Modern Atomic Theory
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Transcript of Modern Atomic Theory
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