Class Notes-Bohr & New Atomic Model (Chapter 4, Section 1) Arrangements of Electrons in Atoms Thursday, October 29th, 2015

The Development of a New Atomic Model Rutherford’s model was incomplete How were electrons distributed? What prevents negative electrons from crashing into positive nucleus? 1900s-New Atomic Model (revolutionary) Relationship between light & electrons

How many Electrons Can Fit in Each Energy Level? Energy level is also called “principal quantum number” Energy Level = n n max. # electrons 1 2 2 8 3 18 4 32 5 50 6 72 Max # e = 2n2 Big Question: Why?

There are two ways to state the location of an electron… Quantum numbers – the location of a specific electron Electron configurations – the addresses of every electron in an atom (read the periodic table like a book!)

Quantum numbers Principle quantum number (n) = identifies the energy level of the electron Angular momentum quantum number (l) = identifies the sublevel Spin quantum number = identifies the orientation in the pair

Properties of Light Before 1900s-thought light behaved as a wave During 1900s-also has particle characteristics Electromagnetic Radiation

Electromagnetic Spectrum High Freq Low Freq High E Low E

Wavelength & Frequency Speed of light = 3.00 x 108 m/s c Wavelength = distance between waves  Frequency = # waves that pass point in  given time c =   as  increases,  decreases as  increases,  decreases (slinky analogy)

Emission of electrons from a metal when light shines on metal Light of minimum frequency was required to eject electron from metal Problem-wave theory of light predicted light of any frequency could eject electron Photoelectric Effect

Is Light a Particle? Max Planck-hot objects emit energy not as waves, but in packets called “quanta” “quantum”-minimum energy that can be gained or lost by an atom E = h E = energy h = Planck’s constant (6.626 x 10-34 Js)  = frequency (in s-1)

Light as a Wave & Particle Einstein’s radical idea! Dual wave-particle nature of light Called particles of light “photons” (no mass) Explained photoelectric effect (photon must have minimum energy) Light is absorbed only in whole numbers of photons Light as a Wave & Particle

Emission Spectra White light -continuous spectrum Hydrogen atoms -line-emission spectrum Big Question -Why did hydrogen atoms only give off specific frequency (colors) of light?

The “Fingerprints” of Atoms

Atomic Spectra The atom is quantized, i.e. only certain energies are allowed.

Neils Bohr Solved the Mystery Electron circled nucleus in “orbit” of fixed energy Absorption-electrons can “hop” from ground state to excited state Emission-when electrons “fall” from excited state to ground state Energy difference corresponds to hydrogen’s spectral lines Ladder analogy

Remaining Mystery Bohr’s model only explained hydrogen atom What about other elements with more than one electron?