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

• Valence electrons are those on the outer level

• Valence e- determine properties ( reactivity )

Energy Level 4 holds 32 e-

Energy Level 3 holds 18 e-

Energy Level 2 holds 8 e-

Energy Level 1 holds 2 e-

NUCLEUS

Electron Energy Levels of Li

• Lithium as example– 3 electrons in Li

• 1 Valence electrons– In level that holds 8

Energy Level 4 holds 0 e-

Energy Level 3 holds 0 e-

Energy Level 2 holds 1 e-

Energy Level 1 holds 2 e-

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 4 holds 32 e-

s, p, d, and f orbitals

Energy Level 3 holds 18 e-

s, p and d orbitals

Energy Level 2 holds 8 e-

s and p orbitals

Energy Level 1 holds 2 e-

s orbital ONLY

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 4 holds 0 e-

s, p, d, and f orbitals

Energy Level 3 holds 0 e-

s, p and d orbitals

Energy Level 2 holds 1 e-

s and p orbitalsOnly 2s orbital used

Energy Level 1 holds 2 e-

s orbital ONLY

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- )orbitalEnergy Level

Number of Orbitals by Type ( for each energy level )

Total Orbitals

2 e- per orbital

Number e- spots

s p d f x2 for all

1 1 1 = 1 2

2 1 3 1 + 3 = 4 8

3 1 3 5 1 + 3 + 5 = 9 18

4 1 3 5 7 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