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 1 st energy level has only s orbital 2 nd energy level has s and p orbitals 3 rd 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 1 st energy level has only s orbital – 2 electrons 2 nd 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 orbitals Only 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 2 nd 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 ) Total Orbitals 2 e - per orbital Number e - spots spdf x2 for all 111 = = = = 1632

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