Bohr's Model of the Atom PDF

Bohr’s model of the Atom LEARNING TARGETS Explain the limits of the Rutherford’s model of the atom Describe the experimental work of Bohr with hydrogen gas Explain the postulates of the Bohr’s Atomic model Explanation of the line spectrum of the hydrogen by Bohr’s...

Bohr’s model of the Atom LEARNING TARGETS Explain the limits of the Rutherford’s model of the atom Describe the experimental work of Bohr with hydrogen gas Explain the postulates of the Bohr’s Atomic model Explanation of the line spectrum of the hydrogen by Bohr’s model Explain the success and failure of Bohr’s model Explain how Bohr used two discoveries to develop his model  Limitations of Rutherford model of the atom There was a significant problem with this model The atom would collapse if the electron spirals into the nucleus Rutherford’ model could not explain why atoms remain stable Two discoveries used by Bohr to develop his model Photoelectric effect Quantum Theory Rethinking atomic structure based on the nature of energy It has been known that atoms could emit light when stimulated by an electric current You can actually identify elements by the set of colours that each atoms of each element emit when stimulated. However they have no idea how or why this occurred You will see how Bohr use this information to develop his atomic model Nature of light Light is a form of energy called electromagnetic radiation Electromagnetic radiation is a form of energy that is produced by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles traveling through a vacuum or matter Light and Matter At the end of the 19 century, matter and energy were considered to be distinct and unrelated Matter – particles and had mass Light – waves with no mass However this change with the discovery of the photo electric effect Photoelectric effect Electrons are emitted from a metal surface when it is exposed to light Photoelectric effect Planck’s Quantum Theory Energy released or absorbed is always in the discrete(separate and distinct) quantity of energy Energy released or absorbed is not continuous stream The energy emitted by a heated solid is quantized One packet of energy is known as QUANTUM of energy  Photon Is a unit of light energy Photoelectric effect can be explained by applying the quantum theory Light can also be viewed as a stream of particles called photons Photoelectric effect can be explained by the light photons colliding with the electrons at he metal surface and ejecting them from the metal Recap -Quantization of Energy Means energy is released or absorbed in discrete or certain amounts Quantized means certain values only Continuous means all values Analogy to illustrate quantization Canadian currency only used certain values - $1 $2 $5 $10 $20 $50 $100 Canadian currency is quantized  Quantization of energy analogy A person walking on a ramp can stand at any height along the ramp – continuous energy A person walking on stairs can only stand at heights at the top of each stair step. They cannot stand at the heights in between the tops of the stair steps. The stair steps are analogous to quantized energy. It can only have certain values  Quantize d energy having a specific amount of energy Bohr’ work - 1911 experimented with applying electricity and thermal energy to hydrogen gas he observed a line spectrum of only certain colors Bohr used Planck’s Quantum Theory to develop his model of the atom To explained the line spectrum of hydrogen gas Bohr proposed that electrons orbit the nucleus in definite energy levels Bohr’s experiment SPECTROSCOPY The analysis of spectra to determine the properties of their source using a spectrometer EMISSION SPECTRUM Spectrum seen when electromagnetic radiation emitted by atoms is passed through a spectrometer results when excited atoms return to a lower energy state from a higher energy state TYPES OF EMISSION SPECTRUM CONTINUOUS SPECTRUM An emission spectrum that contains all wavelengths DISPERSION OF WHITE LIGHT Produces continuous spectrum Contains all the wavelengths of Visible light LINE SPECTRUM Emission spectrum that contains only certain wavelengths Arises when excited electrons emit energy Every element has its own unique line spectrum like a fingerprint Astronomers use line spectra to identify the composition of stars Line spectrum of various elements Bohr’s experiment Hydrogen line spectrum Only lines of certain colors are observed  Bohr model of the atom Bohr used the line spectrum of the hydrogen atom to develop a quantum model of the atom He knew that his model has to account for the experimental evidence provided by the hydrogen line spectrum The line spectrum of the hydrogen atom proves that electron can only have particular discrete energy levels He postulated that the total energy of the atom is quantized  Bohr’s Model- postulates Electrons only exist in certain allowed orbits. Each orbit has a specific quantity of energy associated with it Electrons do not lose energy in this allowed orbits Energy of electron increases with increasing distance from nucleus Bohr model postulates Electrons can jump from and to different energy levels When electron jump from one energy level to another, a specific quantity of energy is emitted or absorbed Bohr’s model of the atom Also called the Planetary model Electrons are like planets moving round the sun Electron transition – the movement of electrons from on energy level to another  How an electron gains energy? When an electron gains energy it jumps to a higher energy level How an electron loses energy? When an electron loses energy it drops to a lower energy level Bohr’s explanation of line spectrum of hydrogen gas  BOHR’s explanation of line spectrum of hydrogen gas Electrons do not emit energy when they are in the same orbit Colors of light emitted by excited hydrogen atoms correspond to the energy changes that electrons experience as they move between energy levels Since only certain colors were observed, Bohr concluded that an atom has only certain specific energy levels Hydrogen Line Spectrum Explanation Video https://www.youtube.com/watch?v=p40n-x o2lT8 Bohr’s model success The Bohr model’s is of great historic importance because it included the quantization of energy of in atoms and paved the way for later theories The energy levels Bohr calculated for the hydrogen atom were very similar to values obtained experimentally from the hydrogen’s atom emission spectrum by spectroscopy Drawbacks of Bohr’s model Bohr was unable to provide any theoretical justification for his assumption that electrons in orbits would not lose energy by radiation. His theory could only explain the line spectrum of hydrogen but not the line spectra of more complex atoms Bohr’s Model of the Atom Video https://www.youtube.com/watch?v=au2 HCVn9IJI  v=xytRf3fs_gY Bohr Model in Brief: The ube.com/watch? planetary model, its https://www.yout connection to emission spectra & quantized electrons. https://www.yout ube.com/watch? v=lJh2Ra1eygA