occupied by one pair of electrons.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

x

y

z

x

y

z

x

y

z

2. Angular Momentum Quantum #Angular Momentum Quantum # ( ll )

There are three p sublevels and thus it has three orbitals that can hold only 2 electrons.

Quantum Numbers2. Angular Momentum Quantum #Angular Momentum Quantum # ( ll )

There are five d sublevels and thus it has five orbitals that can hold only 2 electrons.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Quantum Numbers2. Angular Momentum Quantum #Angular Momentum Quantum # ( ll )

There are seven f sublevels and thus it has seven orbitals that can hold only 14 electrons.

Too complicated to show with drawings.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Principal Energy Levels 1 and 2

For n=1, it can hold a maximum of 2n2 number of electrons: 2 electrons For n=2, it can hold a maximum of 2n2 number of electrons: 8 electrons

ClassworkP 118 # 6 and p122 # 7,8

Quantum Numbers3. Magnetic Quantum NumberMagnetic Quantum Number ( mmll )

Specifies the exact orbital within each sublevel

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Quantum Numbers4. Spin Quantum NumberSpin Quantum Number ( ms )

An orbital can hold 2 electrons that spin in opposite directions.

Indicated by arrows:

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

The arrows indicate 2 electrons spinning in opposite direction

C. Johannesson

General Rules For Writing Electron Configurations

1. Pauli Exclusion Principle

Each orbital can hold TWO electrons with

opposite spins.

In the following diagrams boxes represent orbitals.• Electrons are indicated by arrows: ↑ or ↓.

C. Johannesson

2. Aufbau Principle

Electrons fill the lowest energy orbitals first.

The number represents n, the principal quantum number

C. Johannesson

RIGHTWRONG

3. Hund’s RuleWithin a sublevel, place one e- per orbital

before pairing them.

C. Johannesson

O 8e-

Orbital Diagram

Electron Configuration

1s2 2s2 2p4

Notation

1s 2s 2p

↑ 1s2↓

H ↑ 1s1

Hydrogen has 1 electron. It will occupy the orbital of lowest energy which is the 1s.

He

Helium has two electrons. Both helium

electrons occupy the 1s orbital with

opposite spins.

WRITING ELECTRON CONFIGURATIONS

1s

1s

Superscript indicates number of electrons in orbital

Li

1s22s2

The 1s orbital is filled. Lithium’s third electron will enter the 2s orbital.

↑↓ ↑

↓

1s 2s

↑

1s22s1

Be ↑↓

The 2s orbital fills upon the addition of

beryllium’s third and fourth electrons.

1s 2s

Filling the 2s Sublevel

B1s22s22p1

1s 2s 2p

↑↓ ↑↓ ↑

Boron has the first p electron. The three 2p orbitals have the same energy. It does not matter which orbital fills first.

↑ ↑ ↑N1s 2s 2p

↑ ↓↓ ↑

The third p electron of nitrogen enters a different p orbital than its first two p electrons to give nitrogen the lowest possible energy.

1s22s22p3

Filling the 2p Sublevel

V

1s 2s 2p

3s 3p 4s

3d1s22s22p63s23p64s23d3

Classwork p 128 #14

Electron Dot DiagramsThe electrons in the outer energy level

(called valence electrons) are the most important electrons for chemical reactions.

Lewis electron dot diagrams are used to represent these outer electrons around the symbol of an element.

ExamplesLithium Electron configuration: 1s22s1

Select electrons that are in the outer energy level (the ones with the largest principal quantum number): 1s22s1

Largest principal quantum number is 2 and there is 1 electron in this level Li

1. Symbol of element represents nucleus and all electrons except those in outer level2. Write the electron configuration of element to determine valence electrons.3. Each side of symbol represents an orbital, draw dots to represent electrons in that orbital.

Valence electron

Oxygen: 1s22s2 2p4

Oxygen: 1s2 2s2 2p4

Oxygen: has 6 valence electrons (2 +4)

O

Krypton: 1s22s2 2p6 3s23p6 4s2 3d10 4p6

krypton: has 8 valence electrons (2 +6)

Kr

Krypton: 1s22s2 2p6 3s23p6 4s2 3d10 4p6

Classwork p 130 # 15 (Z= atomic number)