Absorption & Emission

Energy of a photon We can measure the energy of a photon using Einstein’s equation: h = 6.63 x 10-34 Js  Planck constant f = frequency of photon/electromagnetic radiation c = 3 x 108 m/s  speed of light in a vacuum l = wavelength of photon/electromagnetic radiation

Energy Levels So, what causes the hydrogen in the discharge tube to emit just four wavelengths of light? The electrons in the atom can only orbit at certain distances from the nucleus, i.e. the radius of the orbit is “quantized” Each “orbit” represents the energy of the electrons. So, electrons on the lower level (orbit) is at Ground State and have the lowest potential energy, while the other states are called Excited States because the electrons are at a higher potential energy The electric discharge gives the electrons the energy to jump on to higher energy levels Eventually (pretty quickly), these electrons will jump down to a lower energy level, or even the ground level In jumping down the electrons emit a photon of energy equal to the difference in the energy between the two levels

Energy Levels and Photon Emission 2nd Exited State DE = 0.66 eV 1st Exited State DE = 1.90 eV Ground State Calculate the energy of all the photons that can be emitted by this atom. DE is the energy gap between two energy levels. Which of them is not part of the visible spectrum? 1st to ground state 2nd to 1st state 2nd to ground state

Answer: 1st to ground level The energy of a photon is E = hf The energy of the emitted photon equals the energy gap between the two energy levels. DE = 1.90 eV, so:

Answer: 2nd to 1st level The energy of a photon is E = hf The energy of the emitted photon equals the energy gap between the two energy levels. DE = 0.66 eV, so:

Answer: 2nd to ground level The energy of a photon is E = hf The energy of the emitted photon equals the energy gap between the 2nd and the ground energy level. DE = 1.90 + 0.66 = 2.56 eV, so:

Frequencies

Hydrogen Emission Spectrum Hydrogen is the simplest atom and so are its energy levels. When its only electron is excited to higher states, it can fall back to lower energy levels to emit different photons E = 0 eV E = -0.85 eV E = -1.51 eV E = -3.40 eV 1st excited state Balmer Series E = -13.61 eV Ground State Lyman Series

Hydrogen Emission Spectrum What is the difference between the Lyman and the Balmer Series? In the Lyman Series the electron “relaxation” produces the emission of U.V. photons In the Balmer Series the “relaxation” of the electron produces emission of visible light photons Hydrogen Emission Spectrum

Emission Spectra When the electrons of an atom are exited they can jump to higher energy levels, if the energy provided is just right to make the jump. When they jump back down a photon of energy hf = E1 – E2 is emitted. That is why an atom will emit only light with wavelength (i.e. colour) characteristic of the energy levels in the atom. Click on each energy level to reveal the emission spectrum of hydrogen

Absorption Spectra We considered how excited electrons can fall from a higher energy level to a lower one and emit a photon. But, what gave the electrons the energy to get to those higher levels? One way is when an electron absorbs a photon The photon must have exactly the same energy as the energy between the two energy levels What wavelengths of visible light make up white light from, say, an incandescent filament lamp? All the wavelengths between 700 – 400 nm (all colours)

Hydrogen Absorption Spectrum Absorption Spectra So, what would the spectrum of this “white” light be after passing through a gas of, say, hydrogen? All the wavelengths will be present apart from the wavelengths of the emission spectrum of hydrogen, i.e. Balmer Series. In fact, these have been absorbed by the atoms in the hydrogen gas. Hydrogen Atoms Hydrogen Absorption Spectrum

Absorption Spectra and Stars We can study absorption spectra from stars to understand their composition. By looking at the Sun’s absorption spectrum can you tell which element is most abundant? Sun’s Absorption Spectrum Hydrogen Absorption Spectrum Hydrogen is the most abundant element in the Sun. In fact, the darkest lines in its spectrum match the wavelengths of the absorption spectrum of hydrogen.

The atom and energy levels Click on different areas to reveal their properties A photon with energy hf = E1 – E2 hits the e-, which is exited to the next energy level. Electron: atomic particle orbiting around the nucleus. –ve charge and mass 1/1800 of a proton Neutron: neutral particle inside the nucleus. Different no of neutrons for the same element make different ISOTOPES of that element e- falls to a lower level and a photon with energy hf = E1 – E2 is released. E1 Nucleon no: given by the sum of no of protons and no of neutrons. Using this no you can identify the different isotopes of the same element. Ground level: lowest energy state possible for the electrons. Electrons tend to fill this level 1st and “fall” in it after being exited. E2 Proton: equal and opposite charge as e-. Slightly less mass than a neutron. 7 Li e- falls back to ground level hf = E1 – E2 hf = E1 – E2 Exited e- jumps energy gap Proton no: it tells the no of protons (and also electrons) in the atom. This no is a property of the element, so the atomic no tells what element the atom is. 3 Higher energy levels: when not completely filled with e-, they can “host” exited e-.