Warm-Up Lithium has an atomic weight of g/mol. When g of lithium is heated, it emits an energy measured at 262,500 joules. What is the energy given off by one atom?

Properties of Light Wave TheoryParticle Theory Light has measureable characteristics of frequency ( ν ) and wavelength ( λ) When matter is heated, it emits radiation, the wavelength distribution of the radiation depends on the temperature. Planck showed that energy can only be released by atoms in “chunks” of some minimum size (quantum). E=h ν

Properties of Light Wave TheoryParticle Theory Photoelectric effect: light shining on a clean metal surface causes the surface to emit electrons, but only if the frequency of the light is greater than some minimum frequency. Einstein showed that light comes in small energy packets called photons. The photon must have enough energy (E=h ν ) to overcome the attractive forces holding the electron within the metal atom. If the photon does not have enough energy (does not have the minimum frequency), no electron will be emitted.

Properties of Light What happens when white light passes through a prism? It produces a continuous spectrum of color (a rainbow) The white light is dispersed into its component wavelengths – it contains light of all wavelengths and thus a continuous spectrum is produced.

Properties of Light Not all radiation sources produce a continuous spectrum. When different gases are placed under low pressure in a tube, and high voltage is applied, the gases emit different colors of light.

Properties of Light When the light from such a tube is passed through a prism, the resulting spectrum only contains a few wavelengths of light. The colored lines are separated by black space. Each element has its own characteristic (unique) spectrum

Properties of Light A spectrum containing radiation of only specific wavelengths is called a line spectrum. Scientists first detected the line spectrum of hydrogen in the mid- 1800’s, but couldn’t explain it.

Properties of Light The lowest energy state of an atom is called its ground state. A state in which an atom has a higher potential energy than it has in its ground state is an excited state.

Properties of Light When current is passed through a gas at low pressure, the potential energy of some of the gas atoms increases. The atoms go from their ground state to an excited state. Ground state Excited state

Properties of Light When an excited atom returns to its ground state, it releases the excess energy in the form of EM radiation (e.g., light). Ground state Excited state

Hydrogen Line Spectrum Scientists first detected the line spectrum of hydrogen in the mid- 1800’s, but couldn’t explain it. They predicted that the hydrogen atoms could absorb whatever amount of energy was added to them, and thus emit a continuous spectrum of light.

Hydrogen Line Spectrum Explanation of hydrogen’s line emission spectrum led to a new theory of the atom – called quantum theory

Hydrogen Line Spectrum When an excited hydrogen atom falls back from an excited state to its ground state (or lower energy state) it emits a photon of radiation E photon =h ν The energy of the photon is equal to the difference in energy between the atom’s two states

Hydrogen Line Spectrum What they saw: hydrogen atoms only emit specific frequencies of light What it means: the energy differences between the atoms’ energy state are fixed The electron of a hydrogen atom exists only in specific energy states.

Hydrogen Line Spectrum In addition to the spectral lines in the visible part of the spectrum, hydrogen also produces spectral lines in the ultraviolet and infrared regions of the EM spectrum. Scientists still needed to come up with a model of the hydrogen atom that explained this…

Bohr Model In 1913 Niels Bohr proposed a model of the hydrogen atom that linked hydrogen’s electron with photon emission.

Bohr Model The electron can circle the nucleus only in allowed paths, called orbits. The electron (and therefore the hydrogen atom) is in its ground state when it is in the orbit closest to the nucleus.

Bohr Model The electron cannot exist in the empty space between orbits The energy of the electron is higher when it is in orbits that are farther from the nucleus (excited state)

Bohr Model & Spectral Lines An electron can move to a higher orbit by gaining an amount of energy equal to the energy difference between the two orbits.

Bohr Model & Spectral Lines When the electron falls back to the lower energy state, the excess energy is released as a photon of light

Bohr Model & Spectral Lines From the wavelengths of the emitted spectral lines, Bohr was able to calculate the allowed energy levels for hydrogen

Bohr Model & Spectral Lines Bohr’s success in explaining mathematically the spectral lines of hydrogen did not work when trying to explain the spectra of atoms with more than one electron. Nor did Bohr’s theory fully explain the chemical behavior of atoms. A better (and stranger) idea was needed.

Assignment P. 97 #1-5