Atomic Emission Spectrum EQ: Why does each element have a unique absorption and emission spectrum and how does the Bohr model explain it?

Questions to ponder Why do hot objects glow and why does the color of the glow change predictably? Why do solar panels create electricity? Why do gases glow when an electric current is run through them? What gives fireworks their color? What gives fire its color?

Elements can be identified by their atomic emission spectrum Spectrum – an ordered collection of wavelengths When gases are heated they emit light. The wavelengths of the light emitted corresponds to a definite wavelength and frequency represented by a line. Each element in its gaseous state has a unique set of wavelengths in its line spectrum.

Emission Spectra Continuous Emission Spectrum – section of the visible spectrum that is unbroken and includes wavelengths from the entire band Slit White Light Source Prism Photographic Film

Discrete Emission Spectrum Emission – a spectrum made up of only a few narrow bright lines Spectra of Hydrogen Discrete Emission Spectrum Slit Film Low Density Glowing Hydrogen Gas Prism Photographic Film

Absorption – A spectrum that spans the entire visible spectrum but is missing certain wavelengths Spectra of Hydrogen Discrete Emission Spectrum Discrete Absorption Spectrum Slit Hydrogen Gas Film White Light Source Prism Photographic Film

Why does this happen? Bohr Model Electrons can be found in orbitals (quantum levels) around the nucleus. When an electron has energy added to it, it will jump from its ground state to an excited state

Bohr Planetary Model of the Atom 1. EXCITATION Bohr Planetary Model of the Atom

Vocab: Excitation - occurs when an electron in an atom is given energy causing it to jump to a higher orbit. This can happen through: -collisions -photon absorption Ground state (relaxed) – the lowest allowable energy state of an atom. Atoms always return to ground state.

With possible orbits for electrons Here comes a nucleus With possible orbits for electrons e- In this configuration the atom is not excited.

Here comes a photon e- In this configuration the atom is not excited.

e- Now the atom is said to be excited because the electron is in a higher than normal orbit.

e- Now the atom is in a non-excited state again. It emitted a photon when it changed orbits.

The excited atom usually de-excites in about 100 millionth of a second. The subsequent emitted radiation has an energy that matches that of the orbital change in the atom.

Atomic Emission Spectrum

This emitted radiation gives the characteristic colors of the element involved. Video - Flame Colors https://www.youtube.com/watch?v=jJvS4uc4TbU

Matter gains and loses energy the same way Max Planck found that matter can gain and lose energy only in small, specific amounts called quanta If electron is hit with a quanta of sufficient energy, it will be promoted to a higher energy state BUT IF….electron does not receive enough energy to jump to next orbital nothing happens AND quanta CANNOT build up

How it Works In order to make the material glow, an electron needs to absorb enough energy to jump to the next orbital…it needs a frequency of 5.5 x 1014 Hz or higher. Shine 5.4 x 1014 Hz and lots of it all day, NOTHING will happen Hit it with 5.5 x 1014 Hz …HURRAY! BUT electron wants to go back to ground (relaxed) state As it drops it releases the same amount of energy absorbed You get LIGHT!

(h = Planck’s constant = 6.63 x 10-34J) So, when matter glows It has been heated enough and absorbed energy As electron falls back to ground state it releases its energy it particular sized packets called quanta Measure the amount of energy in each of this quanta through this equation E = hf (h = Planck’s constant = 6.63 x 10-34J)

Example Tiny water droplets in the air disperse the white light of the sun into a rainbow. What is the energy of a photon from the violet portion of a rainbow if it has a frequency of 7.23 x 1014Hz?

Example What is the energy of a photon with a wavelength of 5.76 x 10-7Hz?

Exited electron releases energy in the form of light Frequencies and wavelengths are determined by the movement of an electron through quantum levels

Lyman – Balmer - Paschen

Questions to answer Why do hot object glow and why does the color of the glow change predictably? Why do solar panels create electricity? Why do gases glow when an electric current is run through them? What gives fireworks their color? What gives fire its color?