Bohr’s Model of the Atom Topic 15.4

Outcomes You will explain, qualitatively, the characteristics of, and the conditions necessary to produce, continuous, line-emission and line-absorption spectra You will describe that each element has a unique line spectrum You will explain, qualitatively, the concept of stationary states and how they explain the observed spectra of atoms and molecules You will calculate the energy difference between states, using the law of conservation of energy and the observed characteristics of an emitted photon

Bohr Atomic Model Bohr is credited with solving the problem with Rutherford’s planetary model of the atom To do so, Bohr, combined two previously existing ideas: Rutherford's Planetary Model Quantum Mechanics' Discrete Energy

Combining Ideas Bohr suggested that electrons could only exist at certain distances from the nucleus and at each of these distance the electrons had corresponding energies http://phet.colorado.edu/en/simulation/hydrogen-atom Experiments later demonstrated that atoms only ever emit energy at specific frequencies (spectroscopy)

Spectroscopy Spectroscopy involves looking at light from various sources through a diffraction grating and analyzing the colors that are seen Every element gives off its own unique emission line spectrum, which acts like a fingerprint of that element

Types of Spectra Continuous Spectra When the light from a glowing solid or liquid passes through a diffraction grating or a prism, a continuous spectrum is produced In a continuous spectrum, light of all colours (all frequencies) are present - one colour continues into the next.

Types of Spectra 2. Dark-line Spectra (Absorption Spectra): If we shine a continuous spectrum of white light through a cool gas at low pressure, it will absorb the light at the same frequencies as it emits These missing lines from the continuous spectra are called the absorption line spectrum

Types of Spectra 3. Bright-line Spectra (Emission Spectra): In experiments it was discovered that if a high voltage (or heat) is applied to a gas sample of an element in a tube at low pressure When a spectroscope is used to examine the light, it does not emit a continuous spectrum of colors. Instead, each element releases an emission line spectrum made up of individual lines of color at specific frequencies

Types of Spectra Note that the bright-line and dark-line spectra of a certain gas have the lines at the same frequency

Spectrum of the Stars

Bohr’s Hypothesis Bohr theorized that energy in the form of EMR is: Emitted when an electron falls down from one level to a lower level Absorbed to make an electron jumps up from one level to a higher level **As electrons move out from the ground state (or outwards in general) to higher levels photons are absorbed (absorption spectrum) **As electrons move back to the ground state (or inwards in general) to lower energy levels photons are emitted (emission line)

Representation of Energy Levels Electrons that exist in an energy level are considered to be in a stable state and do not give off EMR

Calculations and Energy Levels The energy absorbed or emitted during a transition can be calculated using: The wavelength and frequency of the emitted or absorbed photon can be calculated using:

Homework Concept Q’s: p. 780 #2, 4 & 8 Calculation Q’s: p. 780 #6 & 11 Bohr booklet