Modern Atomic Theory
Physical Science
State Standards
• CLE.3202.Inq.1 – Recognize that science is a
progressive endeavor that reevaluates and
extends what is already accepted
Review of First Models
• Democritus : Used atomos to describe matter
– No experiments, just observation with naked eye
• John Dalton : Experiments to combine gases
– Developed Law of Definite Proportions
• J.J. Thompson : Cathode-Ray Tube
– Discovered electrons / Plum Pudding model
• Earnest Rutherford : Gold Foil experiment
– Discovered nucleus ( protons ) / Modern view
– Electrons orbit nucleus in definite paths ( planets/sun )
What Do We Know Now?
• Model ( conceptual view ) of the atom has
changed significantly since Rutherford’s model
• Now know that electrons are found only in ( at )
certain energy levels – not between levels
• Electrons act like waves
• Exact locations of electrons cannot be determined
Electron Energy Levels
• In 1913, Niels Bohr
suggested that electrons
can reside only at certain
energy levels
• Electrons must gain
energy to move up
• Electrons must lose
energy to move down
ELEVATOR MODEL
Electrons – Particles and Waves
• By 1925, Bohr’s model no longer explained all aspects
of electron behavior
• Electrons act like particles but they also act like waves
too
– Wave-Particle duality
– Smaller the particle, the more it acts like a wave
• Concept of electrons redefined
– Imagine a wave vibrating on a string ( demo? )
Wave-Particle Duality
• In 1923, Louis de Broglie ( a French physicist )
made a hypothesis that led to a statement of the
WAVE-PARTICLE DUALITY of nature.
• It included the present theory of atomic structure
• De Broglie used research by Albert Einsten and
Max Planck to develop an equation relating mass
and velocity of a particle to its wavelength
Electron Energy Levels
• Number of levels filled
depends on electrons
– specific to an element
Energy Level 1 holds 2 eEnergy Level 2 holds 8 eEnergy Level 3 holds 18 eEnergy Level 4 holds 32 e-
• Valence electrons are
those on the outer level
• Valence e- determine
properties ( reactivity )
NUCLEUS
Electron Energy Levels of Li
• Lithium as example
– 3 electrons in Li
Energy Level 1 holds 2 eEnergy Level 2 holds 1 eEnergy Level 3 holds 0 eEnergy Level 4 holds 0 e-
• 1 Valence electrons
– In level that holds 8
NUCLEUS
Location of Electrons
• Imagine the moving propeller of an airplane
– Now determine the location of any blade at any time
– How would you do this?
• Exact position of electrons also hard to calculate
– Moving very fast always ( never stop or slow down )
• Orbitals ( regions where electrons might be ) are
thus used to describe an approximate location
– Different orbital SHAPE for different orbital types
Electron Orbitals
• Four orbitals : s, p, d, and f
• d and f orbitals are more complex
– 5 possible d orbitals ( holds up to 10 electrons total )
– 7 possible f orbitals ( holds up to 14 electrons total )
Energy Levels and Orbitals
• 1st energy level has only s orbital
• 2nd energy level has s and p orbitals
• 3rd energy level has s, p, and d orbitals
Energy Level 1 holds 2 es orbital ONLY
Energy Level 2 holds 8 es and p orbitals
Energy Level 3 holds 18 es, p and d orbitals
Energy Level 4 holds 32 es, p, d, and f orbitals
Energy Levels and Orbitals of Li
• 1st energy level has only s orbital
– 2 electrons
• 2nd energy level has s and p orbitals
– 1 electron
Energy Level 1 holds 2 es orbital ONLY
Energy Level 2 holds 1 es and p orbitals
Only 2s orbital used
Energy Level 3 holds 0 es, p and d orbitals
Energy Level 4 holds 0 es, p, d, and f orbitals
Energy Levels and Orbitals
• Each energy level has a unique orbital layout
• Orbitals in each energy level decide total number of
electrons that can be at each energy level
• Example – the 2nd energy level holds 8 e• has an s ( holds 2 e- ) and a p ( holds 6 e- )orbital
Energy
Level
Number of Orbitals by Type
( for each energy level )
s
p
d
1
1
2
1
3
3
1
3
5
4
1
3
5
Total
Orbitals
f
7
2 e- per
orbital
Number
e- spots
x2 for all
1=1
2
1+3=4
8
1+3+5=9
18
1 + 3 + 5 + 7 = 16
32
Electron Transitions
• Electron locations limited to certain energy levels
• How do electrons move between levels?
• When an atom gains or loses energy
• Ground state – lowest energy level of electron
– At normal (room) temps, electrons at ground state
• Excited state – when electron has gained energy
Atoms Emit & Absorb Light
• Photons are absorbed by electrons to move it
from ground to an excited state
• Photon – a particle of light
• Photons have different energies
• Elevator model – energy of emitted photon
depends on how many levels are crossed
Atoms Emit & Absorb Light
• Energy of photon is related to wavelength of light
– High-energy photons have short wavelengths
– Low-energy photons have long wavelengths
• Wavelengths emitted depend on element
– Since each element has a unique structure
– Atomic “fingerprint”
– Neon signs : Neon ( Ne ) gas produces red light
– other gases emit different colors of light
– Color depends on wavelength of light emitted