Chapter 5• Light• Electrons in atoms

Models of the atomRutherford’s model of the atom did not show or explain chemical properties of elements

Needed some modifications

Along comes Bohr….

THE BOHR MODEL A Danish physicist, Niels Bohr, proposed that

an electron is found only in specific circular paths, or orbits, around the nucleus

This is a combination of Rutherford’s model and Einstein’s famous E=mc2

Each orbital has a fixed energy, known as energy levels

These levels are similar to rungs on a ladder. The lowest rung has the least amount of energy. The highest rung has the most energy

A quantum of energy is the amount of energy needed to move an electron from one orbital to another

When electrons jump from one orbital to another, they release light of different frequencies

These different frequencies result in different colors of light

Light behaves two ways:

1. As a wave2. As a particle

LIGHT: What Is It?• Light Energy

• Atoms• As atoms absorb energy, electrons

jump out to a higher energy level.• Electrons release photons

(bundles of energy released) when falling down to the lower energy level.

Light Waves

• Light pieces are called = PHOTONS• Light travels as a wave and has a QUANTUM of

energy

Wavelength (λ)= distance from crest to crest, meters

crest

trough

λ

Frequency (ν or f)= # of waves that pass a point in 1 sec., Hertz or Hz = 1/sec

ν

Speed of Light (c)= 3.00 x 108 m/sec

c = λ f

ELECTROMAGNETIC SPECTRUM

Light as a Particle*

• photoelectric effect• Emission of electrons from a metal

when light is shined onto the metal.• This helped us understand it also acts

as a particle!

Light EnergyQuantum = minimum amount of

Energy (in joules) that can be lost or gained by an atom

E=energy (J); f=frequency (1/sec, or Hz)

E = h f

Planck’s constant = h = 6.626 x 10-34 J sec

Light Spectra• Ground State – lowest energy state of an atom• Excited State – an atom in a higher energy state• Line Emission Spectrum –is the spectrum of

frequencies of electromagnetic radiation emitted by atoms or molecules when they are returned to their ground state.

• Each element's emission spectrum is unique.

Line Emission Spectrum

Quantum Model Notes

• We don’t know exactly where the electrons are, but we can have a MODEL showing the most likely place for an electron.

Quantum Model- 3D model to identify where electrons exist.

• ENERGY LEVELS= • a possible location for electrons

found around the nucleus • There are seven (maybe more!)

energy levels.

• SUBLEVEL= • For each level, (EXCEPT LEVEL #1!)

there can be orbitals of different shapes, called sublevels.

• We name these sublevels s,p,d,f

Long Periodic Table

Orbitals

• a single, allowed location for the atomic electron.

• Each orbital can hold at most 2 electrons

• can be empty or only half-full

Sublevel # Orbitals Total # of e-(# of orbitals*2)

s 1 2

p 3 6

d 5 10

f 7 14

3. What does the quantum mechanical model determine about electrons in atoms?

4. How do 2 sublevels of the same principal energy level differ from each other?

5. How can electrons move from one energy level to another?

6. How many orbitals are in the following sublevels?1. 3p2. 3d3. 2s4. 4f5. 4p

Aufbau Principle:Electrons fill the lower energy levels firstExample: Climbing a staircase. You have to start at the bottom and move your way up

Pauli Exclusion Principle:Each orbital can hold only 2 electrons of opposite spinExample: Shoes in shoe box

QUESTION FOR DISCUSSION

Modeling the Pauli Exclusion Principle:How are 2 bar magnets similar to electrons’ spins in the same orbitals?

Rules for electron Configurations• Hund’s Rule:• Electrons fill orbitals of equal

energy before sharing orbitals.• Ex: In a house, usually all kids

bedrooms are filled with one kid before their parents make them share a room.

The Energy Levels of an atom

Sublevels = s,p,d,f orbitals =

Energy

nucleusn=1

n=2

n=3

n=4

1s

2s

3s

4s

2p

3p

4p

3d

4d 4f