e-e- e-e- Ground state Excited state Electrons can only be at specific energy levels, NOT between levels.

Color = Energy of Photons Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329

Energy Level Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329 A B CD Ground state Energy Four excited states

An Excited Lithium Atom Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 326 Photon of red light emitted Li atom in lower energy state Excited Li atom Energy

Excitation of Hydrogen Atoms Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 328

Return to Ground State

Frequency A Frequency B Frequency C n = 2 n = 1 n = 3

Frequency A Frequency B Frequency C n = 2 n = 1 n = 3 A B C A + B = C

Bohr Model of the Atom Electrons in Atoms nucleus (+) electron (-) Courtesy Christy Johannesson

Bohr Model electrons exist only in orbits with specific amounts of energy called energy levels Therefore… electrons can only gain or lose certain amounts of energy only certain photons are produced Courtesy Christy Johannesson

Bohr Model Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom Courtesy Christy Johannesson nucleus

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H 2 emission spectrum (top), H 2 absorption spectrum (bottom)

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H 2 emission spectrum (top), H 2 absorption spectrum (bottom) Lower-energy orbit Higher-energy orbit Photon e-e- e-e-

Bohr Atom The Planetary Model of the Atom

Bohr’s Model Nucleus Electron Orbit Energy Levels

Bohr Model of Atom The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation. Increasing energy of orbits n = 1 n = 2 n = 3 A photon is emitted with energy E = hf e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-

Cartoon courtesy of NearingZero.net

Development of the Atom

Planetary Model +

An unsatisfactory model for the hydrogen atom According to classical physics, light should be emitted as the electron circles the nucleus. A loss of energy would cause the electron to be drawn closer to the nucleus and eventually spiral into it. Hill, Petrucci, General Chemistry An Integrated Approach 2nd Edition, page 294

Quantum Mechanical Model Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals). Niels Bohr & Albert Einstein

Development of Atomic Models Rutherford model In the early twentieth century, Rutherford showed that most of an atom's mass is concentrated in a small, positively charged region called the nucleus. Bohr model After Rutherford's discovery, Bohr proposed that electrons travel in definite orbits around the nucleus. Thomson model In the nineteenth century, Thomson described the atom as a ball of positive charge containing a number of electrons. Quantum mechanical model Modern atomic theory described the electronic structure of the atom as the probability of finding electrons within certain regions of space.

Modern View The atom is mostly empty space Two regions – Nucleus protons and neutrons – Electron cloud region where you might find an electron